What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Interesting Read: Vne-TAS or IAS That is the question

Pmerems

Well Known Member
Advertiser
Recent finding from the N174BK accident and interesting discussions on forum about this incident caught everyone?s interest for safety reasons. The report indicated that Vne was exceeded and destructive aerodynamic flutter occurred resulting in the loss of life. I have read many of these posts and one post in particular caught my eye was the comment that Vne is in TAS not IAS.

I had a Dynon D180 in my RV-7A when I built it and I set up all the speed limits in the unit. I didn?t recall there was an option in the setup Vne as TAS. About a year and a half ago I upgraded to the Dynon SkyView system and once again I entered the speed limits in the setup but didn?t recall an option for Vne as TAS.

After reading these more recent posts I talked to another RVer with a Garmin system and he mentioned he had the option to set Vne limit as TAS or IAS. I then researched the Dynon SkyView software update history and found that about a year ago there was a software update that allowed the selection of the Vne limit as TAS or IAS. I then set my Vne as TAS which is now a dynamic limit taking into account density altitude.

After talking with a few other RVers there was still some disagreement on whether Vne should be in TAS or IAS. I even asked a friend at work who retired from the Air Force as a flight test engineer (he attended AF test pilot school) and he thought Vne was an IAS reading. After a few days he kept thinking about it and did some internet searching and came across this interesting article that I thought would be of interest to the RV community. In fact some VAF posts were cited in the article. There is also some interesting flutter videos.

http://flyingdonald.blogspot.com/2011/03/what-flutter.html
 
Recent finding from the N174BK accident and interesting discussions on forum about this incident caught everyone’s interest for safety reasons. The report indicated that Vne was exceeded and destructive aerodynamic flutter occurred resulting in the loss of life.

I don't think that is what the report said at all. Exceeded Vne, sure, but I think it in fact denied evidence of aeroelastic flutter.

The statement in the report "All fractures exhibited characteristics consistent with static overload. No evidence of multiple dynamic loading fractures was found, nor evidence of fatigue characteristics or other preexisting damage" says pretty much exactly that: overspeed, but no flutter.
 
Last edited:
After talking with a few other RVers there was still some disagreement on whether Vne should be in TAS or IAS. I even asked a friend at work who retired from the Air Force as a flight test engineer (he attended AF test pilot school) and he thought Vne was an IAS reading.

From my memory when we added Vne as TAS to SkyView: In certified piston aircraft, Vne must be an IAS number. This means the designer must choose an IAS number that is safe at that IAS at the service ceiling of the plane if the aircraft if flutter (TAS) limited not dynamic pressure (IAS) limited.

Apparently there are some planes out there that have artificially low published ceilings so that the Vne can be higher because they only need the IAS to represent the TAS limit at the published ceiling.

Of course, much of this likely comes from the fact that up until a few years ago, light GA aircraft didn't really have access to TAS numbers in the cockpit on a gauge, so allowing a TAS instead of IAS limit was kind of pointless because the pilot couldn't easily see the limit at that moment.

Now that we can have an EFIS dynamically move the IAS Vne marker based on the lower of the TAS or IAS limit, it would be interesting to hear what an airframe designer would tell you the appropriate limits are for both IAS and Vne if they didn't have to relate them to one another.

One thing I do know- before EFIS units, most people were using Vne in IAS, and doing that at 10K feet, where the TAS was a lot higher. RV's don't seem to come apart when you do that. We've had a few people tell us how restrictive it is to fly an RV with the TAS limit at the published Vne, but then how they realized that they flew for years without this knowledge and the plane was always fine- so effectively they have tested at higher Vne's and proven the design in their experimental, so it was an appropriate speed to use in phase 2.

Based on that data, I'd say that Vans likely chose a Vne that did take some TAS effects into account, but maybe is a compromise and can be exceeded at the ceiling. It doesn't appear that the published Vne that was only your IAS at sea level.

Just my experience working with EFIS units, but not as an airframe designer.
 
I have done some thinking (and Googling) about this controversial question. My understanding is the following. If I make any mistakes or cut any corners in my reasoning (which is quite possible), please let me know!

In theory:

Flutter is caused by aerodynamic forces, which are a function of the dynamic pressure, Q. By definition of Equivalent Airspeed EAS, it is tightly connected with Q: Q=½×ρ×(EAS)² where ρ is the density of air at sea level on a standard day, i.e. a known constant.

Q is also equal to ½×ρ×(TAS)² where ρ is the actual density of the air at your altitude, i.e. not a constant, more complicated to figure out. So Q is numerically "tied" more directly with EAS than with TAS. That's because the relationship between Q and TAS also depends on the density around the airplane, but EAS takes that variable into account. So Q is related directly to EAS but only indirectly to TAS (i.e. the relationship between Q and TAS varies as a function of the local density).

So, assuming that indicated airspeed and equivalent airspeed are very close (or at least roughly proportional, if you have a pitot tube in a good spot and stay well below the speed of sound), then the speed at which you might hit flutter (i.e. VNE times some safety margin, typically ~15%) is a value of IAS, not of TAS. This means that, as you fly higher, you can fly at faster true speeds before you have to worry about flutter.

