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Best glide ratio in a RV-7A

Bavafa

Well Known Member
I was practicing my engine out and checking for best glide and typical distance I can go for every 1000' of altitude. This time from the get go I would pull the prop all out and with that I was getting 100+K speed at 500 fpm. This was impressive as I had been using the 80-85 speed for my calculations in the past.

Does this sound about right or was my plane more slippery today? OAT was cooler then before (60F) but I wonder how much of an effect will have on this lets say for ever 10F.

Thanks in advance
 
i did.......

some testing years ago and came up with 10/1 glide ratio, RV-6.
 
Those numbers don't seem right to me..

I was practicing my engine out and checking for best glide and typical distance I can go for every 1000' of altitude. This time from the get go I would pull the prop all out and with that I was getting 100+K speed at 500 fpm. This was impressive as I had been using the 80-85 speed for my calculations in the past.

Does this sound about right or was my plane more slippery today? OAT was cooler then before (60F) but I wonder how much of an effect will have on this lets say for ever 10F.

Thanks in advance

My 7A with "transparent" prop will glide best at 110.5 mph and sink least at
about 84 mph. Under those conditions, my minimum sink will be about 700 f/m and at best glide speed it will sink about 940 f/m. A dead engine will give you significant drag. That means it would sink faster but you'd need slower speeds. I suspect your engine was still producing power.

The CAFE 6A has better numbers than my 7A. It had a best glide speed with transparent prop of 106 mph and would sink 756 f/m or at minimum sink the speed would be 80.3 and the sink would be around 572. Again, with a dead engine there will be more drag, less speed and more sink. The CAFE 6A had a best glide ratio of about 12.3 at 1650 pounds. Normally, when weight changes, the ratio is the same and the speed goes up or down. 10:1 in a -6 sounds reasonable, given the above.

You can find your speed for a transparent prop using the methods described on my pages for which see links below. That won't tell you your dead engine speeds but it will help you to compare your airplane to the two I described above.


There are many previous posts on this subject and many of those talk about the effects of various pitch settings with CS props.
 
Drag?

A dead engine will give you significant drag.

Any veteran may correct me but...

Perhaps you meant to say a dead engine with windmilling prop will produce signiciantly more drag than a non-turning dead engine. Both of which will produce no thrust of course, compared to an idling engine.

Symmantics maybe, but I'm thinking from the perspective of the newbies for understanding, which I am one of.

Wouldn't it be useful to test for best glide, or min sink with the engine turned off and NOT windmilling? Maybe you can't get the prop stopped until very low airspeeds unless using a CS prop which I have not yet experienced so I don't know but would like to.

Bevan
 
Almost

Any veteran may correct me but...

Perhaps you meant to say a dead engine with windmilling prop will produce signiciantly more drag than a non-turning dead engine. Both of which will produce no thrust of course, compared to an idling engine.

Symmantics maybe, but I'm thinking from the perspective of the newbies for understanding, which I am one of.

Wouldn't it be useful to test for best glide, or min sink with the engine turned off and NOT windmilling? Maybe you can't get the prop stopped until very low airspeeds unless using a CS prop which I have not yet experienced so I don't know but would like to.

Bevan

Even a stopped prop will give you drag as compared to a "transparent" prop. But yes, that will be less than a windmilling prop's drag. In a recent posting, Kevin Horton talked about trying to stop the prop on an RV8. Kevin is a professional test pilot among his other, many virtues. If I recall correctly, he advised against trying to stop the prop with less than 15,000' AGL. I did not succeed trying to stop mine. His is CS, mine is FP.

Many years ago I had a Moni motorglider. With that, a stopped prop was so much better than a windmilling prop that it felt like a new source of thrust. But, 2 or 3 square feet of prop provides drag as compared to it not being there. Take a look at the 1995 work by Norris and Bauer. There's a link to it on the pages linked below.
Also linked on those pages is a comparison that illustrates the difference between a transparent prop and a windmilling prop for a C-152. It's huge.
 
I was playing at engine stopped landings earlier this week. With my fixed pitch wood prop, the prop stops at about 65 knots indicated. No engine power = less airflow over the tail and somewhat less authority in the flare. Also, my prop stops in a horizontal position and causes a minor elevator burble at <60 knots.

