[TThurston]

There have been a number of discussions regarding the importance of practicing power-off landings. With a constant speed propeller, the common wisdom is to set the prop to coarse pitch to minimize drag. The propeller governer should work as long as the propeller is windmilling. Then you pitch for best glide and find a field, and restart the engine if you can.

But I've been reviewing the POH for my current plane (a Diamond DA20-C1), and note the the fixed pitch propeller will stop windmilling around 60 KIAS. This made me wonder about the drag created by that windmilling fixed-pitch prop. If all attempts to restart the engine don't won't work, will I get better glide performance with my prop windmilling, or slowing enough to stop the prop from windmilling, and then lowering the nose back to best glide speed?

I reallize that in an actual engine failure, there's not much time to consider alternatives. But with the recent thread about an engine failure in IMC, I wondered if there might be enough difference in performance with the prop stopped that it might made a difference in whether in how far you can glide.

The only procedures I can find regarding this question for RV's is from the RV-12 POH. The procedure for "Engine Air Restart" contains the note that the prop will not windmill below 80 KIAS. I'm not sure if this means if you are currently windmilling it will stop if you go slower than this speed. It might also meanif you are not windmilling, it will start if you go at least this fast. I think there is a difference. Also in this procedure, it says to maintain at least 60 KIAS. I assume at this speed the prop would not be windmilling. However, in the procedure for "Forced Landing" it suggests two airspeeds, 85 KIAS for best glide distance, and 60 KIAS for minimum rate of descent.

Any thoughts?

(I wasn't sure where to post this, so please move if this isn't the right place.)

 

[n5lp]

There have been a number of discussions regarding the importance of practicing power-off landings. With a constant speed propeller, the common wisdom is to set the prop to coarse pitch to minimize drag. The propeller governer should work as long as the propeller is windmilling. Then you pitch for best glide and find a field, and restart the engine if you can.

But I've been reviewing the POH for my current plane (a Diamond DA20-C1), and note the the fixed pitch propeller will stop windmilling around 60 KIAS. This made me wonder about the drag created by that windmilling fixed-pitch prop. If all attempts to restart the engine don't won't work, will I get better glide performance with my prop windmilling, or slowing enough to stop the prop from windmilling, and then lowering the nose back to best glide speed?

I reallize that in an actual engine failure, there's not much time to consider alternatives. But with the recent thread about an engine failure in IMC, I wondered if there might be enough difference in performance with the prop stopped that it might made a difference in whether in how far you can glide.

Any thoughts?

(I wasn't sure where to post this, so please move if this isn't the right place.)

I did rudimentary glide tests in my fixed pitch RV-6 with the engine off and a windmilling propeller and with the engine off and the propeller stopped. It did glide better with the propeller stopped but the difference was not great. I attribute that to the very course pitch of an RV fixed pitch propeller.

As you point out, you do lose glide while slowing down enough to stop the propeller so there is a crossover point that I didn't try to figure out. As for me I wouldn't stop the propeller unless the engine failure happened quite high, like maybe 4,000 AGL or so. With a warm engine I had to get down to about 60 knots to stop the propeller. It was more like 70 knots after the oil was cooler.

Another factor, depending on the nature of the failure, is that it would probably be much simpler to get the engine running again if it is already windmilling.

 

[Snowflake]

This question came up when I was an Aeronautical Engineering student. We did the math for a small aircraft, and the results were pretty much what Larry found. With the prop stopped, you definitely stretch your glide distance, I don't have my notes in front of me but 10-15% is the number that comes to mind. I remember that the drag on a windmilling prop is almost exactly equivalent to that of a solid circular disc, the same diameter as your prop arc, attached where your prop is.

The problem is that to stop your prop you have to load the prop up by slowing down and raising the nose until it stops turning. This will happen almost immediately with a wood prop (it has very little inertia) but takes more work with a metal prop... You'll be slowed down quite a lot and in some small aircraft probably close to your stall speed. Then you have to push over and accelerate back to your best glide. Done perfectly, you lose nothing in this manoeuver. Done in the stressful situation of an engine failure, you probably lose some altitude you won't get back.

We concluded in class that for an engine failure on takeoff, you wouldn't want to waste time with it, even though you could stretch your glide... The distance gained on the ground wouldn't be worth the added risk. But for an engine failure in flight, you'd most definitely want to stop it, to maximize your opportunities after that point.

In my younger and crazier days I tried this in a light aircraft with a wooden prop. I definitely recorded improved glide performance with the prop stopped. I can also add that I was unable to start the wooden prop windmilling again by diving... That was the only heart-pumping moment during the test, as I hadn't expected it to not want to windmill if I picked up some airspeed again. I used idle mixture to shut down the engine, and planned to give full mixture again to re-start the windmilling engine. But there was no windmilling! I had to use the starter, which had been intermittent on a previous flight. I was quite happy when it worked.

