The impossible turn made possible---simulated engine failure from 600' with turnback

[igorek82]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

This was interesting - Stationary and Windmilling Propeller Drag - http://www.peter2000.co.uk/aviation/misc/prop.pdf

"Conclusion - The main goal of this investigation was simply to determine whether a stationary or a windmilling propeller has more drag. The answer is complicatedly simple: it depends. It is clear that it depends on the pitch and length of the propeller, and it is probably independent of the wind velocity. A crossover point was discovered where the drag forces for the windmilling and stationary states were the same. This crossover point is also dependent on the pitch, the length, and probably independent of the wind velocity."

Does anyone know where we can find actual data for a typical fixed-pitch ~76 inch prop on io-360?

 

[beestforwardspeed]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Are you sure?

Perhaps at some very high altitude, but most GA glide performance charts look something like this:

If your assertion was correct, that should be a curved line, correct? I don't ever recall seeing one with other than a straight line.

In any case, altitude should not affect glide distance or glide angle, as long as the correct speed is used. And the same applies to aircraft weight.

Stipulated the above may not be correct on the edge of space, but that's not what the discussion is about.

Absolutely correct! Density altitude (and weight) have NO effect on glide range. The only reason it is relevant in determining a minimum safe turn back altitude is that your climb angle will be shallower on departure the higher either of these things are.

Thanks for posting the video,leaving yourself open to all the experts and nay Sayers.

Glad to!

I know full well before I post anything that there will be those who disagree with anything.

 

[Captain]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Absolutely correct! Density altitude (and weight) have NO effect on glide range.

That is not a correct statement.

 

[FastEddieB]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

That is not a correct statement.

Source?

 

[nauga]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

double post

 

[nauga]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

That is not a correct statement.

For a GA airplane's flight envelope it most definitely is correct if you're talking about stabilized glide range. Weight affects the speed at which you get a particular glide range and altitude affects the *true airspeed* at which you get a particular glide range but maximum glide range and the spread of possible stabilized glide ranges does not change with either.

edit: That being said you're going to be in a worse spot to begin with so you'll be lower when you get to a stabilized glide at higher DA, all other things being equal.

Nauga,
nondimensionally

 

[Captain]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Source?

Two identical aircraft flying side by side at 7,000 ft msl. Chop the power at the same time and hold best glide speed. Only difference is one is empty save a 65 pound pilot and the other is max gross takeoff weight.

If you're telling me they both glide the same distance then I'm the one who's gonna need a source.

 

[nauga]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Two identical aircraft flying side by side at 7,000 ft msl. Chop the power at the same time and hold best glide speed. Only difference is one is empty save a 65 pound pilot and the other is max gross takeoff weight.

If you're telling me they both glide the same distance then I'm the one who's gonna need a source.

If you *start* at best glide speed for each airplane *at that airplane's weight* then yes they will glide the same distance. Glide ratio is a function of L/D, which can be reduced to lift coefficient/drag coefficient (CL/CD), neither of which varies with weight.
The speed at which you get a particular glide ratio will change, but the range of available glide ranges will not. The heavier airplane will have a higher best glide speed and a higher rate of descent at that speed but the range it makes will be the same as the lighter airplane.

ref: http://wright.nasa.gov/airplane/ldrat.html

Nauga,
who prefers decent descent

 

[kyleb]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If you *start* at best glide speed for each airplane *at that airplane's weight* then yes they will glide the same distance. Glide ratio is a function of L/D, which can be reduced to lift coefficient/drag coefficient (CL/CD), neither of which varies with weight.
The speed at which you get a particular glide ratio will change, but the range of available glide ranges will not.

ref: http://wright.nasa.gov/airplane/ldrat.html

Nauga,
who prefers decent descent

You're just saying that because you do flight test for a living...

 

[FastEddieB]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If you're telling me they both glide the same distance then I'm the one who's gonna need a source.

Since you asked nicely...

http://www.askacfi.com/1759/effect-of-weight-on-glide-ratio.htm

 

[Gucci Pilot]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Also spelled out in Aerodynamics for Naval Aviators

 

[Captain]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Since you asked nicely...

http://www.askacfi.com/1759/effect-of-weight-on-glide-ratio.htm

This has been a nice refresher. However, the reference chart you posted shows a static speed of 65kts. No range of speeds to compensate for L/D max. So, my position of varying ranges seems correct.

I will admit however I was advocating a position that was, at its core, less than 100% correct.