Most aerodynamicists will agree with this intuitively: The thinner the air is (i.e. the lower ρ is at your altitude), the faster the air needs to go in order to have enough energy to induce the forces that are induced by more dense air at lower true speeds. Nearly all analysis of aerodynamic forces is based on only the Q of the air (i.e. only on EAS, or on TAS and density) as long as the airflow is well away from Mach 1 everywhere, and the Reynolds number (i.e. speeds and sizes of components) does not change much. (Either of those things - shockwaves, or changes in Re - can alter the shape of the airflow and the pressure distribution around the component of the airplane). Some people, e.g. here and here, have tried to argue why flutter-based VNE should be a TAS rather than an IAS, but I do not find these arguments convincing. These arguments seem to come from people who have never done an aerodynamics analysis using Q, i.e. from people who do not appreciate that aerodynamic forces are determined directly by EAS (which is very close to IAS) but only indirectly by TAS.

Conservatively:

A lot of flight-training material, and a lot of manufacturers' documentation (such as the text from Van's mentioned earlier in this thread and in other discussions about this topic) recommend that you treat VNE as a value of TAS, rather than as a value of IAS or CAS.

Basically, this is conservative: You find what TAS might give you flutter at low-ish altitudes, you subtract ~15% to give you VNE, and you stay below that many KTAS. The higher you go, that same TAS corresponds to lower IASs i.e. to lower dynamic pressures. So the higher you fly, the lower your IAS must be (in order to stay below that VNE TAS), and the bigger that "~15%" margin gets.

From the point of view of minimizing the risk of accidents (i.e. safety and liability), I can see why this approach a good idea. It is what I personally do in practice, just to be safe. But it does force the pilot to fly at speeds that are quite a bit slower than what are actually the fastest safe speeds.

(It also forces the pilot to be able to calculate TAS from IAS. If you have an external temperature sensor, then your EFIS probably does this automatically based on the local air pressure and temperature, but many airplanes do not have this capability and would thus require the use of tables, calculators, or other computing aids so that the pilot can determine TAS).

In reality:

The best discussion I have found on this topic is here. It is the only text I have seen that acknowledges that... (1) Aerodynamic forces are a function of Q, not just of TAS... (2) Flutter behavior is also dependent on the damping effects of the airflow, which are smaller when the air is less dense... (3) Despite fact "1", a lot of the literature for pilots recommends that VNE be treated as a TAS, i.e. the VNE IAS goes down as you fly higher... (4) The only way for sure to settle the issue is with data.

The bottom line:

As you may know, most aerodynamic forces are scaled with the dynamic pressure, Q ...

But flutter is different, because of the inertial coupling and the damping effect of the air. So flutter speed does not remain a constant indicated airspeed as you increase altitude. The flutter speed (expressed as an IAS) decreases slowly as you increase altitude. But nowhere near as fast as the indicated airspeed would decrease if you kept the true airspeed constant and increased altitude. A really good rule of thumb turns out to be half way between the two.

There are two sources that you can refer to as documentation for this ... A fellow at McDonnell Douglas Helicopter in Tempe, AZ ... had some helicopter flight test data showing the effect of altitude on blade flutter ... What he said was true of the true airspeed for flutter. It does increase with altitude, but not as fast as the airspeed at a constant indicated airspeed, which also increases with altitude. So, I took his data from his letter to the editor and converted it to indicated airspeed, and sure enough, it matched my "half-way" rule of thumb very nicely. The second source is the new book out of Germany called Fundamentals of Sailplane Design, by Fred somebody, sorry I can't remember his last name... He has a plot of sailplane flutter boundary measured at one of the Akafliegs, plotted on a flight envelop, and it shows the same "half-way" rule that I have been suggesting. So I was happy to see that he found that independently.

I would treat it as a rule of thumb, and remember there is nominally a 15% margin for flutter on top of that, which can make up for many sins like instrument error, wear in control surfaces ninges, incomplete mass balance, etc.

For example: Say an RV-6 has a VNE of 185 knots. At sea level, that's 185 KIAS and 185 KTAS. But then say you climb to an altitude where, around that speed, the IAS is about 24 knots below the TAS. (That's roughly in the ballpark for RV speeds at 8000-ish feet on a standard day). An aerodynamicist might say you can fly at up to 185 KIAS, i.e. 209 KTAS. The manufacturer might say you can fly at up to 185 KTAS, i.e. 161 KIAS. In reality, apparently you can probably split the difference and fly at up to about 173 KIAS, 197 KTAS.

(But, again, for the record, I personally follow the manufacturer's recommendation anyways, to be safe).

Edits: Formatting and links.
 
Last edited:
For example: Say an RV-6 has a VNE of 185 knots. At sea level, that's 185 KIAS and 185 KTAS. But then say you climb to an altitude where, around that speed, the IAS is about 24 knots below the TAS. (That's roughly in the ballpark for RV speeds at 8000-ish feet on a standard day). An aerodynamicist might say you can fly at up to 185 KIAS, i.e. 209 KTAS. The manufacturer might say you can fly at up to 185 KTAS, i.e. 161 KIAS. In reality, apparently you can probably split the difference and fly at up to about 173 KIAS, 197 KTAS.

(But, again, for the record, I personally follow the manufacturer's recommendation anyways, to be safe).

Edits: Formatting and links.

When phase one was being flown off with the RV-7A in 2003, I was not spun up on the TAS restriction with RV's. The 7A was gradually taken to 200 KIAS from 8000' in a gentle WOT dive...the airplane was stable and did not come apart or flutter.

Was I lucky or do RV's have an unknown safety margin above Vne 200 KTAS?

I did not do that with the 8.
 