I use 80-90 knots as best glide. The performance difference within that airspeed band isn't big, and the airplane generates a ~10:1 glide ratio based on the ASI, the altimeter, and the second hand on a watch.
 
Even a stopped prop will give you drag as compared to a "transparent" prop. But yes, that will be less than a windmilling prop's drag. In a recent posting, Kevin Horton talked about trying to stop the prop on an RV8. Kevin is a professional test pilot among his other, many virtues. If I recall correctly, he advised against trying to stop the prop with less than 15,000' AGL. I did not succeed trying to stop mine. His is CS, mine is FP.

During a practice a friend of mine in his RV7A tried stopping his prop and it took him two tries of bringing the plane to near stall speed before the prop stopped wind milling. The amount of lost altitude may or may not be worth doing this depending on how high you are.

Also, what is meant by a "transparent" prop

Lastly, I should have noted that during the practice today, my engine was running at idle speed.
 
What is a Transparent Prop?

During a practice a friend of mine in his RV7A tried stopping his prop and it took him two tries of bringing the plane to near stall speed before the prop stopped wind milling. The amount of lost altitude may or may not be worth doing this depending on how high you are.

Also, what is meant by a "transparent" prop

Lastly, I should have noted that during the practice today, my engine was running at idle speed.

A transparent prop is producing neither thrust nor drag. Jack Norris and Andy Bauer were able to "prove" that their "zero thrust device" actually did correctly produce that. Their articles from 1995 are linked on my page which is lined below. That enables you to get the correct drag curve for the airplane that applies to all speeds in flight. It is not, however, a good way to determine engine-out speeds except as a limit; that's how I used it. The CAFE 6A used the zero thrust device in the test.

An engine at idle could be producing thrust. That's not so easy to know except if you stop the engine. That's a risk you have to take but not one that I would recommend.
In any case, it seems very unlikely to me that an RV7 would have engine out glide performance as good or better than an RV9.
 
I tested my RV7 yesterday with the Garman 396 using the Glide Ratio feature it has. It works great and makes it easy to adjust speed at engine idle and watch how the glide ratio improves or decreases. I need to test more but 85-90 mph seemed the best with the prop in the fine position. I know it improves in the course position from previous tests but I need to duplicate this when I have more time. Does any one know if an engine wind milling will have adequate oil pressure to allow one to get the prop to go course? I guess the next question is if it does go course will it be too little pitch to stop the engine or will it reach an equilibrium? Sorry I am not brave enough to turn my engine off.
 
I tested my RV7 yesterday with the Garman 396 using the Glide Ratio feature it has. It works great and makes it easy to adjust speed at engine idle and watch how the glide ratio improves or decreases. I need to test more but 85-90 mph seemed the best with the prop in the fine position. I know it improves in the course position from previous tests but I need to duplicate this when I have more time. Does any one know if an engine wind milling will have adequate oil pressure to allow one to get the prop to go course? I guess the next question is if it does go course will it be too little pitch to stop the engine or will it reach an equilibrium? Sorry I am not brave enough to turn my engine off.

Our test in my friends RV7 showed that with the turned off engine and only wind milling, there will be enough oil pressure to set the pitch and go full coarse but will not stop it the win milling unless you slow down to the stall speed.

I am curious to know what best speed/glide ratio you will get with your prop at full coarse.
 
I have a video made by Wide World of Flying where the prop is first allowed to windmill at fine pitch, then with the prop at coarse pitch, and finally with the prop stopped. The increase in glide ratio is pretty incredible. This was using a C-182.
I can't find the video right now, but I seem to remember the glide ratio increasing somewhere around 27% with the prop stopped.
 
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Just found the video and watched it.
With the prop windmilling in fine pitch, glide ratio is 12:1.
With the prop windmilling in coarse pitch g/r is 13.7:1 (14% increase).
With prop stopped, prop in coarse pitch g/r is 15.23:1 (27% increase).

Again this is a C-182.
 