 

[FastEddieB]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

This has been a nice refresher. However, the reference chart you posted shows a static speed of 65kts. No range of speeds to compensate for L/D max. So, my position of varying ranges seems correct.

I will admit however I was advocating a position that was, at its core, less than 100% correct.

Nice response! Seriously.

I think the chart shows a constant IAS because the weight is assumed to stay constant. And is normally shown at max gross. At lighter weights one would use a lower IAS to go the same distance. And, of course, the TAS would be higher at altitude.

And no "range of speeds" because L/D Max remains constant as well - unless high lift devices or spoilers come into play.

All pretty counterintuitive, but good to know. Catches a lot of very experienced pilots.

 

[nauga]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

However, the reference chart you posted shows a static speed of 65kts. No range of speeds to compensate for L/D max. So, my position of varying ranges seems correct.

Funny thing is, the "range of range" is not necessarily intuitive. If that chart and speed were for, say, max gross and you were to use that speed in the airplane with the 65 lb pilot, then the light airplane would have *less* range. Gliding at L/Dmax gives you max range. Any amount fast or slower than L/Dmax gives less performance.

This is just another reason why AOA in the cockpit can be a beautiful thing, if applied and interpreted correctly. One AOA for L/Dmax, any weight.

Nauga,
by the numbers

 

[denverpilot]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Several have mentioned the effects of density altitude being a big consideration.



I don't get it, what changes with a departure engine failure at a high DA? I can see a partial failure, but assuming we are talking complete failure where is the DA consideration?



I get it that at high DA you could argue less altitude might be gained before failure but the purpose of this drill is to establish and AGL number where you would consider turning back.



Someone else used 180 degree turn to establish altitude needed but 180 won't get you back on the runway you departed, a tear drop or a left 90 followed by a right 270?

Lower climb rate and lower engine power of not turbo-normalized, means you'll be further from the runway by the time you hit that AGL number, so glide angle being equal at the "best" airspeed, you won't make it back from the same AGL number as at sea level. You'd need a higher AGL number.

 

[CharlieTango]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Lower climb rate and lower engine power of not turbo-normalized, means you'll be further from the runway by the time you hit that AGL number, so glide angle being equal at the "best" airspeed, you won't make it back from the same AGL number as at sea level. You'd need a higher AGL number.

If i do a normal takeoff ususally around 9,000' DA and I try the impossible turn at 500' or higher I will have trouble not overshooting the runway. I am better off if I am farther from the runway.

All things considered the numerous posts pointing to DA as one of the most important considerations is wrong.

 

[Captain]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If i do a normal takeoff ususally around 9,000' DA and I try the impossible turn at 500' or higher I will have trouble not overshooting the runway. I am better off if I am farther from the runway.

All things considered the numerous posts pointing to DA as one of the most important considerations is wrong.

In my defense, my main point (before getting side tracked on DA / weight) was the difference between an engine at idle and a dead engine.

If anyone has access to a plane they KNOW they can restart I'd suggest a test. Start a few thousand feet up over some runway (safety in case you can't restart) and do it twice. Once at idle and once with a dead engine. Compare a steady VSI with both at best glide speed. My hunch is a dead engine will be at least 200 fpm more.

 

[CharlieTango]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

In my defense, my main point (before getting side tracked on DA / weight) was the difference between an engine at idle and a dead engine.

If anyone has access to a plane they KNOW they can restart I'd suggest a test. Start a few thousand feet up over some runway (safety in case you can't restart) and do it twice. Once at idle and once with a dead engine. Compare a steady VSI with both at best glide speed. My hunch is a dead engine will be at least 200 fpm more.

If I lost my engine but have enough altitude to maybe glide to the field should I let my prop windmill or stop it?

 

[Captain]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If I lost my engine but have enough altitude to maybe glide to the field should I let my prop windmill or stop it?

At best glide it'll do one or the other. Which ever it does do that.

 

[Palmpilot]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Lower climb rate and lower engine power of not turbo-normalized, means you'll be further from the runway by the time you hit that AGL number, so glide angle being equal at the "best" airspeed, you won't make it back from the same AGL number as at sea level. You'd need a higher AGL number.

If high DA degrades the climb performance enough to cause the climb angle to be smaller than the glide angle, a higher AGL number makes things worse, not better, because the higher AGL comes at the expense of being farther from the runway, which aggravates the problem of the climb angle being smaller than the glide angle.

 

[Palmpilot]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If i do a normal takeoff ususally around 9,000' DA and I try the impossible turn at 500' or higher I will have trouble not overshooting the runway. I am better off if I am farther from the runway.