When phase one was being flown off with the RV-7A in 2003, I was not spun up on the TAS restriction with RV's. The 7A was gradually taken to 200 KIAS from 8000' in a gentle WOT dive...the airplane was stable and did not come apart or flutter.

Was I lucky or do RV's have an unknown safety margin above Vne 200 KTAS?

Flutter does not happen EVERY time the flutter speed is exceeded....and that is the insidious part of it. You can get away with it more often than not - but when you don't you have lost your tail (or other significant airframe component).

In all honesty, the odds are probably on the side of those who slightly exceed the limits - but the consequences of losing the bet are fatal. The only way to be sure to win is...not to play.
 
When phase one was being flown off with the RV-7A in 2003, I was not spun up on the TAS restriction with RV's. The 7A was gradually taken to 200 KIAS from 8000' in a gentle WOT dive...the airplane was stable and did not come apart or flutter.

Was I lucky or do RV's have an unknown safety margin above Vne 200 KTAS?

I did not do that with the 8.

For flutter to occur it often needs something to excite it.

The rough and tumble method is to attain test speed and then sharply rap the stick (or rudder pedal) in the axes being tested to provide a movement to start the system oscillating.

If that is not done, it is possible you were at a speed it would oscillate, if it had an outside influence (turbulence, etc.).
 
Flutter does not happen EVERY time the flutter speed is exceeded....and that is the insidious part of it. You can get away with it more often than not - but when you don't you have lost your tail (or other significant airframe component).

In all honesty, the odds are probably on the side of those who slightly exceed the limits - but the consequences of losing the bet are fatal. The only way to be sure to win is...not to play.

AMEN to that! At the very least one should know the rules.
 
AMEN to that! At the very least one should know the rules.

David, At least there was a ground vibration test (GVT) performed on the RV8 over 10 years ago with the N58RV accident. We don't have the results, but that test defines the vibration modes and frequencies of the airframe and it's extremities. The RV8 rudder is lighter, has a thicker skin, and a CG 3/4" closer to the hinge line as compared to the 7 tall rudder. The FAA 23.629-1b says the gvt defines the frequencies, and if it is greater than 10HZ then thumping the controls would not be sufficient to excite the event. It indicates that the air testing should excite the surfaces and progressively go to higher speeds to define when the damping is lessened, approaches zero and approaches what they call "divergence". Divergence means - when it starts, it proceeds to failure, always. This is the reason it is not wise to test to Vd ourselves. Flight testing is done carefully (with instrumentation, and small progressions in speed) to avoid a crash.

It (FAA Doc) says that "divergence" should not be imminent at Vd (dive) speed and, if not, that (Vd) becomes the basis for Vne (- .9*Vd) as pointed out in other threads.


Disclaimer: at least this is from what I have been reading.
 
I
In theory:

Flutter is caused by aerodynamic forces, which are a function of the dynamic pressure, Q.
The bottom line:



I][/SIZE]

I am an Aero Eng and I am involved with flight test (handling qualities - NOT flutter). I have covered flutter testing but I am not an expert in that field. Very few people are. That knowledge tends to get passed on via on the job training by specialists. What I am writing here is very top level.

Flutter is a function of true airspeed, not Q. It is really a frequency thing. As true airspeed goes up, the frequency of the excitation (not the magnitude, which is a Q thing) starts to move and can approach one of the harmonics of the structural bending frequencies. When freqencies meet bad things happen. That's why they mass balance surfaces, to change the frequency of their oscillations to move it away from a structural frequency.

So VAN, or whoever, will do his flutter testing up to a certain altitude with a given engine and airframe. If he shows positive flutter margin at that altitude, in a dive up to Vne + some margin, then for that altitude and below if you respect Vne in IAS corresponding to the TAS that was tested you are good, and down low you are even better (TAS is less). You go above that altitude, or put in a bigger engine, you become a test pilot doing envelope expansion, without the benefits of expert analysis, instrumentation or any of the other benefits of a controlled test situation. I suggest you wear a chute!
 
regarding excitation, the level required likely depends on whether the structural damping is neutral, slightly divergent or highly divergent. I think youtube has a great video of some light twin exactly at the neutral speed and the whole thing is twisting and wiggling but not getting any worse. It was done at Edwards and there is a chase plane and some Test Pilots are having lots of fun.

If it is divergent enough, then the slightest input will trigger it. There were some interesting videos on the Air and Space magazine website that supplemented an article they did a bunch of years back. They showed a wind tunnel model sitting there fat dumb and happy and then.....BAM... it was gone. It disintegrated instantly (or so it seemed). That was not the intent - WT models are very expensive. That was a very $$$ data point.

So if somebody decides to go above and beyond the book, my question would be: "do you feel lucky punk"? :eek:
 
Thank you Scott, for an excellent explanation.

It seems to me that what is lacking is a clear suggested VNE from Vans, expressed conventionally (what most pilots are used to) in IAS and altitude, since not all aircraft have TAS indicators. e.g., "recommended VNE is 190 KIAS at density altitudes of 10,000 ft or less; and declining 2% per thousand feet for higher altitudes", or whatever the real numbers are.
 
Aftermath . . .

Boeing_B-52_wo_VS.jpg


It landed safely.

Edit: Not flutter see post next page.
 
Last edited:
Thank you Scott, for an excellent explanation.

It seems to me that what is lacking is a clear suggested VNE from Vans, expressed conventionally (what most pilots are used to) in IAS and altitude, since not all aircraft have TAS indicators. e.g., "recommended VNE is 190 KIAS at density altitudes of 10,000 ft or less; and declining 2% per thousand feet for higher altitudes", or whatever the real numbers are.