As Larry points out, do the "try to restart" checks first, and exhaust all options before stopping the prop. Even though I know there are benefits to it, i'd still only consider it a last resort once I was certain that an engine-out landing was unavoidable.

 

[Indigo]

Most propeller governors require a minimum of 50psi incoming oil pressure to operate the propeller, if the the engine is not running I doubt there would be enough pressure to work the propeller eneough to make any difference in the pitch.





James Dean
Chief Inspector
American Propeller

 

[Guy Prevost]

Check Kevin Horton's site. He did some fairly careful testing on this. I believe his conclusion was that unless he started at a very high altitude, he would loose more glide distance by slowing to stop the prop than he would gain back with the increased glide ratio caused by the stopped prop.

Guy

 

[Bavafa]

A good friend of mine did this test in his RV7 with CS prop to get a good feel for such situation. He was definitely able to change the pitch of his prop and found to give him a much advantage glide ration by doing that. He also tried stopping the wind milling and it took him two tries, nearing the stall speed before he was able to stop the prop. But with the stopped prop, his glide ratio had improved but not that significantly compared to full course pitch.

 

[nucleus]

I gained altitude stopping the prop

I gained altitude stopping the prop, just converted speed into height. I believe it was somewhere around 65-70 knots where it stopped. RV200 prop.


Hans

 

[skylor]

I gained altitude stopping the prop, just converted speed into height. I believe it was somewhere around 65-70 knots where it stopped. RV200 prop.

Hans

You might reduce sink rate or gain altitude initially when you convert speed into altitude, but depending on the prop there will be a period of time when you're flying well below best glide (i.e. high sink rate) waiting for the prop to lose momentum and stop, and then you will convert altitude back into speed when you push the nose down to get back to best glide speed...and at 65-70 knots while trying to increase speed the sink rate will be very high indeed. All this means that overall, the altitude required to stop the prop is not going to be "free". Also, any rapid pitch changes to achieve the above will momentarily increase drag, resulting in further energy loss.

Skylor

 

[B25Flyer]

Internet flight instruction

There is lots of internet flight instruction going on here... The first thing is that internet flight instructors do not have to have any ratings, or have any first hand experience.

Much of what has been posted here is just someones idea about what might happen, and some of it is just plain wrong.

Take your airplane up high over the airport and pull the mixture. If you are afraid to do that, find an instructor who will do it with you. If the instructor is afraid, find a different instructor... If all else fails, buy a case of beer and come to Iowa for a visit....

If you are afraid to pull the mixture 5000 ft above a 5000 ft runway, how do you think things will go when it quits unexpectedly at 1500 AGL?

Getting comfortable in this regime will help develop excess mental capacity. That will provide the mental horsepower needed to make good decisions when it really happens....

With all that said. Stopping the prop will almost never help in an airplane like the RV. If you have a normal C/S prop, pulling the prop back to high pitch/low rpm will help if you have oil pressure to control the prop. If your engine is healthy it will govern. If it blows up, all bets are off.

If you have an aerobatic C/S prop it will still govern if your engine is healthy, and pulling it back will help alot. If you lose oil pressure, an aerobatic prop will still fail to high pitch/low rpm.

Either way, once the landing area is made, a C/S can be used as a speed brake to slow down quickly.

If you have a fixed pitch prop there is not much you can do anyway, but trying to stop it would be a waste of effort. I have stopped the prop on my wood prop RV-4 but I don't have any way to measure accurately enough to prove a difference. I doubt anyone else does either....

The important point here is most of this is moot. Anyone who has not flown their airplane with the engine out, will not have the excess mental capacity to think through all these options.

Tailwinds,
Doug Rozendaal

 

[scsmith]

In this case, almost all of above is correct

Much of what has been posted here is just someones idea about what might happen, and some of it is just plain wrong.
Doug Rozendaal

In this case, it happens that almost all the comments here are correct.

Stopping a prop will help most if you do it at high altitude. From low altitude, there is little gain, and much risk.

Stopping a fixed pitch prop will be easier, and should help more.
Stopping a C/S prop may be harder, and it may not be worth it compared to getting to coarse pitch.

Getting to coarse pitch with a C/S prop is always a good idea if you can.
If the engine is broken enough that it won't windmill enough to make oil pressure, maybe you will be lucky and it will stop.

 

[Snowflake]

In this case, it happens that almost all the comments here are correct.

I agree, having tried it both with an instructor and solo, in fixed pitch aircraft.

I'll qualify that by saying that I haven't tried it in an RV... Yet. When i'm more comfortable with mine I will probably do that, and in the meantime I hope it won't come up unexpectedly....