Glide Ratio Model IV

[Bud Davidson]

I like the "for what it's worth" notes. This is one. I frequently flew my Aeronca Champ with engine shut down like a glider. Restart of the engine required diving to red line airspeed. That was with a 65 HP Continental. Turning the prop on the 912 before startup suggests STRONGLY that it would require restart with the starter ONLY, not something I would want risk unnecessarily. I trust those who suggest engine out flight requires airspeeds over 60ias should be believed. Glides to an unexpected forced landing most likely will follow an engine failure. So...
Has anyone charted glide with a stationary prop??

[PapuaPilot]

That's good advice David. Where I work we teach our pilot's if the engine fails to pull the prop control full out to course pitch (Cessna 206). From some real life scenarios we know it makes a considerable difference in the glide ratio.

This was brought home even more when I started flying the Cessna Caravan which has the PT6 engine. With a PT6 you can position the prop anywhere from fine pitch to feather. The Caravan has about a 9 foot disc area and the angle of the prop blades make a huge difference in parasite drag. By bringing the power to idle you can push the nose over and get a very controllable 1500-2000 FPM decent and keep your airspeed where it was. The other extreme is feathered, which we have our pilots do once during training to demonstrate how a Caravan glides after an engine out. With the prop is feathered you glide at 80-85 knots and have a 600 FPM rate of decent. The Caravan has about a 15:1 glide ratio; from 10,000 feet you can go about 30 miles and will have almost 20 minutes before you will be landing. When you get to your key point you fly almost the same patten as you normally do with power.

For engine failures in a Kitfox I would suggest:
-If you have a controllable pitch prop go to course pitch and keep the prop spinning.
-For a fixed pitch prop try to get the prop to stop turning (if you need the extra range). Slow the aircraft down, even consider stalling it then go back to Vgl.
-If you have a landing spot safely within reach it doesn't really matter what the prop drag is, just maintain Vgl, get to your key point.
-Keep a little extra altitude and maintain your glide speed to the flare. Slip it control your rate of decent.
-DON'T get slow and DON'T dive and pick up extra speed. Vgl in my KF is 65 mph and I try to maintain 65-70 all the way down till the flare.

[av8rps]

Even though my Kitfox 4-1200 is on a set of 14 ft long amphibious floats that are very draggy and weigh 230 lbs, it glides well. I can't tell you specifics as I've never measured it, but compared to most light aircraft I feel the glide is much better. And since the geared engines don't windmill, in my opinion that is a big advantage. I would agree with a 10 to 1 ratio, like new Kitfoxes. That is probably conservative.

In the mid to late 80's when I started flying Avid Flyers I actually logged almost 4 hours of gliding time with the engine turned off (early 2 strokes were not very reliable, so I practiced flying with a "failed" engine). And every time I shut down the engine I also finished with a landing (I did the same practice on floats). I may have more "real" gliding time in this type of an airframe than anyone?

So what did I learn from all that engine off time?

Based on my actual experience, I learned there is for sure a difference between an engine that is shut off verses one that is just pulled back to idle. Especially if it is a geared engine. Prior to actually shutting down the engine, I had tested emergency procedures repeatedly with the engine at idle, simulating a wind milling prop. So when I finally shut down the engine and the prop stopped, I was initially surprised how much better the glide was. No scientific numbers to support that, but I felt it was significant.

Consequently, I think there would be a significant difference between our Kitfoxes flown with geared engines verses non geared engines in a true engine out scenario. The geared engine would glide better.

I've (unintentionally) had to glide (and land) a couple Lycoming powered aircraft (certified types) after complete engine failures. IMHO, with the engine producing no power that windmilling prop is the equivalent of having a 6 ft circular piece of plywood bolted to your crankshaft. Unlike the geared engines where the prop stops, there is a lot of drag coming off that disc. But unless you have flown one with the engine actually OFF, you won't grasp how much drag is there, as simulated power off is just not the same. At idle you are still making some thrust. Just stand behind any typical aircraft when it's idling...there's a lot of wind.

Another thing I learned from my engine out practice is that you really need to know what your best glide speed is. In my early Avid Flyer that number was 60 mph, even though the stall was only 22 mph. I tested that again and again and found that 60 was the magic number. And after years of flying most of all the derivatives of that early Avid, 60-65 is still a good number for most. The only difference I would say is that later, heavier models may need a bit more speed as you get close to the ground, as you need to have enough energy to be able to control the sink rate of these "high-lift-but-high-drag-at-slow-speeds" STOL wings. And you need to have enough elevator to flare. Sink rate can easily get away from you if you allow speeds to get under 60 mph. If I were going to dead stick my Model IV on floats I would aim for 70 near the surface. You can always bleed off extra speed, but you can't get it back.

I also learned that the first time you experience an engine out with a geared engine, that your brain will need some time to adjust to things. Because there is no prop turning, or engine running, most all of the noise and vibration you are used to will be gone. All that will be replaced with just wind whistling noises. And the controls will suddenly feel smoother than you ever felt before. So anticipate a few seconds for your brain to accept these changes when the engine fails. (hopefully that won't be on takeoff when you don't have extra seconds...)

For those wondering if it is fair to compare the gliding ability of an old super light Avid Flyer to a modern Kitfox, I think it is. While the Avid is much lighter, that undercambered airfoil would be much draggier than the Riblett airfoil used on newer Kitfoxes. So in effect, the modern Kitfox would actually glide better. But of course the new Kitfox is heavier than that early Avid, so maybe when all is said and done the early Avid and the modern Kitfox would glide similarly? If I had to guess, I'd guess the Kitfox would have the better glide. It is just so much aerodynamically cleaner than the older planes.

