T

TThurston

Well Known Member
In my spare time I've been re-reading the FAA's "Airplane Flying Handbook". I just finished reading several pages on gliding. The handbook emphasies the importance of understanding and being proficient in gliding not only for power-off accuracy landings, but also normal landing landing approaches, and forced landings after engine failure.

It occured to me that I don't know much the glide characteristics of various RV models. Van's web site publishes numbers for climb, cruise, and stall, but I can't find anything about glide. I remember a few stories about new RV's with engine problems (the recent emergency landing of the new RV-10, and Paul's Dye's experience with his new RV-3). It seems like it would be good to have some idea about best glide speed before flying. So I have a few questions:

1. What is the best glide speed for various RV models?

2. What is the descent rate at best glide speed?

3. How is the glide affected by flap settings? For example, if I'm on approach at best glide speed and drop some flaps, how will my glide change? How much will it change my touchdown point?

4. How is glide affected by weight and balance?

I'm currently building a RV-9A, but I'm not so far into it that I might consider changing to something that I could fly sooner, like a -12, a -14, or an already flying -6.
 
That's what phase I flight testing is all about!
Every airplane is different.
 
My Rv-3 glides at L/D 10.5 @ 95mph with prop stopped. I don't know the rate of sink but you could work it out from the numbers. Flaps: I don't know the L/D at various flap settings. My thinking on flaps in an engine out landing is if the field is made carry some extra speed with 10* flaps on downwind, increase to full flaps on final and if short, retract the flaps and convert the extra speed to distance. If high slip with flaps. One thing to be careful about is retracting flaps while slow because you may stall. Don't get low and slow with flaps down. Glide slope, L/D, is not affected by weight. More weight just speeds you up, the polar is shifted to a higher speed.
 
I've established that best glide on my RV-6 with prop stopped is ~100 mph. That is in agreement with the tests the CAFE foundation did several years ago. The glide ratio is ~10:1, and minimum sink is 700 fpm at 80 mph. Given the accuracy of my measurements, any of those numbers could be +/-10%.
 
wilddog said:
My thinking on flaps in an engine out landing is if the field is made carry some extra speed with 10* flaps on downwind, increase to full flaps on final and if short, retract the flaps and convert the extra speed to distance.
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I don't recommend "retracting the flaps" on final.
My thinking is, never go to full flaps until you KNOW the field is made.
Retracting flaps does NOT result in an immediate speed increase; only a reduction in lift.
 
Mel,
Holding off full flaps until you're sure the field is made is certainly the best advice. But we all screw up and if you find yourself short on final with engine out and have the extra speed you can retract the flaps and extend the glide. I'm thinking about this like a glider pilot, they carry extra speed with flaps and/or spoilers in the pattern and if short, retracting flaps/spoilers is just like adding power in a power plane. A RV would not have the dramatic change in L/D that a glider does but it should be enough to help extend the glide some. Other than retracting flaps while gliding at altitude I have not tested this to see if it really would extend the glide of my -3. My glider goes from an estimated L/D of 5:1 with 90* flaps to about 40:1 when I clean it up.
 
Your RV must fly different than mine.
If I dump flaps on most any airplane with conventional flaps, I get a sudden loss of lift and an immediate drop in altitude. Gliders and spoilers are totally different. I agree with you on that.
Raising flaps does not increase airspeed. Airspeed is only increased by lowering the nose, again loosing altitude. This is not something I want to do on final approach.
This is my experience. Yours may vary.
 
I must not have been clear in my thoughts. I agree with you raising flaps will not increase speed. But if you already have a bit of excess speed, both raising them and converting the existing excess speed to distance will extend the glide. I have tested raising flaps in a glide and as long as I have plenty of speed, there is no loss of altitude. Now if you are gliding along slow with full flaps and raise them, that's a different story and you are correct. If my engine quits in my RV, it becomes a low performance glider with flaps and might as well be flown like one. Also a -3 is the only RV i've flown so I don't know anything about how this would work in the other models.
 
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Full flaps quite likely has more drag than say 20 degree but similar lift. So removing some flap may reduce drag quite a lot, without affecting the lift.

Does a British Airways B777-200 ring a bell? Capt Peter Burkill scanned the cockpit looking for anything to save the plane, reducing flap was his only bright idea. Subsequently proven to be the only course of action available and able to save them all from a fiery crash.

Different flap types perform differently so YMMV
 
#4 is the same for all conventional airplanes.
Balance makes only a small difference, aft cg is better.
Best glide airspeed varies as the square root of the weight, so if you are 20% below gross best glide is 10% lower than the speed at gross.
Fly the same indicated airspeed regardless of altitude.
Previous post is correct, you will always be flying at max L/D.
Since drag matters, it also matters whether the prop is windmilling or stopped, or fine or coarse pitch for a CS.
And, in the real world, where you are trying to cover some distance over the ground, "best glide speed" depends on the wind.
 
CAFE test of 6A

Kyle Boatright said:
I've established that best glide on my RV-6 with prop stopped is ~100 mph. That is in agreement with the tests the CAFE foundation did several years ago. The glide ratio is ~10:1, and minimum sink is 700 fpm at 80 mph. Given the accuracy of my measurements, any of those numbers could be +/-10%.
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If you mean the test that was done with a 6A and the Norris-designed zero thrust device, then you could not get agreement. Your data is reasonable, but the CAFE test was done with "transparent" prop and the glide ratio that they found was either 11.4 (their calculation) or 12.3 (calculated from their data: 134 pounds drag into 1650 pounds). Their best glide speed was 106 CAS mph. Whether your indicated airspeed is accurate for CAS is something you can know if you calibrate, but you can't assume it's good because of the high angle of attack.