Does any one know if an engine wind milling will have adequate oil pressure to allow one to get the prop to go course?
For my engine at least there is adequate oil pressure.
These are numbers I recorded some time ago with a dead engine:

At 70 kts with the mixture at idle cut-off and prop control full in:
Oil pressure 55 psi
RPM 1350

At 70 kts with the mixture at idle cut-off and the prop control fully out (coarse):
Oil pressure 50 psi
RPM 900

Fin
9A 0-320, 9:1 CR, Hartzell, MT governor.
 
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I am curious to know what best speed/glide ratio you will get with your prop at full coarse.
On my RV-8, with three bladed CS MT prop, I got a best glide ratio of about 8.7:1 with engine OFF and prop control pulled full aft, at a speed of about 104 KIAS. I could just get the prop to stop, if I held it about one kt above the stall, with full flap, for about 2 minutes. And it would start turning again if I increased the speed faster than about 85 kt. The glide ratio with prop stopped was about 9.4:1 at about 80 kt. More info and data plots here.
 
Keven, It is interesting that the glide ratio is best at about 105kts but the minimum decent rate is best at 88 kts. So which is best?
At 10,000 ft will take 8.3 minutes at 88 kts (least decent) to reach the gound and the distance traveled will be 12.2 kts.
At 10000 ft will take 14.4 kts traveled at 105kts (best ratio) to reach the ground and will consume 8.2 minutes!
The time is almost the same! Amazing. Someone should check my math.
I will try to get some better data for my RV7 but it should be close to the RV8 since everything else seems to be similar.
 
Keven, It is interesting that the glide ratio is best at about 105kts but the minimum decent rate is best at 88 kts. So which is best?
At 10,000 ft will take 8.3 minutes at 88 kts (least decent) to reach the gound and the distance traveled will be 12.2 kts.
At 10000 ft will take 14.4 kts traveled at 105kts (best ratio) to reach the ground and will consume 8.2 minutes!
The time is almost the same! Amazing. Someone should check my math.
I will try to get some better data for my RV7 but it should be close to the RV8 since everything else seems to be similar.

John,
I am coming up with a different result. Using 8.7:1 glide ratio @ 105K speed, 14.4 k of traveled distance or 16.5 miles but
Using 7:1 glide ratio @ 80K speed the traveled distance is down to 11.5K of traveled distance or 13.2 miles.
 
It all depends on.....

Keven, It is interesting that the glide ratio is best at about 105kts but the minimum decent rate is best at 88 kts. So which is best?

whether you need distance or time.

If you need distance to make an airport, then best glide ratio is better.

If you are close to the airport and want more time, then best decent rate is better.
 
Keven, It is interesting that the glide ratio is best at about 105kts but the minimum decent rate is best at 88 kts. So which is best?
At 10,000 ft will take 8.3 minutes at 88 kts (least decent) to reach the gound and the distance traveled will be 12.2 kts.
At 10000 ft will take 14.4 kts traveled at 105kts (best ratio) to reach the ground and will consume 8.2 minutes!
The time is almost the same! Amazing. Someone should check my math.
I will try to get some better data for my RV7 but it should be close to the RV8 since everything else seems to be similar.
John,

I think you need to check the math again. Starting at 10,000 ft, there is roughly a mile difference in glide range between speed for best glide and speed for lowest rate of descent. The descent rate will decrease as the altitude decreases (the glide angle stays the same, but the TAS decreases for the same IAS, and thus the rate of descent decreases). Thus it is hard to give an accurate difference in descent time, but the difference between best glide speed and lowest rate of descent speed would be roughly 50 seconds.

As Mel said, which speed is better depends on the scenario.
 
Best glide speed is the speed for best L/D but as modified by propeller drag. The speed for minimum descent rate occurs at 76% (3^-1/4) of best L/D, the speed for minimum power to stay aloft. Best L/D occurs when induced drag and parasite drag are equal V^4=4W^2/(rho^2 S^2 Ap pi e)
W=weight, S=span, Ap is parasite drag area which for an RV-6 is about 2.2, e is about 0.81 for the typical RV-6 wing tip with less than a 25? rake angle. An RV-6 wingtip of 1' span at the trailing edge has about an 11? rake angle.
 