Well Bob, it isn't very complicated - Van gives us a Vne that convention would tell us is at a Standard Atmosphere, and every pilot at one time or another knows how to convert to TAS given altitude and temperature - so....go ahead and build the table (or rule of thumb)!

Paul
 
Last edited:
As I recall the Concorde accident was caused by a "runway 'gator".
i.e. pieces of a blown tire.
 
The Concorde accident was not caused by flutter. What makes you think it was?

59211499.jpg

The Concorde disaster had as much to do with flutter as did the B-52 shown in the picture above or the RV accident cited in the original post.

It's interesting that we as a group get so concerned about exceeding Vne because of possible flutter when there are other potentially catastrophic consequences. It's almost as if, when an RV goes down and it was known to have been beyond redline, we assume it must have been flutter. While flutter is a very real and dangerous thing, incorrectly assuming that it was somehow involved in an accident blinds you to other possibilities.

That does not promote safety.
 
Last edited:
As I recall the Concorde accident was caused by a "runway 'gator".
i.e. pieces of a blown tire.

It was. And the image may well have been doctored for effect, many "take off" fire images showing up on the internet over and over are fabrications.

Back to the subject of this thread, vibration and flutter are not the same issues. I saw canard tips in motion up and down very rapidly, probably 4-6 inches, when a propeller blade departed in cruise. I thought the canard and elevators were in a flutter mode but they were not. It was the result of severe vibration. It could have led to flutter, it being excited by the vibration, but it did not. The speed in normal cruise was not near Vne when this happened, that may have been what saved the day. I wonder about it.

Certainly the risk meter goes up when flying at or above Vne....something to think about when upgrading HP beyond what was available when the limit was established and using that HP for more speed. Or when letting the airplane fall out of the envelop doing aerobatics, it is very easy to slip through Vne. I did it once in a screwed up spin recovery in a C-150, it was through the red line in about 2 seconds. It is that easy to loose it when the nose is pointed down, even more so with an RV. Some RV's have come to their end when this happens.

Vne = Very near end. :)
 
Last edited:
59211499.jpg

The Concorde disaster had as much to do with flutter as did the B-52 shown in the picture above or the RV accident cited in the original post.

Difference being that under the picture of the B-52, the caption said that it was not flutter.
Caption above the Concorde picture indicated that it WAS caused by flutter!
 
Let?s add some fuel to the fire

I started the thread because I was under the impression that Vne was IAS and one day I pushed the nose of my RV down at full power and tried to get close to Vne. Came within 15 knots and decided that was good enough. Well after reading another post about getting close to Vne during a slow decent to an airport and noticing his Vne concern was much lower than Van?s published Vne, I started reading more about Vne and found the information I posted in the beginning of this thread.

With the recent finding from the accident and how Vne was exceeded and parts had separated from the aircraft I really wanted to understand the Vne numbers.

There were comments that many RV?s have flown faster than Vne (IAS) and if they did they most likely flew faster the Vne (TAS) and have not come apart. I believe this to be true. But as others have commented that once a disturbance is started, flutter could occur.

But what flutter are we talking about? Wing? Aileron? Elevator? Horizontal Stab? Vertical Stab? Rudder?

So now let me add something to the fire, How many RV?s have properly mass balanced elevators?

When I followed the assembly instructions for my elevator and a friend?s elevator, there wasn?t enough lead provided to properly counterbalance the elevators. I had added a nut plate in the forward counterbalance rib tooling hole so I could add extra weight externally if necessary, which I did need to balance the elevators. That was before the aircraft was painted. Once the RV was painted I needed to add the additional weight to properly balance the elevators.

So for those much more aerodynamically knowledgeable then me, does a underbalanced elevator increase your chances of flutter when approaching Vne?

How many of you reading this thread have properly balanced elevators?
 
So for those much more aerodynamically knowledgeable then me, does a underbalanced elevator increase your chances of flutter when approaching Vne?

How many of you reading this thread have properly balanced elevators?


Generally speaking, the critical point is that the CG of the surface is at or ahead of the hinge point. If your elevators have a CG aft the hinge point, then the possibility of flutter would increase. How much? Who knows.

My rudder/elevators are balanced. They were actually balanced twice - once before paint, and then once afterwards. There was a small but noticeable difference.
 
Last edited:
So now let me add something to the fire, How many RV?s have properly mass balanced elevators?
Or rudders.

It was the opinion of the TSB report into the crash of RV-7A C-GNDY (think I have that right) that the rudder likely failed due to flutter during manoeuvers at speeds in excess of Vne.

They found that the aircraft had flown for a while in an unpainted state, then was sent to the paint shop. Before painting, the rudder surface was smoothed out with significant quantities of bondo. There was no evidence in the logs or on the aircraft that it had been re-balanced, and if I recall correctly a new weight and balance wasn't performed either.
 
<snip>

. . . But as others have commented that once a disturbance is started, flutter could occur.

But what flutter are we talking about? Wing? Aileron? Elevator? Horizontal Stab? Vertical Stab? Rudder?

So now let me add something to the fire, How many RV?s have properly mass balanced elevators?

. . . <snip>
From my research, not experience - read that twice.

1. As Paul Dye as pointed out, flight in smooth air, beyond the flutter speed can occur, but if flutter begins, it has a range of results. Physically, the result will depend on the damping of the system and strength of various components. The result could range from buzz to separation.

