Best glide with windmilling prop or stopped prop

[Larry Vrooman]

I used to own a motorglider and the glide ratio significantly improved with the propeller stopped. On the negative side, you had to slow to just above stall to stop it. On the other hand, it wasn't that far below the best glide airspeed, and once stopped you needed a lot of airspeed to get it spinning again.

That said, if I lose an engine in a fixed pitch GA single, I am NOT going to muck around slowing down enough to stop the propeller. In a non-motor glider that will put you way back on the drag curve where the loss due to induced drag is potentially costing you as much as more than you gain by stopping the prop.

Worse, it serves as a distraction when you should be focused on 1) identifying a place to make an emergency landing and getting set up to make the field, and then with that done and if time allows, 2) determining if a restart is possible (and having the judgement to know when it's not or when you don't have the time).

You see similar issues with accidents after landing gear failures. Gear up landings on a hard surface runway are a non event from a death and serious injury perspective, and you have to look really hard to find even one.

Unless of course the pilot did something stupid like attempting to shut down the engine(s) before landing to save the engines and props, or landing on grass in the mistaken belief it'll do less damage to the aircraft (It usually does more as the sod balls up under and in front of the wings and does far more airframe damage, as well as producing a much faster stop.) Those efforts, when they go wrong, are the accidents where you find injuries in landing gear failure events.

In short, don't get fancy. Just slow (or pitch over) to best glide speed, identify where you intend to land, and fly the airplane with no heroics.

 

[Rob58]

In short, don't get fancy. Just slow (or pitch over) to best glide speed, identify where you intend to land, and fly the airplane with no heroics.

Good wisdom here.

 

[Dana]

...I lose an engine in a fixed pitch GA single, I am NOT going to muck around slowing down enough to stop the propeller. In a non-motor glider that will put you way back on the drag curve where the loss due to induced drag is potentially costing you as much as more than you gain by stopping the prop.

It depends on the altitude. If you're at 1000' AGL, just land the plane wherever you can. If you're at 10,000', the gain in glide will likely offset the sink due to near-stall flying stopping the prop. In between? Somewhere there's a dividing line, which will be different for every aircraft.

 

[Larry Vrooman]

It depends on the altitude. If you're at 1000' AGL, just land the plane wherever you can. If you're at 10,000', the gain in glide will likely offset the sink due to near-stall flying stopping the prop. In between? Somewhere there's a dividing line, which will be different for every aircraft.

Well sure, if you are high enough the juice will eventually be worth squeeze. On the other hand, if you are high enough for it to be worth the initial altitude loss, you've probably got more than enough altitude to glide to a suitable landing field anyway.

 

[Dana]

Well sure, if you are high enough the juice will eventually be worth squeeze. On the other hand, if you are high enough for it to be worth the initial altitude loss, you've probably got more than enough altitude to glide to a suitable landing field anyway.

Most likely, yes, unless you're way out over the ocean or a forest.

 

[Thom Riddle]

Best glide with windmilling prop or stopped prop?

In every airplane I've owned with a fixed pitch prop, I've tested glide at idle speed and with engine/prop stopped. In EVERY case, the glide is not nearly as good with prop stopped as it is at idle speed. Note this is with a fixed pitch prop. Here's why it is true. Think of your airplane sitting on the ground with the engine idling. Is wind going past the cockpit? If so, then the prop at idle is producing thrust. ANY thrust improves glide vs no thrust.

I now own an RV8A with c/s prop. This particular prop does not have counterweights so if oil pressure is lost (engine stopped) it goes to flat pitch with is effectively a speed brake. If it had counterweights, it would go to max pitch with engine stopped which would improve glide considerably.

 

[Larry Vrooman]

Best glide with windmilling prop or stopped prop?

In every airplane I've owned with a fixed pitch prop, I've tested glide at idle speed and with engine/prop stopped. In EVERY case, the glide is not nearly as good with prop stopped as it is at idle speed. Note this is with a fixed pitch prop. Here's why it is true. Think of your airplane sitting on the ground with the engine idling. Is wind going past the cockpit? If so, then the prop at idle is producing thrust. ANY thrust improves glide vs no thrust.

I now own an RV8A with c/s prop. This particular prop does not have counterweights so if oil pressure is lost (engine stopped) it goes to flat pitch with is effectively a speed brake. If it had counterweights, it would go to max pitch with engine stopped which would improve glide considerably.

There's a missing factor here.