All things considered the numerous posts pointing to DA as one of the most important considerations is wrong.

...for your airplane. Not all planes have good enough high altitude climb performance to get that result.

 

[Palmpilot]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If I lost my engine but have enough altitude to maybe glide to the field should I let my prop windmill or stop it?

My concerns with that are that trying to get the prop stopped when you're that close to the ground could be dangerous, and that doing whatever maneuvers you have to do to get it stopped could cause additional altitude loss.

On the other hand, I've been taught that if you have a controllable-pitch prop, setting the prop control to the low-RPM position does provide some additional glide performance, by increasing the pitch.

 

[FastEddieB]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

I was in under the impression that due to the gearing, if a ROTAX 912 quits the prop will virtually always stop.

Is that not correct?

 

[denverpilot]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If high DA degrades the climb performance enough to cause the climb angle to be smaller than the glide angle, a higher AGL number makes things worse, not better, because the higher AGL comes at the expense of being farther from the runway, which aggravates the problem of the climb angle being smaller than the glide angle.

That's exactly what I said. Taken to the extreme for an example, if you're ten miles out and have only gained 50' to clear the trees, you're not going to make it back. If you're now later in the same climb at 500' and 100 miles out, and you tested 500' as your turn back altitude at sea level, you ain't gonna make it.

On a hot day it's not uncommon to see 65% BHP or less up here. If time to climb is hideous and you want the option to return, you'd better circle the airport.

No test pilot here with great aerodynamic math skills but I know on a hot high day, you aren't going to make it back after a slog of a climb at the same altitude and time to climb as you'd have at sea level.

I wouldn't attempt a turn back at LXV while still passing between the gap in the trees just northwest of the northbound runway during an "oh crap this thing is underperforming!" takeoff. Not a chance in hell you'd make it back to the runway. Even on a good performance day you won't make pattern altitude until you're leaving the pattern southbound toward Buena Vista.

If you need the safety blanket of returning to the runway for your departure plan up there, you'd have to do an early crosswind turn and stay near the runway by transitioning into a downwind. The climb is too shallow to do it straight out and expect to come back with a sick engine.

 

[MAKG1]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

On the other hand, I've been taught that if you have a controllable-pitch prop, setting the prop control to the low-RPM position does provide some additional glide performance, by increasing the pitch.

It sure does….

Last time I did a high altitude engine out in a CAP 182, I pulled the prop right after establishing best glide (at 3500 feet) and did three full circles around South County before lining up and landing. That's about right for a 172, but you can't do that much in a 182 with the prop forward.

You can feel it. It feels like adding a little throttle!

 

[dtuuri]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

If you *start* at best glide speed for each airplane *at that airplane's weight* then yes they will glide the same distance. Glide ratio is a function of L/D, which can be reduced to lift coefficient/drag coefficient (CL/CD), neither of which varies with weight.
... The heavier airplane will have a higher best glide speed and a higher rate of descent at that speed but the range it makes will be the same as the lighter airplane.

ref: http://wright.nasa.gov/airplane/ldrat.html

Not saying this is wrong, but my math-challenged mind devised a thought experiment (too lazy to actually try it out) and it failed to give the expected result. I folded a piece of paper into an airplane and tossed it across the room, noting where it landed. Then I retrieved it and stapled a shoelace to the estimated CG location, which added weight to the glider. I expected with the added weight it would glide approximately the same distance, but it did not. Should I have detached the shoe first?

dtuuri

 

[jsstevens]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

Not saying this is wrong, but my math-challenged mind devised a thought experiment (too lazy to actually try it out) and it failed to give the expected result. I folded a piece of paper into an airplane and tossed it across the room, noting where it landed. Then I retrieved it and stapled a shoelace to the estimated CG location, which added weight to the glider. I expected with the added weight it would glide approximately the same distance, but it did not. Should I have detached the shoe first?

dtuuri

No, just throw it harder to match the increased glide speed with more weight.

 

[igorek82]

Re: The impossible turn made possible---simulated engine failure from 600' with turnb

The speed at which you get a particular glide ratio will change, but the range of available glide ranges will not. The heavier airplane will have a higher best glide speed and a higher rate of descent at that speed but the range it makes will be the same as the lighter airplane.

ref: http://wright.nasa.gov/airplane/ldrat.html

Nauga,
who prefers decent descent

Nauga is correct, the range remains the same. What does change is the time spent gliding, which is decreased with increase in weight, thus leaving you with less time to attempt engine restart, checklists, mayday calls, etc.