2. From literature, ground vibration test animation, it is all the above. wings, torsion/bending of aft fuse, HS, VS and rudder. Oh - and all the control surfaces. All will participate, like one side of a tuning fork won't naturally vibrate without the other.

3. Rudders.

Original 7 rudder is the same as the 8. Also referred to as the short 7 rudder.
The current 7 rudder uses 9 leading numbers. AKA - tall rudder.

The weight is without upper and lower fiberglass. No paint. With balance wt.
short:
Wt = 3472 grams
Aft CG = 3.18"
skin = .020"
area = 718 in2

tall:
Wt = 3855 grams
Aft CG = 4.03"
skin = .016"
area = 936 in2

Both use the same counterweight. 1.85 lb. or 840 grams.

Fiberglass caps and paint will push CG further aft.

This is just a comparison, I could not possible quantify what it means.
FWIW - the Lancair rudder is balanced before and after paint.

DO NOT assume that adding more weight is the thing to do. That would just lower the VS vibration frequency and with all the interactions - who knows if that is acceptable.

Does anyone have similar information for the 6 rudder with .016" skin?
 
Keep your eye on the ball

I've been hesitant to contribute to this thread, but just a couple of points to ponder from an old fighter pilot...

If you carefully read both of these mishap reports, although flutter is discussed, it is apparent that in both cases the pilot likely removed the tail of the airplane using the stick (i.e., aircraft handling error). In the case of the Canadian mishap, a pilot was attempting BFM (basic fighter maneuvering, or 1 v 1), oversped the airplane, and applied excessive G resulting in structural failure. In the second example, it appears as though the pilot may have attempted a high speed split-S. Unless G and buffet (i.e. aerodynamic limit) are carefully managed, any split-S can result in dangerous speeds in RV-types.

Some airplanes are built stoutly and have high drag characteristics (e.g., Pitts) that allow more margin for handling error when attempting to max perform the airplane. Other airplanes are simply more stoutly built (e.g., Extra) and have more strength margin.

At Vne, my RV-4 is capable of generating over 14 G's. If I apply rapid G under those conditions, I can generate G far in excess of ultimate design load limit (9.0, assuming no construction error or fatigue damage) and cause structural failure. If I'm applying asymetric G (i.e., maneuvering about more than one axis simultaneously), G limits are further reduced (i.e., it takes even less G to remove the tail). The ability to "generate G" is a function of airplane and flight control design as well as "q" (dynamic pressure, or more simply, airspeed). Snatching the stick (i.e., an abrupt pull) can generate more than 9 G's in less than a second in the typical RV at speeds above 165-170 MPH CAS (typical cruise speed, still inside the green arc). Smooth application of flight controls and proper use of G/buffet is critical during maneuvering flight to avoid exceeding limits.

Vne is effectively a TAS limit, as has been discussed. If you have advanced flight displays, it may be indicated in the cockpit, otherwise a simple table can be generated to give you some very good IAS rules of thumb. The yellow arc does not even exist on turbine powered airplanes, so piston airplanes have a privilege to use up some of the structural design margin not available under more conservative certification/design rules. I'm very congnicent of airspeed during cruise descent, especially from "high" altitude (at or abov 8000'). Smokey's article referenced earlier in this thread has an excellent description of the importance of properly controlling airspeed during descent.

RV-types have three handling characteristics that need to be understood to avoid handling mishaps: low drag charactersitics that result in the aircraft accellerating rapidly when the velocity vector is below the horizon (i.e., the nose is down)--moreso if the lift vector is also below the horizon (i.e., the pilot's head is point down toward the ground)--especially for airplanes with fixed-pitch propellers; reduced static margin (i.e., pitch stability), particularly at aft CG and/or high pitch angles/power/reduced speed and limited stall warning.

It's important to understand maneuvering speed, symetric or asymetric G application and learning to live "inside the green arc" during maneuvering flight. It's very seldom that I exceed Vno (max structural cruising speed or the top of the green arc) under any conditions. During maneuvering flight, speed in excess of maneuvering speed only provides energy for going up, going down is best conducted at corner velocity (maneuvering speed) or less to make sure that you have maximum G allowable for maneuvering. Maneuvering speed is not a fixed value, it varies with gross weight and whether or not manuevering is about one or more axis.

RV's are outstanding sport airplanes. They are very well designed, engineered and have excellent handling characteristics. If operated within the envelope using proper handling techniques, they are some of the sweetest flying airplanes that are affordable for average folks like us. They are not, however, bullet-proof. Proper handling training is critical or proper experience that can be carefully applied during testing is necessry to learn how to fly them.

Story: It was my second flight in my new to me RV-4. My insructor was in a Christen Eagle and was chasing me for the flight (normal technique utilized for checking out in single seat airplanes in the military). At the conclusion of basic handling training, we set up for fighting wing (i.e., flying formation in a loose chase position, think "rat racing" or following another airplane around during maneuvering flight mantaining a defined area behind them). At one point during maneuvering (loose lag roll to the outside for folks that understand what that means), I noted excessive wind noise. A quick glance at my airspeed indicator showed I was well in excess of red line. My attitude at that point was about 10-15 degrees nose low and about 210 degrees of bank or so (i.e., inverted, nose low, right roll past vertical). As soon as I recognized the overspeed condition, I smoothly unloaded (reduced G to 1/4 or so) rolled up right, and carefully applied minimum G to get the nose tracking back up above the horizon to allow the airplane to slow back down. Our maneuvering floor and altitude at that point gave me the luxury of proper control application, assuming I didn't encounter any flutter as a result of the over-speed. That was my first introduction to RV acelleration characteristics and taught me that cross-checking airspeed is an integral part of maneuvering flight--even when maneuvering "eyes outside of the cockpit" it's necessary to know what your energy (airspeed) is at all times.

