Glide in an RV

TThurston · #1 01-12-2013, 03:10 PM

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.

Mel · #2 01-12-2013, 03:15 PM

That's what phase I flight testing is all about!
Every airplane is different.

wilddog · #3 01-12-2013, 04:02 PM

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.

Kyle Boatright · #4 01-12-2013, 04:17 PM

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%.

Mel · #5 01-12-2013, 04:17 PM

Quote:

Originally Posted by wilddog

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.

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.

Brantel · #6 01-12-2013, 04:30 PM

Full slips can be your friend in an RV...especially in an engine out situation.

wilddog · #7 01-12-2013, 04:43 PM

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.

Mel · #8 01-12-2013, 04:55 PM

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.

wilddog · #9 01-12-2013, 05:33 PM

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.

RV10inOz · #10 01-12-2013, 05:38 PM

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