Feathered vs Windmilling Prop
- Thread starter chuckles1225
- Start date
Chief Captain
Well-Known Member
Ignore Mike H
Guys, no need to get wound up over Mike H. Given what he's said on this thread, and the fact that I happened across some of his other posts here, I doubt he's a pilot.
Fancy someone with this on his profile not knowing what a stop and go is.
Ratings - ASEL, AMEL, Commercial, Instrument
Flight Time - 400
Myth busted.
Guys, no need to get wound up over Mike H. Given what he's said on this thread, and the fact that I happened across some of his other posts here, I doubt he's a pilot.
Fancy someone with this on his profile not knowing what a stop and go is.
Ratings - ASEL, AMEL, Commercial, Instrument
Flight Time - 400
Myth busted.
Mike H
Well-Known Member
Really, it seems like I'm the only one NOT getting worked up over etc, etc,
I learned years ago not to take internet disagreements seriously- or personally.
Sometimes it takes being challenged to get a more refined, thought out explanation out of some people. Apparently, tgrayson (and a few others) don't respond favorably to having what they say challenged. It really isn't my goal to get so deep into somebody's dome that they feel a need to search my old posts for something, anything to try to discredit me. It sure says somebody is getting a lot more worked up over "the battle" than I am, though.
whatever tgrayson is trying to explain about windmilling props and reverse thrust, he's explaining it poorly IN MY OPINION.
///AMG
Well-Known Member
I think this was mentioned previously, but here goes:
You can consider a prop to be a lift generating device, just like the wings of an airplane. The energy generating that lift is normally provided by the engine of the aircraft. When that engine is no longer in operation, it is simply the free stream of air moving the prop. Considering that there is a sum total of energy available to the aircraft at a given moment, moving this prop with some of it takes away from the rest of the energy available. Therefore you must sacrifice some altitude or airspeed to make up for this. If you want to remain level, that energy will come in the form of forward velocity/airspeed otherwise known as kinetic energy. If you want to keep that precious airspeed, you will need to descend and lose altitude/potential energy in the process. Clearly having no thrust exacerbates this problem, but that extra energy lost will add to the sum of your energy deficit. Not to mention the induced drag from the slipstream from the prop. Normally the energy from the motor is what counteracts this force, but without it, you are simply taking even more energy from the aircraft. Make no mistake, an unpowered airplane is eventually coming down, but the question is how much more quickly it does with the prop unfeathered. The last prop I flew descended at nearly 3 times the rate with the prop unfeathered that it did with a feathered prop. I hope that is a clear scientific-like explanation of the problem.
You can consider a prop to be a lift generating device, just like the wings of an airplane. The energy generating that lift is normally provided by the engine of the aircraft. When that engine is no longer in operation, it is simply the free stream of air moving the prop. Considering that there is a sum total of energy available to the aircraft at a given moment, moving this prop with some of it takes away from the rest of the energy available. Therefore you must sacrifice some altitude or airspeed to make up for this. If you want to remain level, that energy will come in the form of forward velocity/airspeed otherwise known as kinetic energy. If you want to keep that precious airspeed, you will need to descend and lose altitude/potential energy in the process. Clearly having no thrust exacerbates this problem, but that extra energy lost will add to the sum of your energy deficit. Not to mention the induced drag from the slipstream from the prop. Normally the energy from the motor is what counteracts this force, but without it, you are simply taking even more energy from the aircraft. Make no mistake, an unpowered airplane is eventually coming down, but the question is how much more quickly it does with the prop unfeathered. The last prop I flew descended at nearly 3 times the rate with the prop unfeathered that it did with a feathered prop. I hope that is a clear scientific-like explanation of the problem.
TUCKnTRUCK
That guy
It seems that a rather simple question opened one hell of a can of worms here!
I think I read on the first page, it's hard to turn the prop through by hand, and likewise is hard to do so in the air. The energy to turn that prop in flight doesn't just appear, think of it as tapping the total kinetic/potential energy of the airframe to turn the prop.