Part 3 of the transition training manual posted in the sticky at the top of this Safety section has an extensive discussion of limits, handling characteristics and maneuvering flight that may help shed some light on this topic for folks that might want more information.

Forgive the jargon!

Fly safe,

Vac
 
Last edited:
More Margin

Wondering what the weakest link is always a thought while I build. Cruising at 180mph true, one doesn't want a strong gust of wind to separate the tail or whatever the weakest link is on our build. If I knew that adding a gusset here, or a stiffener there, would increase the margins, I would probably do it just to give a peace of mind at 180mph. Granted, the tails came off in the already mentioned incidents under different circumstances, but the thoughts of increasing margins is ever present.


From my research, not experience - read that twice.

1. As Paul Dye as pointed out, flight in smooth air, beyond the flutter speed can occur, but if flutter begins, it has a range of results. Physically, the result will depend on the damping of the system and strength of various components. The result could range from buzz to separation.

2. From literature, ground vibration test animation, it is all the above. wings, torsion/bending of aft fuse, HS, VS and rudder. Oh - and all the control surfaces. All will participate, like one side of a tuning fork won't naturally vibrate without the other.

3. Rudders.

Original 7 rudder is the same as the 8. Also referred to as the short 7 rudder.
The current 7 rudder uses 9 leading numbers. AKA - tall rudder.

The weight is without upper and lower fiberglass. No paint. With balance wt.
short:
Wt = 3472 grams
Aft CG = 3.18"
skin = .020"
area = 718 in2

tall:
Wt = 3855 grams
Aft CG = 4.03"
skin = .016"
area = 936 in2

Both use the same counterweight. 1.85 lb. or 840 grams.

Fiberglass caps and paint will push CG further aft.

This is just a comparison, I could not possible quantify what it means.
FWIW - the Lancair rudder is balanced before and after paint.

DO NOT assume that adding more weight is the thing to do. That would just lower the VS vibration frequency and with all the interactions - who knows if that is acceptable.

Does anyone have similar information for the 6 rudder with .016" skin?
 
The problem with increasing the margins a bit is that our airplanes have multiple load cases and issues to consider. There are also very many parts. Since we do not have access to the test results and analysis we haven't a clue how to improve margins - except for this:

Build it to the plans and keep it as light as possible. Operate it strictly within the limitations that the factory suggests.

Dave
 
Good post Mike.

Correct me if I'm wrong here, but is the chance at these type of accidents greatly incresed by the pilot's reaction to get the nose up and slowed down right away (NOW?).

Taking your example - nose down, high bank, overspeed. Many pilot's reaction would be 'oh cra*', gotta get pointed up, resulting in a quick pull/roll, high'er' G move to get the nose up and bleed off airspeed, resulting in structural failure in the process.

Observing an expert trainee such as yourself, the better reaction would be to recognize that it may/is not best to get slowed down 'right now' by use of G force to get nose up. Rather, recognize that even if above VNE the airplane will more likely stay together by using as little input/stress load as possible to get to nose up attitude, although that very well may mean a fair amount more time spend above the red line?


Also on the debate on counter-balanced control surfaces... I notice the Nemesis NXT flying at 400mph does not have counter balanced tail surfaces unless there is some other mechanism of doing so.
 
Last edited:
Another reaction that should be ingrained if in an overspeed situation in an RV with a fixed pitch propeller is to chop the throttle.

I've fallen out of some bother maneuvers and ended up nose down and even with the throttle pulled to idle used up a lot of altitude easing out of the dive while keeping the G low and the acceleration (speed increase) to a minimum.

Its the sudden jerk on the controls that will do you in here. Smooth control inputs are the order of the day.
 
Another reaction that should be ingrained if in an overspeed situation in an RV with a fixed pitch propeller is to chop the throttle.

I've fallen out of some bother maneuvers and ended up nose down and even with the throttle pulled to idle used up a lot of altitude easing out of the dive while keeping the G low and the acceleration (speed increase) to a minimum.

Its the sudden jerk on the controls that will do you in here. Smooth control inputs are the order of the day.

Agree, except for keeping the G low. The best way to slow the aircraft is to load up the wing. With the throttle at idle, a quick-but-smooth pull to >5G (assuming you are under aerobatic gross) will definitely chip away at any energy surplus while you're recovering to normal flight.
 
By G low I meant under the limit. So yes a hard enough pull or push or whatever to load things up (and increase the drag doing so) is in order.

Its the panic hard pull that grossly over stresses things. Frankly for most I suspect if you pull smoothly to a point where you are still awake (assuming a positive G here) you will be OK.

I guess another thing Im saying is that conditioning oneself not to panic as the speed starts to increase and do things in a methodical way as opposed to a reactionary, panic driven way is key to survival.

Probably not a bad idea to jet start out easing the nose down from a slow airspeed and learn just how fast these things accelerate is a good place to start learning what is is all about. Reduce power, ease out of dive with a decent G pull.
Rinse, repeat.
 
