Gliding with the prop stopped
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A stopped prop is still an aerofoil being dragged through the air or water. It creates lift, and therefore some associated induced drag, as well as form drag. So does the windmilling prop. I can see no reason why in principle one should be draggier than the other.
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but not one where the rotor has stopped.
FarmStripFlyer, The BMW engine with clutch set up you describe is similar to one in a friend's Skyranger microlight. Seeing the propeller stationary after the engine has started certainly takes some getting used to!
I think the clutch is there to stop the gearbox seeing the torque reversals from the boxer engine at low rpm, so it is probably not a good idea for the engine to be in the transition RPMs between clutch disengaged and fully engaged for extended periods, as clutch slip and gear chatter may occur.
Some experimenting on the ground to determine clutch engagement and disengagement RPMs would be well worthwhile, bearing in mind that the rpm to overcome prop inertia with the clutch engaging will be higher than the disengaging rpm.
I think the clutch is there to stop the gearbox seeing the torque reversals from the boxer engine at low rpm, so it is probably not a good idea for the engine to be in the transition RPMs between clutch disengaged and fully engaged for extended periods, as clutch slip and gear chatter may occur.
Some experimenting on the ground to determine clutch engagement and disengagement RPMs would be well worthwhile, bearing in mind that the rpm to overcome prop inertia with the clutch engaging will be higher than the disengaging rpm.
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Thinking a bit about it. We have three conditions.
First, the engine is driving the prop, the prop is using engine power to accelerate air and thus produce thrust. The air is moving over the prop airfoil in a controlled manner, in this case it is the way it was designed to work so fairly efficiently.
Second, the engine is producing no power and the prop is windmilling. The ier flowing through the prop is having energy extracted from it to provide the power to turn the dead engine. There would still be some sort of controlled airflow over the prop airfoil for this to work although the whole process would be pretty inefficient. I suspect that a windmill airfoil would look a lot different from a prop airfoil. But in any case drag is produced.
Third case, the prop simply produces drag and, as someone pointed out earlier, some torque which has to be countered by aileron at the cost of more drag. It's just another bit of profile drag from the point of view of aircraft performance.
From the various comments it would seem that there is not a lot of real world difference between the windmilling condition and the prop stopped condition.
Now the thing that I am wondering about is this. Could it be that in the case of an airplane which glides best at a low IAS, which would be a high aspect machine, might the stopped prop produce less drag? Conversely, in the case of an aircraft which glides best going faster, low aspect ration, might the windmilling prop produce less drag?
First, the engine is driving the prop, the prop is using engine power to accelerate air and thus produce thrust. The air is moving over the prop airfoil in a controlled manner, in this case it is the way it was designed to work so fairly efficiently.
Second, the engine is producing no power and the prop is windmilling. The ier flowing through the prop is having energy extracted from it to provide the power to turn the dead engine. There would still be some sort of controlled airflow over the prop airfoil for this to work although the whole process would be pretty inefficient. I suspect that a windmill airfoil would look a lot different from a prop airfoil. But in any case drag is produced.
Third case, the prop simply produces drag and, as someone pointed out earlier, some torque which has to be countered by aileron at the cost of more drag. It's just another bit of profile drag from the point of view of aircraft performance.
From the various comments it would seem that there is not a lot of real world difference between the windmilling condition and the prop stopped condition.
Now the thing that I am wondering about is this. Could it be that in the case of an airplane which glides best at a low IAS, which would be a high aspect machine, might the stopped prop produce less drag? Conversely, in the case of an aircraft which glides best going faster, low aspect ration, might the windmilling prop produce less drag?
All very interesting. But experience has shown that if the propellor has stopped turning, for whatever reason, unless you have plenty of altitude, the field will select you. Good luck.
From the various comments it would seem that there is not a lot of real world difference between the windmilling condition and the prop stopped condition
If it ever happens and I have the altitude where it make sense I will stop the prop.
Just for the sake of the discussion an assumption:
For my imaginary airplane the glide ratio is 10:1.
The stopped propeller gives me a 10% increase in glide range.
Glide range at 2000' altitude is 3 miles ( 10 x 0.3 miles)
Glide range at 2000' altitude is 3.3 miles with the propeller stopped.
Barely the distance of the field I'm aiming for.
Glide range at 12000' altitude is 20 miles ( 10 x 2)
Glide range at 12000' altitude is 22 miles with the propeller stopped.
Two miles is a significant difference so it will be worth the trouble of slowing down to stop the prop, even if you lose some of your gain due to slow flight or stall to stop the prop.