Testing today

I did some RV7 testing today looking for best glide ratio as determined with the 396. At 3000 ft I got about 11:1 with the prop in fine pitch at 85 mph and idling at 1280 rpm. At course pitch it idled at 790 rpm and could get 14:1 glide ratio at 95 mph. I tried to get best rate of decent using the vsi. That doesn't work so well due to the delay. Compareable data points were 500 fpm fine pitch and 350 fpm at course. It was just me aboard so I was well below gross. I have a BA hartzell prop with an 0-360. The engine was running at idle. You can feel a surge going from fine pitch to course.
YMMV :)
 
That's hard to believe that these planes can get such a great glide ratio.

If you were at gross, your ratio would have been even better.
 
Idle power > Zero

I did some RV7 testing today looking for best glide ratio as determined with the 396. At 3000 ft I got about 11:1 with the prop in fine pitch at 85 mph and idling at 1280 rpm. At course pitch it idled at 790 rpm and could get 14:1 glide ratio at 95 mph. I tried to get best rate of decent using the vsi. That doesn't work so well due to the delay. Compareable data points were 500 fpm fine pitch and 350 fpm at course. It was just me aboard so I was well below gross. I have a BA hartzell prop with an 0-360. The engine was running at idle. You can feel a surge going from fine pitch to course.
YMMV :)

I am convinced that the airplane in this example is producing power even at idle. I base this on the glide ratio. Of course, I'm assuming you were not flying downwind. BTW - the 396 will give you rate of descent as well as glide ratio. It should be more accurate than the VSI for many reasons. Do keep in mind that the glide ratio as measured by the GPS is wind-sensitive.

If you plug the 95 mph, 14:1 numbers into my glide triangle spreadsheet you will see that even at 1400 pounds, the (minimum) drag is less than 100 pounds. Since the CAFE 6A had 134 pounds at 1650 it would have had 114 pounds at 1400 pounds. The 6A was tested using a zero thrust device which means there was no drag and no thrust from the prop. The ratio was about 12.25:1 and that doesn't change with weight. I'm just assuming 1400 pounds because that is what Van's uses for solo weight and you are not likely to have been significantly lighter.

So what, you ask? Only to say that the numbers you are getting may be very wrong for when you do have a dead engine!

If you want to compare your plane directly to CAFE results, you can try to build a zero thrust device or you can use that '396 to follow the steps in my presentation (PowerPoint) from AirVenture2010, linked below. But that still won't tell you anything about glide with a dead engine. Well, yes, it will tell you the upper limit; your dead engine drag will be greater than zero. Bank on that. Only direct trials will tell you how much more, though.
 
"Do keep in mind that the glide ratio as measured by the GPS is wind-sensitive." Good point!! It was very calm today but I forgot that wind can be a factor using the GPS.
 
That's hard to believe that these planes can get such a great glide ratio.
If you were at gross, your ratio would have been even better.

Glide ratio does not change with weight! Only the speed at which the best glide ratio is achieved changes.
 
That's hard to believe that these planes can get such a great glide ratio.
Given that the engine was still running, and the prop was probably putting out some thrust, I don't think we can call the measured results a glide ratio. I'm betting the rate of descent would increase noticeably if the mixture was pulled to OFF.
 
Reawakening an old thread with some data that may be of interest to others.

I did glide testing in my newly completed RV-7A. I have a Catto 3-blade fixed-pitch propeller.The IO360 engine was at idle of about 900 RPM. I used the autopilot to hold indicated airspeed while I recorded the time to pass each altitude.

I flew at altitudes up to 15000 MSL. The density altitude was about 2000' higher in the California summer.

At 78 KIAS, single pilot, half fuel, I saw descent rates of about 900-100 fpm above 10k and 700-800 fpm below 10k, or an average of 860 fpm from 17k to 5k DA. TAS was 90-100 at high altitude and 80-90 at low altitude. I averaged 1.8 nm / 1000' of descent, or a glide ratio of 10.9:1.

At 80 KIAS, nearly gross weight, on a different day I saw an average of 900 fpm from 17k to 5k DA, or 1.7 nm / 1000' of descent.

For emergency purposes, I will plan for 1.5 nm / 1000' AGL to determine which airports are within gliding range, expecting that my piloting skills in an emergency may not be as good as I could achieve in controlled conditions.
 