Its the panic hard pull that grossly over stresses things. Frankly for most I suspect if you pull smoothly to a point where you are still awake (assuming a positive G here) you will be OK.

Right on - I think we're in complete agreement.
 
The smooth straight pull to 5g is not always the issue, but the bank angle as well. We all read the stories on here of the 'rolling g's'.
 
Fire walled throttle

The smooth straight pull to 5g is not always the issue, but the bank angle as well. We all read the stories on here of the 'rolling g's'.

The other issue seems to be a lack of instinct to pull back the throttle. It seems that many loss of control accidents include wreckage found with the throttle all the way forward. I believe that people learning to fly aerobatics should initially train in a plane with a fixed pitch climb prop. This way they become very aware of throttle position on "down lines" and pulling it back becomes second nature. This should be the first response when faced with an overspeed (VNE) condition.

Skylor
RV-8
 
Study on the mechanics of energy in aerobatics, particularly the loop, seem to be in order. When the first reports came out about the latest inflight disassembly, I ordered and read "Better Aerobatics" by Alan Cassidy. One of the basics is to pull G's on the down side to get the attitude level without adding a lot of energy from altitude loss. Actually, minimal altitude loss is with full throttle, something that took some thinking to wrap by head around in RV terms. And, minimal loss is a balance of entering airspeed (split S) not so high as to manage the peak speeds and loop size, and not so slow as to limit initial G loading due to stall.

Lots to learn on the pilot side (for me anyway). The only loop I performed in a T6 seemed pretty easy actually.

Separating the pilot side, and getting to margin, building to plans is good, I just get the feeling that some Vne margin has been unknowingly lost with the 7/9 rudder, and we (the naive among us, me) have felt confidence with the stories of exceeding Vne with 6's, 8's and Rockets. Like it has been said, if there were a few known (proven?) modifications to the HS/VS/Rudder that would push that margin back to support the stories, then I would likely do them and still make every attempt to stay within the stated operating limitations.
 
Fascinating

Some excellent articles and I enjoyed reading the blog.

These are great little airplanes and as usual people think they can do more with them than the designer engineered..... because, no just because! My question is why?

As built by the designer the aircraft is safe, so why change it?

Probably because that is what makes us build our own airplane I guess is that we are willing to take risks...... mine will be calculated, and resolved by building and flying as the designer suggested.

Really good article on TAS and IAS I understand it more than when I took my PPL.
 
Cumulative Stress

Another point to consider is repetitive over stressing of the airframe; structural fatigue.
I'm afraid some think that just because a limit has been exceeded and nothing has happened, that is the new 'real' limit. Assuming nothing has happened to the structure and it is okay to continue to play in that airspeed/'G' load arena. Over stressing the airframe is cumulative. The final load that causes the airframe to breakup may be within limits if it has been overstressed before.
That is not to say that one can't literally pull the wing off on the first flight.
 
Somewhere I heard this, not sure of the source or accuracy.

When testing certified airplanes, the structure is allowed to be permanently deformed/damaged once 100% of the design strength is exceeded. However it's not allowed to fail until 150% is exceeded.

So if you load the airframe to 120%, small permanent deformations or cracks can have formed and thus weakening the structure. Now on the next flight, or a flight a week/month/year from now, you reach 90% of the original design strength, and the tail breaks off despite not having reached 100%.

Here's a graphic example of a structural overload/overspeed. I'll bet that wing isn't as strong as it was when they took off. https://www.youtube.com/watch?v=OWuPmGPBW8o
 
Somewhere I heard this, not sure of the source or accuracy.

When testing certified airplanes, the structure is allowed to be permanently deformed/damaged once 100% of the design strength is exceeded. However it's not allowed to fail until 150% is exceeded.
Correct. At or above 100% of design limit load (DLL) No failures or cracks anywhere but structure can, and most likely will, have some permanent yielding (deformation) and a full inspection of the structure should be performed. Cracks can happen at ultimate load which is DLL times the safety factor which is typically 1.5 for aircraft.
So if you load the airframe to 120%, small permanent deformations or cracks can have formed and thus weakening the structure. Now on the next flight, or a flight a week/month/year from now, you reach 90% of the original design strength, and the tail breaks off despite not having reached 100%.
No cracks should occur at 120% DLL. But it is true that yielded structure (has exceeded 100% DLL) may not be able to carry the same load as that of un-yielded structure. This is due to changes in geometry of the structure (buckling) and material strength is less after it has been stressed past yield. This is why a full inspection should occur if DLL is ever exceeded.
 
Last edited:
Great Discussion!

Lots of good points brought up so far! A quick summary:

