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Tail stall RV8

phi75

Member
Hello i Try somme stalls with and without flaps
The stabilizator stall before the wing
Is that normal ?
Thanks
 
With my RV8, I experience what i believe is a shadowing of the horizontal stabilizer by full flaps under some conditions. It causes an elevator control pressure reversal. This typically occurs when I have an aft cg, such as a passenger.
 
Was only the pilot inside and 29 gallons
Had an RV4 before and was the wing stall before like many aircraft
 
With my RV8, I experience what i believe is a shadowing of the horizontal stabilizer by full flaps under some conditions. It causes an elevator control pressure reversal. This typically occurs when I have an aft cg, such as a passenger.

It is not from 'shadowing of the tail by the flaps', but rather a large change in onset flow angle because of the downwash induced by the trailing vortex wake from the flap tip and wingtip.
 
Grab the search function on strakes.
High AOA and gear legs are likely the culprit.
Normal.
 
Hello i Try somme stalls with and without flaps
The stabilizator stall before the wing
Is that normal ?
Thanks

I don't think you can stall the tail. You can, however, run out of elevator authority, and/or get a force reversal.
 
Still have elevator mouvement even if the travel is not so big.
I heard from other RV8 owners The same about the tail stall before the wing.
We should ask Van’s directly maybe
 
phi75: Are you saying to recover from the nose pitching down, you need to pull the stick aft? That would be a tail stall recovery.
 
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Salut, Si la queue venait à caler, alors le nez se cabrerait de manière incontrôlable, et la récupération serait douteuse sans une goulotte de traînée pour redescendre le nez. Regardez le F-101 Voodoo pour référence.

Hi,

If the tail were to stall, then the nose would pitch up uncontrolably, and recovery would be doubtful without a drag chute to get the nose down again. Look at the F-101 Voodoo for reference.
 
Salut, Si la queue venait à caler, alors le nez se cabrerait de manière incontrôlable, et la récupération serait douteuse sans une goulotte de traînée pour redescendre le nez. Regardez le F-101 Voodoo pour référence.

Hi,

If the tail were to stall, then the nose would pitch up uncontrolably, and recovery would be doubtful without a drag chute to get the nose down again. Look at the F-101 Voodoo for reference.

I don’t think that’s the way RVs would behave in a tail stall. These planes aren’t susceptible to tail stalls, and the only time that I’ve even heard of tail stalling in conventional type aircraft is when encountering tail plane icing. While using flaps towards the higher limit of flap speeds, the angle of incidence on the tail goes more negative, and with ice could stall. Since the tails of these planes produce down force, “stalling” the tail and losing this downforce would not cause the nose to rise, but cause the nose to drop. The OP is thinking he’s got tail stall because the main wing isn’t the tail stalling yet he’s got buffeting.. a better description would be to say that he’s lost elevator authority.
 
If the tail were to stall, then the nose would pitch up uncontrolably, and recovery would be doubtful without a drag chute to get the nose down again.

No - lots of people think the tail is like a little wing that supports the light end of the airplane and that the main wing supports the heavy end of the airplane in flight. It doesn't work that way. The horizontal tail doesn't keep the tail up inflight, it keeps it down. Anyone who ever played with those Guillows balsa gliders as a kid understands this. ;)
 
Have buffet ... can’t stall my wing ! I will do more stall next week to see

What does the VSI say during the buffet? The pitch down is very slight in a RV. Pick an altitude in level flight, power to idle, pitch as needed to maintain altitude- when the buffet happens, see what the altimeter does as you continue to attempt to maintain altitude with pitch.
 
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I don’t think that’s the way RVs would behave in a tail stall. These planes aren’t susceptible to tail stalls, and the only time that I’ve even heard of tail stalling in conventional type aircraft is when encountering tail plane icing. While using flaps towards the higher limit of flap speeds, the angle of incidence on the tail goes more negative, and with ice could stall. Since the tails of these planes produce down force, “stalling” the tail and losing this downforce would not cause the nose to rise, but cause the nose to drop. The OP is thinking he’s got tail stall because the main wing isn’t the tail stalling yet he’s got buffeting.. a better description would be to say that he’s lost elevator authority.


I disagree that if the tail were to stall the nose would go down. I don't have the time for an aerodynamic lesson, but without the taildownward force the nose would rise x look at the center of pressure as the wing stalls stalls. .
 
I disagree that if the tail were to stall the nose would go down. I don't have the time for an aerodynamic lesson, but without the taildownward force the nose would rise x look at the center of pressure as the wing stalls stalls. .

You’ve got it backwards... The tail generally pushes down to give a nose up pitching moment and offset the nose down pitching moment from the wing.

A tail stall (or failure of the horizontal stabilizer) would result in a pitch down.
 
I disagree that if the tail were to stall the nose would go down. I don't have the time for an aerodynamic lesson, but without the taildownward force the nose would rise x look at the center of pressure as the wing stalls stalls. .

It is true.

The horizontal tail induces a down load to counter the nose down pitching moment of the wing.

Here is a link to an article that explains it.
https://www.boldmethod.com/learn-to...alancing Act,within your aircraft's CG limits).

BTW, this is the primary factor that causes any given airplane to be a bit faster as the CG position moves aft. The closer the C.G. is to the center of lift the less down force that is required for trimmed flight. That trimming force is undesirable in the context of drag, but desirable in the context of stability.
 