On a J-3, if you shut off the mags in flight, and pull up, I believe rotation stopped near 40 mph? after doing some spins, gliding etc, it required a dive to nearly 120 mph to get the prop turning again. The sink rate with the prop stopped was very minimal, but at the same speed with it windmilling, it was easily 2x as much. ( of course we were doing it near an airfield, sod farm etc... plan was just to land if it didn't want to start.)
I think I read on the first page, it's hard to turn the prop through by hand, and likewise is hard to do so in the air. The energy to turn that prop in flight doesn't just appear, think of it as tapping the total kinetic/potential energy of the airframe to turn the prop.
On a J-3, if you shut off the mags in flight, and pull up, I believe rotation stopped near 40 mph? after doing some spins, gliding etc, it required a dive to nearly 120 mph to get the prop turning again. The sink rate with the prop stopped was very minimal, but at the same speed with it windmilling, it was easily 2x as much. ( of course we were doing it near an airfield, sod farm etc... plan was just to land if it didn't want to start.)
tgrayson
New Member
Yes, but that still seems to imply it's the drag of the engine sucking the energy from the airframe. Remember that the rate of energy loss isn't due to torque, it's due to torque * RPM. The higher the RPM, the higher the loss of energy per unit time. That's what power is, change of energy per unit time.
On an airplane, rate of climb is maximized when power is maximized, which occurs as the highest RPM. Likewise, the rate of descent during a power off glide is maximized when RPM is the highest. The latter happens when the drag of the engine is as small as possible, just as the maximum RPM of a power on condition occurs when the drag of the prop is as small as possible, low pitch.
Think for a minute about your engine failure training...dead foot dead engine. What happens to the amount of rudder you need once you feather the dead engine's prop? It decreases. When the engine is driving the prop, the prop it setup to command a pitch setting to match power output. When the engine fails, the reverse happens because the prop is now driving the engine with the only energy source it has, airflow.
So as it turns, it turns the engine. Problem here. The engine still operates on the suck, squeeze, bang, blow process. It is the compression process of the cylinders that inhibits the ability of the prop to turn "freely" and create less drag. Were the plugs to be removed and the compression nulled, the drag would significantly reduce. Regardless, the propeller spinning in this fashion creates more drag because it is no longer being driven by a power source other than the wind, which is insufficient to spin it at the same rate as the prop on the good engine.
With an engine like the PT6, which is a free spool turbine, things are a bit different. With no direct connection to the turbine, an engine failure is detected by the autofeather system which senses a negative torque and automatically drives the prop to feather.
As far as letting a prop stop during training in single engine? We used to do it years ago. We'd set our students up on a normal approach to a long runway (9000') and brief them to the emergency of an engine failure. Over the runway threshold and landing assured, we'd have them complete the list...mags & fuel off, door unlocked. The prop would stop windmilling at about 65 knots as I recall...and they would focus on that. It was a very clear lesson to them that you can't control the prop, but you can control the landing...IF you maintain your airspeed. Of the many students we taught that to, every one said that it was probably the most important lesson they ever got. Training and checklists are vital tools. But unless you know what to expect, you can still have problems.
So as it turns, it turns the engine. Problem here. The engine still operates on the suck, squeeze, bang, blow process. It is the compression process of the cylinders that inhibits the ability of the prop to turn "freely" and create less drag. Were the plugs to be removed and the compression nulled, the drag would significantly reduce. Regardless, the propeller spinning in this fashion creates more drag because it is no longer being driven by a power source other than the wind, which is insufficient to spin it at the same rate as the prop on the good engine.
With an engine like the PT6, which is a free spool turbine, things are a bit different. With no direct connection to the turbine, an engine failure is detected by the autofeather system which senses a negative torque and automatically drives the prop to feather.