1. Smooth control inputs are critical, and yes, an abrupt input at high airspeed/q can cause structural damage or failure. This is why training is so important...it is the human startle response at high airspeed that may have contributed to both of the mishaps referenced in this thread.
2. Fatigue damage is cumulative, happens over time, and reduces margin to an unknown degree. It is so insidious as a matter of fact, that a fatigued structure can fail at loads below design limits (and do so without warning).
3. "Rolling G" tolerance is lower than a "straight pull." A 4 G limit is a good rule of thumb in a typical RV operated within design limits to avoid an asymmetric over-G condition.
3(a). The concept of "unload, roll, set, pull" is a very important handling technique to understand--it is the proper way to set bank angle/lift vector prior to applying maximum structural load to the airplane (if that is your objective). Because, however, the answer in aviation is always "it depends" sometimes it's desirable when fighting or flying aerobatics to maneuver about two-axis simultaneously. Nothing wrong with that, but the pilot has to understand that different structural limits and handling techniques apply.
4. Maneuvering speed is not a fixed number. It's fairly easily derived by multiplying stall speed by the square root of G allowable (design G limit), which for RV-types, varies by weight.
4(a). Maneuvering speed DECREASES as weight DECREASES, and "asymmetric maneuvering speed" is EVEN LOWER than normal maneuvering speed. The definition is also "full deflection of one flight control about one axis one time" not "full, abrupt flight control input," subtle difference; but if you make say two inputs, you are outside of the definition (e.g., American Airlines Airbus that lost the vertical tail after takeoff from New York).
5. It's always a good idea to observe designer's limits and remember that any margin those limits provide belong to the engineer, not the builder or pilot! In fact, they are intended to save the pilot's butt.
6. Studying a loop (vertical turn) is a great way to understand the dynamics of maneuvering flight. It is also important to study emergency dive recovery.
7. Turn performance is based solely on G and TAS. A maximum performance turn occurs at maneuvering speed/corner velocity, maximum power with G and AOA balanced to put the airplane right between the aerodynamic (lift) limit and the structural (G) limit. G's at the design limit can only be sustained in a vertical turn (e.g., split-S) with the lift vector at or nearly inverted in RV-types. At all other attitudes, there is insufficient specific power to sustain high G loads (i.e., airspeed bleed and G-reduction occurs). As was noted above, the fastest way to slow down an airplane is to load up the wing.
8. A planned vertical turn (aerobatic maneuver, dog fighting, etc.) where energy is managed throughout the maneuver is different than an emergency dive recovery or nose-low unusual attitude with airspeed increasing. In the latter case, airspeed may already be beyond limits or exceed limits during recovery. In a nose-low case at any speed greater than corner velocity/maneuvering speed (as is the case here), power should be initially reduced to IDLE to begin the recovery.
9. Once you've passed redline, you are now a test pilot (more so than usual since technically, you are ALWAYS a test pilot flying an experimental airplane). Flutter can simply happen or it can be induced by control input. Some handling techniques can reduce the probability of inducing flutter, but there are simply no guarantees in this region. Having altitude available for recovery is a great aid (just as it is when you are exploring the other end of the speed range and stalling and/or spinning). A properly computed maneuvering floor will always provide you with options you don't have at low altitude.
10. There is a flutter margin designed into the airplane (think over-speed protection), but it is reduced with altitude. This margin, just like the margin provided by the aerobatic aft CG limit should always be considered sacrosanct unless you are smarter than the folks that designed the airplane, were a pilot graduate from test pilot school, are conducting a properly planned flight test, are ready to attempt an emergency egress during flight and have a world-class flight test engineer backing you up.
11. The three handling characteristics that are very important for all RV-pilots to understand are: A. Low drag/rapid acceleration; B. Reduced static margin (pitch stability); and C. Limited stall warning characteristics.

There is more extensive discussion on each of these topics in the transition manual Part 3 and some of the confidence maneuvers (as well as advanced handling maneuvers) are designed to familiarize transitioning RV pilots with these handling characteristics. Based on the discussion in this thread, in the next draft, I'll expand or revise the handling discussion and briefings with increased information about drag characteristics and static margin.

Again, forgive the jargon.

Fly safe,

Vac
 
Last edited:
....No cracks should occur at 120% DLL.....This is why a full inspection should occur if DLL is ever exceeded.

Cracks might occur at 120%. But the structure is still required to carry the load until at least 150%. At 150%, if in a static test, it must be able to support that 150% load for 3 seconds without failing.

Note that these overloads are expected to be a one-time event, following which the aircraft is either repaired or scrapped. And the loading condition that's tested is usually:
a) Loading to 100%,
b) Unloading,
c) Inspection and assessment of the structure, and then if everything is still okay,
d) Followed by an increasing load to 150%,
e) unloading after 3 seconds,
f) Inspection and assessment of the structure.

Test structures are usually scrapped after this testing.

I'd regard any instance of exceeding limit load (100%) as requiring a major airframe inspection with appropriate consultation with Van's and repair as needed, since cracks or other defects may have been generated by the loading event, and there might be permanent detrimental deformation.

Both fatigue analysis and crack propagation analysis assume that there are no load events above 100%. So if you exceed 100% there is fatigue damage or even crack propagation development that hasn't been assessed.

Crack propagation analysis or testing - this differs from fatigue analysis - generally isn't required for non-pressurized aircraft, as far as I remember. Basically, the analysis assumes that there already is a certain size and shape crack in the part, and applies the various load cycles until the part either fails (in the computer) or a certain number of cycles is achieved. Then the analysis is repeated for another possible flaw geometry for the same part.

It's quite time-consuming and requires additional expertise that many stress analysts don't have.

Dave
 
Last edited:
Cracks might occur at 120%. But the structure is still required to carry the load until at least 150%. At 150%, if in a static test, it must be able to support that 150% load for 3 seconds without failing.

If you mean that the structure should be able to go to 120% repeatedly and then still reach 150% without failure, I disagree.

Anything over limit load (100%) will accelerate the fatigue lifespan and as already mentioned, may have caused permanent deformation. Depending on the circumstances, this could easily compromise the ability top sustain an ultimate load condition.
 
thank you

slightly off topic but Mike, thank you so much for the training syllabus. I am planning on using it as the basis for my next flight review. Lot's of great material in there!
 
Back
Top