When I am sitting on the ground with the engine at idle, the prop is producing a breeze because the engine is running.

Similarly, if I pull the throttle back to idle in flight, the engine is still producing power and will still be generating some thrust, or at least will be generating less drag.

However, in flight, after engine failure (or if I pull the mixture to idle cut off and/or switch off the mags) the propeller is now windmilling due to the force of the air pushing the blades and causing the engine to turn over. There is no power being produced by the engine at all and the drag will be higher than if the prop were stopped.

 

[flyingcheesehead]

A stopped prop creates much more drag than a windmilling one.

As you have since learned, that is not the case. The prop does create drag, but unless you have a way to eject it, you're kinda stuck.

Even though you've got some big speed brakes at that point, the drag of a windmilling prop is roughly equivalent to the drag of a solid disc the size of your prop arc, which is certainly more than just stopped blades, even if they're in coarse pitch.

In most single engine airplanes, the question "stopped or windmilling" is more academic in nature, isn't it? I mean, it's not like we have a choice. Depending on the reason for the engine outage, something is either mechanically blocked and thus preventing the prop from turning, or not, in which case the prop can and will continue to turn. Slowing down to stop a windmilling prop is possible in theory, but will cause a significant altitude loss just for that maneuver, so I doubt this makes much sense in real life.

Depends how high you are. I know a guy (no it's not me, yes he is here) who did this once - IIRC he had to get the plane nearly to a stall before the prop stopped, and he had to push it nearly to Vne to get it windmilling again without the starter.

So yes, it's possible to stop. It's not going to be worthwhile if you're very low, but it's anyone's guess how high you really have to be to make it worthwhile, and that will depend greatly on the airframe involved. Someday, when I have a glass panel that can log data for me, I'd like to do some flight testing on this.

What we DO have to play with in case of a windmilling prop is the prop lever (assuming constant speed prop). Pull it back and see the glide performance improve.

For sure.

Now, anyone done high key/low key?

 

[sarangan]

Best glide with windmilling prop or stopped prop?

I recently was flying an S-35 Bonanza with an IO 550-B engine. At 4, 500 feet the engine came apart and the prop came to a very sudden stop. I immediately made a 180 turn headed to a dirt runway I was aware of. The best glide in this bird is flaps and gear up, prop to full pitch, and 110 knots. The rate of descent is alarming, to say the least. I initially felt I had the runway made but it became clear the drag the stopped prop was creating was like having three, 3-foot long speed brakes deployed. I ended up landing 1.5 miles short from the target airport on a little dirt road next to a fence. My airspeed bled off to 80 MPH almost immediately after my initial nose over and lowering the nose to 110 mph would have put me into a lake next to the road I landed on. Long story short, no one was injured and I didn't bend any metal! A stopped prop creates much more drag than a windmilling one.

The descent rate was alarming compared to what? If it is compared to idle power glide, then you may be right. An idling engine creates power. A stopped engine creates no power.

 

[Jeff767]

A prop stopped creates less drag than windmilling. Not sure why it’s even a discussion. Far more important however is the wind. The OP does not even mention it but I suspect the 180 put him into a headwind. Wind also changes your best glide speed far more than a stopped verses windmilling prop.

 

[flyingcheesehead]

There's a paper on windmilling or stationary propeller drag here: http://www.peter2000.co.uk/aviation/misc/prop.pdf The conclusion in that paper, of course, is that 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."

I decided to give this a read. Unfortunately, I don't think it has much relevance to the discussion here, and certainly doesn't change my mind on anything. He did NOT test propellers that were attached to engines. Since a lot of the drag of a windmilling propeller is due to it being attached to, and rotating, an engine, I don't think the free-spinning method he used is truly representative of the real-world situation we're discussing.

 

[Lindberg]

This may be a dumb question, but if a stopped prop is obviously so much less draggy that a windmilling one, why doesn't the prop stop on its own when the engine dies?

 

[flyingcheesehead]

This may be a dumb question, but if a stopped prop is obviously so much less draggy that a windmilling one, why doesn't the prop stop on its own when the engine dies?

Because there's air going through it. Instead of the prop turning and pushing the air, the air is pushing the prop and making it turn. Slow the plane (and thus the air) down enough, and it won't keep the prop going.

 

[Sinistar]

A prop stopped creates less drag than windmilling. Not sure why it’s even a discussion. Far more important however is the wind. The OP does not even mention it but I suspect the 180 put him into a headwind. Wind also changes your best glide speed far more than a stopped verses windmilling prop.