As far as letting a prop stop during training in single engine? We used to do it years ago. We'd set our students up on a normal approach to a long runway (9000') and brief them to the emergency of an engine failure. Over the runway threshold and landing assured, we'd have them complete the list...mags & fuel off, door unlocked. The prop would stop windmilling at about 65 knots as I recall...and they would focus on that. It was a very clear lesson to them that you can't control the prop, but you can control the landing...IF you maintain your airspeed. Of the many students we taught that to, every one said that it was probably the most important lesson they ever got. Training and checklists are vital tools. But unless you know what to expect, you can still have problems.
TUCKnTRUCK
That guy
Not drag, but turning the engine via prop windmill is in essense running a vacume pump, powered by a wind turbine. Unfortunately all you do is turn that energy into heat and noise in the engine. Once the engine stops turning, the only losses are due to drag on the prop, which is minimized when it's feathered. I think we are both in agreement, I was just trying to oversimplify it.
///AMG
Well-Known Member
Not really adding to the discussion, but I actually flew a variant of the PT-6 that did not have autofeather, and this was a boldface item for engine failure.
tgrayson
New Member
I'm not sure that is the case. One of the points I was making is that increased internal engine friction actually reduces drag on the airplane because it reduces the equilibrium RPM for the windmilling propeller. If the engine seized, internal engine friction is effectively infinite and the stopped prop produces much less drag than a freely turning one.
TUCKnTRUCK
That guy
?
Yes, the rotating engine, and windmilling prop cause more energy loss than a stationary prop. The frictional loss from the engine, as well as the drag induced from windmilling the prop, or generating negative thrust, in addition to the plan form drag of the prop is more than just the planform drag of the stationary prop.
tgrayson
New Member
The airplane does not see frictional forces, it only sees aerodynamic ones. Only the reverse lift vector of the windmilling propeller can have any effect on the airplane. The only use of the frictional forces is determining the equilibrium RPM, which then determines the AoA seen by the blade and hence the reverse thrust. If you want to argue differently, you will have to show a mechanism that connects the internal engine friction to the production of pressure differences around the airplane.
TUCKnTRUCK
That guy
You seem to think I am arguing a point in which I am not. I never said the airplane sees friction. My point, simply was, if your 172 looses it's engine, it becomes a glider, with a very finite amount of energy available. Unless you are saying that you are better off with a windmilling prop, then your best chance to maximize your glide is with the prop stopped.
That is all.
tgrayson
New Member
You said
I was asking you to show how that can affect the pressure differences around the airplane. If you cannot, then you should leave it out of the drag contribution.
TUCKnTRUCK
That guy
it can not affect the pressure differences around the airplane. I only used frictional losses because you did. My argument is based on the energy available/used by a gliding aircraft, in which case, anything that converts energy, ie, friction, is valid...
I am not arguing that a windmilling prop has more or less drag than a feathered prop, because it does. Yes, I know that the dynamic drag of a prop is affected by the aoa.
Once again, energy, not drag is what I am discussing, and for your average ppl, makes more sense. I have no idea why you want to argue this, but you can have the top of the irrelevent information hill... I really have no need to go round and round with you trying to tell me that my point is not actually my point.
You have already well covered the drag aspects of the orriginal question, and I was not atempting to add to or change what you discussed. If you look, I tied to a real world event in which we stopped the prop to increase our max. Glide, a scenario where we are worried obout total energy, not just the induced drag on a windmilling prop. I agree with your breakdown of the drag forces of a windmilling prop, which becomes a sorce of energy loss when you are trying to minimize your energy loss during glide
Good point, I should not have overgeneralized on that.
That said, I'll take a PT6 over a "Garrett grenade" any day of the week!
I remember when Atlantis took delivery of the first new Jetstream 31 back in 1983. They went around the system showing it off and at every stop, they had to go out and rotate the damn props every few minutes so the shafts wouldn't heat soak.
Well, they forgot to do it at one stop. Brand new airplane, two trashed engines.