Reawakening an old thread with some data that may be of interest to others.

I did glide testing in my newly completed RV-7A. I have a Catto 3-blade fixed-pitch propeller.The IO360 engine was at idle of about 900 RPM. I used the autopilot to hold indicated airspeed while I recorded the time to pass each altitude.

I flew at altitudes up to 15000 MSL. The density altitude was about 2000' higher in the California summer.

At 78 KIAS, single pilot, half fuel, I saw descent rates of about 900-100 fpm above 10k and 700-800 fpm below 10k, or an average of 860 fpm from 17k to 5k DA. TAS was 90-100 at high altitude and 80-90 at low altitude. I averaged 1.8 nm / 1000' of descent, or a glide ratio of 10.9:1.

At 80 KIAS, nearly gross weight, on a different day I saw an average of 900 fpm from 17k to 5k DA, or 1.7 nm / 1000' of descent.

For emergency purposes, I will plan for 1.5 nm / 1000' AGL to determine which airports are within gliding range, expecting that my piloting skills in an emergency may not be as good as I could achieve in controlled conditions.


Correct me if I’m wrong, but this is not the same as determining best glide (max range). What you tested for would give you max TIME (min descent rate).
 
Correct me if I’m wrong, but this is not the same as determining best glide (max range).

Well, this method measures glide ratio. He should repeat it at various speeds to determine which one is best (highest). Then he’ll have best glide speed, at the tested weight, no wind.
If you want maximum time (max endurance) then you need to find the airspeed that gives the minimum vertical descent rate, which is not what he did here.
I could critique the method a bit. There’s a lot of averaging going on. It would be better to make the calculations at each measurement point (every 1000 ft of vertical altitude, for example) (IAS to TAS, ratio of TAS to vertical speed) and then average all the glide ratios. I also think there are some miscalculations (e.g., if the lowest DA was 5000’ and the IAS was 78 KIAS, then the TAS can never be as low as 80 knots).
 
Reawakening an old thread with some data that may be of interest to others.

I did glide testing in my newly completed RV-7A. I have a Catto 3-blade fixed-pitch propeller.The IO360 engine was at idle of about 900 RPM. I used the autopilot to hold indicated airspeed while I recorded the time to pass each altitude.

I flew at altitudes up to 15000 MSL. The density altitude was about 2000' higher in the California summer.

At 78 KIAS, single pilot, half fuel, I saw descent rates of about 900-100 fpm above 10k and 700-800 fpm below 10k, or an average of 860 fpm from 17k to 5k DA. TAS was 90-100 at high altitude and 80-90 at low altitude. I averaged 1.8 nm / 1000' of descent, or a glide ratio of 10.9:1.

At 80 KIAS, nearly gross weight, on a different day I saw an average of 900 fpm from 17k to 5k DA, or 1.7 nm / 1000' of descent.

For emergency purposes, I will plan for 1.5 nm / 1000' AGL to determine which airports are within gliding range, expecting that my piloting skills in an emergency may not be as good as I could achieve in controlled conditions.
For reference, I did glide perf testing at idle, and also with engine off. On my aircraft (RV-8 with IO-360-A1B6 and three-blade MT aerobatic constant speed prop), the glide ratio with engine off was about 15% worse than with engine at idle. The best glide speed with engine off was about 8 kt faster than with engine at idle. The speed for minimum rate of descent was over 20 kt faster with engine off than with engine idle.

Bottom line - testing with engine at idle is good practice to help develop a good test technique, and the data will serve you well if the throttle ever jams at idle. But if you want to know how the aircraft will perform after engine failure, you can only learn that by testing with engine off.
 
Bottom line - testing with engine at idle is good practice to help develop a good test technique, and the data will serve you well if the throttle ever jams at idle. But if you want to know how the aircraft will perform after engine failure, you can only learn that by testing with engine off.

Very pertinent info Kevin, thanks for all :cool:

Having had a con-rod failure on my O-320 Falco a few (too many) moons ago, oil and therefore prop control lost, the windmilling prop resulted in a harrowing ROD to maintain proper glide speed... fotos here: http://www.aerofun.ch/falco-1.html
 
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