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Regardless of the real numbers, I believe the biggest factor is the pilot's mind - stopped prop is just not right and if you're much more likely to produce errors than in the case of windmilling one you're used to... 
If I'd have the engine failure, I would not waste time (and altitude) to stop the prop, but I'd rather take each foot of altitude as my advantage.
Having some altitude I would try to restart the engine anyway...
If I'd have the engine failure, I would not waste time (and altitude) to stop the prop, but I'd rather take each foot of altitude as my advantage.
Having some altitude I would try to restart the engine anyway...
Last edited by konradeck; 22nd Aug 2012 at 13:38.
I watched the video and I personally do not support doing what I saw in the video. First I think that pilots underestimate the difficulty of starting an engine with a stopped prop in flight. The engine doesn't like having a 65kt wind stuffed down its throat very much at all so the idea that you can just restart the engine if the approach goes badly may be illusory.
Until recently Transport Canada required that an actual engine shutdown, feathering and restart be demonstrated in flight. I had several instances when the restart process required several attempts with the starter and consumed several minutes. Once the engine simply refused to start and I was forced to do an actual single engine landing
. Fortunately Transport Canada came to its senses and removed this stupid requirement from the ME training requirements.
But the take away for me is if you deliberately stop the prop don't expect you can just give the key a quick twist and the engine will immediately roar back to life every time
Secondly to do a stopped engine glide and then restart the engine on the landing roll and immediately go to full power on a cold engine is a very very poor practice. You are just begging for cracked cylinders and this sets a terrible example to the student on proper engine handling
Until recently Transport Canada required that an actual engine shutdown, feathering and restart be demonstrated in flight. I had several instances when the restart process required several attempts with the starter and consumed several minutes. Once the engine simply refused to start and I was forced to do an actual single engine landing
. Fortunately Transport Canada came to its senses and removed this stupid requirement from the ME training requirements.But the take away for me is if you deliberately stop the prop don't expect you can just give the key a quick twist and the engine will immediately roar back to life every time
Secondly to do a stopped engine glide and then restart the engine on the landing roll and immediately go to full power on a cold engine is a very very poor practice. You are just begging for cracked cylinders and this sets a terrible example to the student on proper engine handling
Last edited by Big Pistons Forever; 22nd Aug 2012 at 14:50.
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Oh my god! He restarted it, and took off without warming it up!
. Fool.
The only fixed pitch propeller I have ever stopped in flight is that of the C 150 I own. I consider doing this on the verge of abusive to the engine if not done very gently, and I would not do it to someone else's airplane without good cause. Fooling around is not good cause. And, if you got things wrong at the end, and damaged the plane for lack of power, it would be hard to explain.
I am required to [stop engines] and feather propellers during flight testing on twins (usually the left), and then do a number of maneuvers. This can involve both a windmilling or feathered propeller, depending upon what is being done. I'm always happy to do this work on a PT-6 powered aircraft, where I can feather the prop without stopping the engine.
Piston engines can be grumpy when restarting in these conditions. If you cannot unfeather and windmill start, a starter start of a feathered prop will usually shake the engine a lot, and if not handled well could result in an overspeed. I always allow an extra thousand feet or so to continue a descent if needed, while the restarted engine is warmed up gradually, and its ability to deliver power smoothly is confirmed.
During twin training I was trained to shut down and restart in flight, though I agree with Big Pistons, that it is not a necessary enough exercise for a new pilot, to warrant the risk and possible harm to the engine.
. Fool.The only fixed pitch propeller I have ever stopped in flight is that of the C 150 I own. I consider doing this on the verge of abusive to the engine if not done very gently, and I would not do it to someone else's airplane without good cause. Fooling around is not good cause. And, if you got things wrong at the end, and damaged the plane for lack of power, it would be hard to explain.
I am required to [stop engines] and feather propellers during flight testing on twins (usually the left), and then do a number of maneuvers. This can involve both a windmilling or feathered propeller, depending upon what is being done. I'm always happy to do this work on a PT-6 powered aircraft, where I can feather the prop without stopping the engine.
Piston engines can be grumpy when restarting in these conditions. If you cannot unfeather and windmill start, a starter start of a feathered prop will usually shake the engine a lot, and if not handled well could result in an overspeed. I always allow an extra thousand feet or so to continue a descent if needed, while the restarted engine is warmed up gradually, and its ability to deliver power smoothly is confirmed.
During twin training I was trained to shut down and restart in flight, though I agree with Big Pistons, that it is not a necessary enough exercise for a new pilot, to warrant the risk and possible harm to the engine.