How does wind change your best glide speed? Best glide is IAS...no? I can see wind changing best glide distance.

 

[Dana]

In a

How does wind change your best glide speed? Best glide is IAS...no? I can see wind changing best glide distance.

In a headwind, you'll glide farther by speeding up (less time for the wind to push you backwards), conversely, slowing down in a tailwind means more time for the wind to help you.

 

[write-stuff]

Calling Rich Stowell...

 

[write-stuff]

In a headwind, you'll glide farther by speeding up (less time for the wind to push you backwards), conversely, slowing down in a tailwind means more time for the wind to help you.

This is a joke, right? (Just making sure...)

 

[Lindberg]

This is a joke, right? (Just making sure...)

No. https://en.wikipedia.org/wiki/Polar...ct_of_wind,_lift_and_sink_on_best_glide_speed

 

[Lindberg]

How does wind change your best glide speed? Best glide is IAS...no? I can see wind changing best glide distance.

It depends on what you mean by "best glide speed." If you mean the speed that results in the longest distance for altitude lost, then it does change with wind. L/Dmax doesn't change, but to get best glide you have to fly faster than L/Dmax into a headwind and slower with a tailwind.

 

[Hang 4]

This is a joke, right? (Just making sure...)

Actually, it's exactly right. Imagine a headwind that is the same as your best glide speed. You will be making your "best glide" through the air, but not moving at all relative to the ground. If you speed up, you'll have increased drag, decreasing your glide ratio through the air, but making forward progress over the ground. All works the other way if you have a tail wind, with a big assumption that your minimum sink speed is materially slower than best glide.

 

[write-stuff]

Actually, it's exactly right. Imagine a headwind that is the same as your best glide speed. You will be making your "best glide" through the air, but not moving at all relative to the ground. If you speed up, you'll have increased drag, decreasing your glide ratio through the air, but making forward progress over the ground. All works the other way if you have a tail wind, with a big assumption that your minimum sink speed is materially slower than best glide.

He said "In a headwind, you'll glide farther by speeding up". A headwind will decrease ground speed causing a reduction in glide distance.

Ok, wait, I see the gotcha in his wording. It sounded as if he was saying that the headwind would increase the "speed" of the aircraft.

 

[sarangan]

I used to own a motorglider and the glide ratio significantly improved with the propeller stopped. On the negative side, you had to slow to just above stall to stop it. On the other hand, it wasn't that far below the best glide airspeed, and once stopped you needed a lot of airspeed to get it spinning again.

That said, if I lose an engine in a fixed pitch GA single, I am NOT going to muck around slowing down enough to stop the propeller. In a non-motor glider that will put you way back on the drag curve where the loss due to induced drag is potentially costing you as much as more than you gain by stopping the prop.

Worse, it serves as a distraction when you should be focused on 1) identifying a place to make an emergency landing and getting set up to make the field, and then with that done and if time allows, 2) determining if a restart is possible (and having the judgement to know when it's not or when you don't have the time).

You see similar issues with accidents after landing gear failures. Gear up landings on a hard surface runway are a non event from a death and serious injury perspective, and you have to look really hard to find even one.

Unless of course the pilot did something stupid like attempting to shut down the engine(s) before landing to save the engines and props, or landing on grass in the mistaken belief it'll do less damage to the aircraft (It usually does more as the sod balls up under and in front of the wings and does far more airframe damage, as well as producing a much faster stop.) Those efforts, when they go wrong, are the accidents where you find injuries in landing gear failure events.

In short, don't get fancy. Just slow (or pitch over) to best glide speed, identify where you intend to land, and fly the airplane with no heroics.

Many years ago I had a landing gear that refused to lock, and I had people on the ground telling me to stop the prop and land on the belly. After thinking long and hard, I rejected their advice and landed with normal power. The prop was destroyed, and the engine had to be torn down, but I walked away without a scratch. If I had to do it again, I would not do anything differently. One emergency is bad enough, you don't want to introduce new variables into that equation.

 

[whifferdill]

How slow do you have to get to stop the windmilling? In some airplanes you’ll have to slow down to almost stall speed to stop the prop.

I was flying acro in a buddy's Pitts over the airport when the throttle cable broke with the engine wide open. Pulled the mixture and landed, but the metal prop didn't come to a complete stop until the very end of the landing roll after I'd turned off the runway onto the midfield taxiway, practically crawling.