I was thumbing through some old photos I took of my Avid gliding with the prop stopped. One shows me at 8800 AGL with a comment that I glided for 22 minutes before landing, or 400 fpm sink. But there were some thermals. So at 60 mph glide, or a mile a minute, I could have glided 22 miles!

Another was 8,000 AGL for 17 minutes (470 fpm - or 17 miles) with no comments about thermals. So they're good gliders for being a draggy little STOL plane.

For any of you that want some proof, check out this video of another Avid derivative gliding off the side of a mountain. Oh, and fwiw your Kitfox will glide better...

https://www.youtube.com/watch?v=jeQP-H_31JQ

[wannafly]

Great thread. I’ll have to go try this for some hard numbers. I’ve always used 60 mph with engine at idle and gotten close to 2 miles per 1000’. I know going over the water between PEI and Nova Scotia I climbed to 10000’ just to be safe over 18 miles of water.
Keep your thoughts coming

[David47]

........... So when I finally shut down the engine and the prop stopped, I was initially surprised how much better the glide was. No scientific numbers to support that, but I felt it was significant......
https://www.youtube.com/watch?v=jeQP-H_31JQ

A prop that's stopped will have a lot less drag than one that's windmilling such as at idle. And there are scientific numbers to prove what you've experienced, although surprisingly you have to do some research to dig them out. So you aren't imagining it !!

Phil, sounds like your flight training in the Caravan is designed to be very practical . And I like your tips for engine out. Early in my flight training I recall one of my Instructors screaming at me "Fly the $%^&&*&* airplane" when I once turned toward the field and was more focused on making the correct radio call than doing anything else ...... those exact words still ring in my ears to this day ...!

[rosslr]

This is an interesting discussion. to add another variation, I ( like a few others here) have an Airmaster CSP unit that has a Feather function. Must admit I have never used it other than on the ground and I have not felt comfortable to turn off the engine off and and try it - the engine off would be the only time it would be useful I imagine). Have others tested the Feather function with engine out?? I would be interested in hearing how much difference it makes. Now with about 340 hours on the plane I am only just starting to feel confident enough to give this a go and learn from it. Would like to now from others what to expect if they have already tested this.

cheers

r

[Av8r3400]

Wise words by the immortal Frank Beagle: "You haf-ta AFTA*!"


*Always Fly The Airplane

[Esser]

This is an interesting discussion. to add another variation, I ( like a few others here) have an Airmaster CSP unit that has a Feather function. Must admit I have never used it other than on the ground and I have not felt comfortable to turn off the engine off and and try it - the engine off would be the only time it would be useful I imagine). Have others tested the Feather function with engine out?? I would be interested in hearing how much difference it makes. Now with about 340 hours on the plane I am only just starting to feel confident enough to give this a go and learn from it. Would like to now from others what to expect if they have already tested this.

cheers

r

I have the beta option instead of the feather on my AirMaster. I wonder if I could still feather with the manual control?

[av8rps]

Just a little recommendation / disclaimer here to the group;

If you are considering doing an actual "shut down the engine", before you reach over and kill the engine, make sure you are mentally committed to land the plane if the engine doesn't restart. The first time I shut down the engine it wouldn't restart, so my first shut down ended in a dead stick at the local airport.. . (Murphy always has a way of letting you know he's out there still doing what he does best).

It was none other than Dean Wilson, the Avid airplane designer that suggested I learn how to fly my Avid Flyer (his prototype) with the engine off, over concerns about the reliability of the powerplant (a Scorpion 400 snowmobile engine hooked up to a 3:1 gearbox he made from a Ford C3 transmission). You see, in 1982 when the Avid prototype first flew Rotax wasn't even heard of for use as an aircraft engine. They came on the scene later when Avid and Kitfox sales were pretty much ruling the recreational aviation world. Anyhow, after Dean heard a fearless 26 yr old (me) telling him how much fun I was having flying his plane in some really sketchy STOL environments, I think he got a bit nervous. I'm sure he was just trying to make sure both me and his beloved prototype stayed in one piece. And while some might argue it is unsafe to suggest shutting an engine down and gliding around, his advice was invaluable as over the next 30 years of my flying I had 5 engine failures in 5 different planes. Oh, and 3 of those were on takeoff. And I not only walked away from every one of those, but I didn't even put a scratch on any of those planes. (Ironically, the Scorpion powered Avid was the first of the five failures, so you can say I learned early on not to trust any engine in an airplane).

All that said, I still feel Deans' advice was really good for me. But I'm not so sure everyone is up to playing test pilot. So I just want to remind everyone here on this great forum to play it safe. Stay within your aircraft, and personal limits. Fly safe.

[Slyfox]

I've had 3 engine outs. once with my older 912ul engine. I had Cheryl with me. this one was on take off, I was about 200ft off. I calmly came back to the runway and landed. the second was with the Rv7 I have. I had about 10hrs on it. I had it happen on take off, I was about 300ft off on climb out. I came back for the runway, not a problem, but I did have to slip and full flaps to make it on the next parallel runway and touched down at the last turn off. no damage on either landing or aircraft. I did get both restarted, but they ran like crap all the way back to the hangar. the rv had injectors that were carboned up. too much rich for the first hours on her. cleaned them and been great since. so I guess engine off can happen at any time. be ready at any time.