If you add drag you get a lower speed number - such as yours. Their test weight was 1650. If you go lighter you get a lower number. And so on.

You could see if your numbers really agree with CAFE by using Norris's alternate method of pre-determining your revs-per-TASmph for zero thrust based on your prop's actual shape and then re-test for speed at minimum sink. Your best glide would then be at 1.32 x the minimum sink speed. The speed will change with weight, but the glide ratio should not. There is a bit of math in that and I have already built a spreadsheet for the prop calculation. Email or PM me if interested in checking this out.

The CAFE tests give a "true" glide ratio, but are sadly useless for dead-engine situations. Real world testing is the only right way to answer that question and it's worth doing for SAFETY.
 
Be careful with comparisons to sailplanes.. They have spoiler dive brakes, which come up and out of the wing kiiling lift immediately and increasing drag significantly, thus letting you steepen the approach immediately. The converse happens when you close the spoiler/dive brakes, airflow immediately reattaches and you get lift in an instant and the drag disappears. With my spoilers closed, it is perfectly all right to modulate the manual flaps on my sailplane from 30 to15 degrees without any drop in lift if you smoothly increase nose attitide with flaps being raised. With a plane flap on the sailplane and the RVs, beyond 15 degs is mostly drag because the airflow can't make that sharp of a turn to stay attached.

In the RV, Use your flaps but keep your speed up near best L/D even with full flaps.. Beyond 20 degs you are getting more drag than lift, so keep your speed up, raise the flaps from full to intermediate adjust with a little nose attitude to make up for slight lift loss from the flaps, as speed is stabilized raise them to clean and adjust with nose attitude to balance that lift loss. The key is to keep your speed at best L/D, you can coordinate the nose attitude increase with the flap rate. If you have manual flaps, then you can smoothly coordinate the flap up rate with nose attitude to have no impact on your glide path. If you have electric flaps, then modulate accordingly in increments to control rate. The bottom line is if you aren't making the field, by all means raise the flaps. As stated above, start with a slip and if you need a steeper glidepath, pull on the flaps. The great news with the RV is you can practice this and I do routinely to get the sight picture and coordination down between flaps and bring on a bit of AOA.

I love manual flaps and have the type in my RV-4 that hooks on notches on the left side of the pilot seat, which I hate! I've had the handle pop out every now and then in turbulence and do worry that might happen on short final or the flair. Anyone have plans for a manual flap handle that looks like a parking brake with button release similar to the Pawnee and SuperCub I fly? Thanks in advance.
 
wilddog said:
But if you already have a bit of excess speed, both raising them and converting the existing excess speed to distance will extend the glide.
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Carrying extra speed is also reducing your glide, flaps up or down. You should be flying on speed or you're just not getting your max glide distance. You are increasing your glide distance because you are going from fast with flaps down to on speed clean. Carrying extra speed is only shooting yourself in the foot if your energy state is one where you need to be at your max glide a/s. If you are high on energy (field well within your max glide), then you can bump your knots a bit, but I would be careful about that. Better to bleed off energy end game rather than watch your aimpoint drift UP your windscreen wishing you had just stayed on speed.

My recommendation for the situation you presented is to GET ON SPEED. That will increase your glide distance with no loss of lift.
 
Speeds for RV12

I remembered that Van's has an RV-12 POH posted on their website. It gives the following:

Maximum gliding distance: 85kts
Minimum rate of descent: 60kts

It's interesting to me that these same speeds are found in the climb performance checklists.

Best angle: 60kts (flaps 1st decent)
Best rate of climb: 75kts (flaps up)
Cruise climb: 85kts (flaps up)

I still haven't seen many reports of speeds for RV's other than the -6. I know Mel says that all RV's are unique and the numbers for each aircraft are determined during phase 1 testing. But I'm hoping that the numbers for each model are close enough to give one a ballpark idea before first flight.
 
Man my 6a with cs at idle drops like a rock. Wish I could feather the prop if it were to quit. Of course if it weren't spinning it wouldn't seem so bad. Do you guys all land with full flaps or just 3/4 of full ?
 
erikpmort said:
Man my 6a with cs at idle drops like a rock. Wish I could feather the prop if it were to quit. Of course if it weren't spinning it wouldn't seem so bad. Do you guys all land with full flaps or just 3/4 of full ?
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What we did in the T-6 was fly with a LITTLE power on to simulate a feathered prop. It wasn't much, just enough to negate the drag of the unfeathered prop. I'm sure they just figured out how much power would give them the same glide profile as a feathered prop.
 
Keep in mind, as you reply to this thread that the -9(A), -10, and 12 all glide much differently than the short wing RV's.

I have had more than one RV pilot try to land my FP -9 and find themselves going around on a 5,000 foot runway.

The -9 just likes to fly and fly and fly!

For example, with full flaps and 55 knots (63 MPH) the decent rate in my -9 is between 450 and 500 fpm with one person on board. While not difficult to land, it is different than the others.
 
N941WR said:
Keep in mind, as you reply to this thread that the -9(A), -10, and 12 all glide much differently than the short wing RV's.
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I wonder whether the -14 will glide like the short wing RV's. I imagine a few people have had test flights in it, but I'm guessing the test flight didn't include any gliding.
 
TThurston said:
I wonder whether the -14 will glide like the short wing RV's. I imagine a few people have had test flights in it, but I'm guessing the test flight didn't include any gliding.
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In the pattern, with the power pulled back, it is hard to tell the difference between the -14 and the -9.
 
plans? you don' need no stinkin' plans!

Silent_Flyer said:
......Anyone have plans for a manual flap handle that looks like a parking brake with button release similar to the Pawnee and SuperCub I fly? .......
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.....you mean, a volkswagen beetle emergency brake ratchet? :)
 
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