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tail trimming forces

Lots of misconceptions here. The commonly held belief that the tail must produce a downforce for the airplane to be stable is incorrect.

The load on the tail varies with speed. At high speeds, low angle of attack, the tail does almost always create a downforce when trimmed. This is largely due to the wing airfoil camber producing a nose-down moment, which increases in proportion to V^2, regardless of c.g. position. At high speed, the large pitching moment pitching moment requires a lot of tail download to trim.

BUT

- an interesting characteristic of the wing airfoil used on RV's is that they have almost zero section pitching moment! That was one of the NACA design goals back in the 1930s. This was (and is) attractive exactly because it reduces the tail trimming forces.

Our c.g. range is roughly from about 20% to 40% of wing chord. If you have the c.g. near the forward end of the range, you may have a downward tail force over the whole speed range. But if your c.g. is farther aft than 25% of wing chord, then at low speeds, when the wing airfoil section pitching moment is very small, you have a small upload on the tail to trim. This is because the effective center of lift is at 25% of wing chord. If the c.g. is behind that point, then you will have an upload on the tail to trim. As the c.g. moves farther aft, the amount of upload on the tail at low speeds increases.

The neutral stability point is approximately when the tail is producing the same lift-per-unit-area (same lift coefficient) as the wing. There are some complicating factors that make this not exactly true, including induced downwash on the tail from the wing wake, the effect of wing section pitching moment, are the main ones. So the tail lift coefficient at the neutral point is a bit less than the wing lift coefficient, but still an upload. Since our wings have almost no section pitching moment, the tail lift coefficient at the neutral point is very nearly the same as the wing. (i.e. the same lift per unit area)

With flaps down, what you have done is increase the wing section pitching moment (quite a lot) so there is more tail download at the upper end of the max flap speed, and less tail upload at low speed (near stall), but still some tail upload if you have the c.g. toward the aft end of the allowable c.g. range.

Also, be careful not to confuse the effect of flaps on the steady-state trim condition with the effect of deploying flaps as a transient when the airplane is in trim without them. In the transient, whether the airplane pitches up or down depends on the c.g. position. But that response has little to do with the effect of the flap on the steady-state trim.
 
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Steve
Scott's observation still applies to the original poster's situation of solo, minimum fuel.

Sam
 
Steve
Scott's observation still applies to the original poster's situation of solo, minimum fuel.

Sam

Maybe not....
I wasn't actually replying to the original post so wasn't thinking about the context of minimum speed near stall.
I was just pointing out that when talking in general about the effects of the trimming force of the tail, it is generally a down force which is contrary to what someone previously posted.
 
Maybe not....
I wasn't actually replying to the original post so wasn't thinking about the context of minimum speed near stall.
I was just pointing out that when talking in general about the effects of the trimming force of the tail, it is generally a down force which is contrary to what someone previously posted.

And remember, with RV airfoils having very low section pitching moment, If your c.g. is between, say, middle and aft end of the allowable range, you could easily have an upload on the tail all the time.

In any case, it is extremely difficult to stall a horizontal tail. High-performance sailplanes with very narrow elevators do sometimes suffer from elevator stall while maneuvering, or on winch launches (which can produce large nose-down moments). But in a power plane, it would be very rare. A C-177 with fwd c.g., yes. An RV? No.
 
Scott, Steve,
I'm thinking of the original poster (we are trying to help him, right?)- solo, 25g of fuel; in my RV8 with a Hartzell composite, I'm right at the forward CG limit. This would give my HS positive down force at stall, according to Steve's observations. Don't know what the original poster has in the baggage compartment, but if not much, then it's a reasonable assumption (let's not split hairs) that he has positive down force. Again, trying to help out a guy wondering how to approach what he is seeing. I was trying to help him build a test to verify the wing is in fact stalling, even though the pitch change is slight, by looking at altitude loss to indicate a stall. I'm thinking that is what is tripping him up. The tail stall is just a distraction.

Sam
 
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Scott, Steve,
I'm thinking of the original poster (we are trying to help him, right?)- solo, 25g of fuel; in my RV8 with a Hartzell composite, I'm right at the forward CG limit. This would give my HS positive down force at stall, according to Steve's observations. Don't know what the original poster has in the baggage compartment, but if not much, then it's a reasonable assumption (let's not split hairs) that he has positive down force. Again, trying to help out a guy wondering how to approach what he is seeing. I was trying to help him build a test to verify the wing is in fact stalling, even though the pitch change is slight, by looking at altitude loss to indicate a stall. I'm thinking that is what is tripping him up. The tail stall is just a distraction.

Sam

I agree.

At forward c.g. I can hold the nose up and just mush. Very slight buffet. I hear funny wooshing sounds along the side of the fuselage. I think, gee, is it going to stall or not. Then I look at the VSI and I am coming down at 2500 ft/min.

This behavior is mostly related to the low aspect ratio rectangular wing. And be careful, it can be a killer. A classic example: You are just a little slow turning base to final. You have a cross wind so the base leg has a tail wind. This makes you overshoot the runway. You bank more and pull. Perfectly coordinated but slow, you may easily see extremely high sink rates. Since you are banked steeply, the high sink translates to added apparent drift, so you pull even harder. This viscous circle can lead to a really bad day.
 
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