Windmilling propellers
 

I cannot convince a non-aircrew colleague of mine that on large supercharged piston engines with constant-speed-propellers, if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm with accompanying boost (manifold pressure) and oil pressure. The only indication of the engine failure -if you missed the initial hiccup of parameters - would be the falling cylinder-head or coolant temperature and yaw and decay of airspeed.
Can anyone point me in the direction of getting written confirmation of this ???

hi
this stuff is complicated, you cant simply find it on wikepedia and so on, I think you should send an email to an aviation authority and get some confirmation
I agree with you, I know its true through my training

Have you considered the control range of the pitch mechanism ?

Unless you have an autofeather, autocoarsen, NTS, etc system operating in cruise flight, you are more likely to end up with a significant drag load on the failed side rather than, as you appear to be suggesting, a magical low drag system ? Typical end result is yaw, roll, and a really good view of the ground below - all of which probably might alert the pilot to an impending problem.

You wont find a written confirmation because the statement is complete wrong.The prop has to go to feather position otherwise the drag will
be too high.The engine will not turn anymore-means no more indications.

Like so many ground instructors have advised: RTFQ, people.

The poster wants info to support his statement that this type of engine would continue to rotate at the selected RPM with a supercharger providing Manifold pressure, etc.

His point: the engine will spin at the same RPM, but you'll get yaw and drag so watch out for the other indications.

(I'm not a piston guy, so I'm not one to judge the veracity of his statements.)

Ok I read the question :)and sorry no I can not point you to source where you can find information on this. But I think you are getting things muddled up.
If an engine would fail due to fuel starvation, the engine`s power output would start to get less there for the prop governor will decrease pith from coarse pitch to fine pitch to maintain RPM.(reducing the force it takes to turn the propeller) just like when you reduce power during descent/approach. You can even hear the the propellers running to fine pitch (change in sound and vibration). Secondly no piston engine will keep on windmilling due to the internal forces of turning the engine (try turning a big six cylinder engine on the ground...) It might be different for turbo prop engine's but I have no experience with these.

And when the engine is not working no supercharger will work or turbocharger.
And Boost Indication will show 0 Boost on ground if at sea level and ISA(When engine is stopped). So inflight you would find that it would indicate the actual air pressure(when engine is stopped) there for being less then 0 Boost. Normal Cruise power setting on my Airplane being around -0,5 to +1 Boost. So yes you would notice it.
And as the engine is not working Oil pressure would be 0psi, but Oil temp would only slowly decrease as well as Cylinder heat temperature. But they would initially stay the same.
And last but not least as John has said you would notice the failure as the props being in fine pitch would mean your really creating a lot of drag. Therefore creating yaw. Hopes this helps:OK:

The key is to follow the prop response to torque from the motor (be it turbine or piston).

Constant speed propeller systems regulate a positive torque output from the motor and convert that torque into a regulated speed. Any increase in torque results in an increase in thrust by virtue or regulating a specified rpm limit with the prop control.

In your scenerio an engine is now attempting to regulate a negative torque (not a reduction in torque). In the case of a negative torque the engines prop control responds by attempting to increase prop rpm. HOWEVER the attempt to increase rpm is in relation to a positive torque applicaiton, since you now have a negative torque deviation the actual result is a reduction in rpm since there is now a negative torque being applied and the blades are commanded to a flatter pitch rather than a higher pitch which would be required to increase rpms...

Since your scenerio has resulted in a negative torque deviation, the prop (by virute of its design to regulate positive torque) will attempt to increase rpm by commanding a reduction in blade angle in relation to the relative wind. This reduction in blade angle will therefore reduce engine rpm with a significant increase in drag. In a negative torque situation the blade angle will always be the inverse of what is required to actually increase the rotational speed again because the system is simply designed to respond to torque from the engine.

In cruise, if you were to reduce the torque or in this case the manifold pressure prop rpms will rise despite the response to a reduction in torque, however it is a REDUCTION in positive torque being applied. As the MP is reduced the blade pitch will gradually return to a flatter pitch to compensate for the reduction in available torque. Eventually and as somebody has already mentioned the blades mechanical limits will be reached and the props ability to compensate for rotational rpm will result in a gradual reduction in engine rpm. If airspeed is slow enough a windmilling prop will cease to rotate, however the extreme rise in drag as a result of the flat blade pitch will more than likely cause a loss of directional control.

Because of the requirement to regulate positive torque, multi engine prop systems have to be fitted with a feather position that permits an expanded range of movement relative to blade angle past a flat pitch detent. This allows the prop to move from the flat blade pitch detent to a pitch that follows a rotational direction that reflects a negative torque angle until the blade is positioned in a low drag position relative to the airplanes relative wind.

A feathering mechanism requires a spring that forces the blades into the negative torque position, again there exists no ability for the hydraulic forces to act negatively upon the props internal mechanisms. The springs will force the prop beyond the limits of the hydraulic system thus feathering the prop and greatly reducing the resulting drag.

It has obviously been a long time since anyone here flew pistons! :8

virgo isn't suggesting this at all, john - merely pointing out that a constant speed prop is just that. The control range of the pitch is relevant however.

The prop is feathered by the pilot. The question is about what happens before the prop is feathered (and the engine is thus still turning.)

Sorry,Micky - the air we fly through can provide a lot of force (enough to lift the entire aircraft into the air ;) ). All piston engines keep windmilling after a failure (discounting mechanical lock-up) until the pilot feathers the prop.
Superchargers are directly geared to the drive train. If the engine is windmilling, the supercharger is turning, and thus doing it's job to compress the intake air. Turbochargers are powered by the extra energy in the exhaust, if the fire isn't burning, a turbocharger will stop working - but that isn't the question here.
After the engine has stopped (after being feathered by the pilot) yes, this is the case. While the engine is windmilling, the oil pump is still being driven, so oil pressure is still normal.

Sorry, absolutely incorrect. You are confusing torque sensing (for auto-feather systems in turbines) with prop governors. Prop governors sense prop RPM, and adjust the pitch to compensate. If the RPM drops, the governor commands a finer pitch - if the engine is running, this reduces prop angle of attack, and thus prop drag and the RPM increases back to the governed level. If the engine is not running, the RPM drops, the governor commands finer pitch, this increases angle of attack (relative wind is striking the front face of the prop in a windmilling situation) and the relative wind thus drives the prop faster.

So, back to the original question:

Supercharged engine, with a non-mechanical engine failure. Prop governor maintains prop RPM. Supercharger & oil pump is mechanically linked to the crankshaft so they still operate normally.

Indicated RPM, oil pressure & manifold pressure all indicate the same after the failure as they did before, and this will continue until the propeller gets to the fine pitch stop, the point of which depends on the airspeed of the aircraft.

The fire has gone out, so Cylinder Head Temp. reduces rather quickly, oil temperature reduces less quickly and (as the prop is now being driven by the airspeed) the aircraft yaws into the failed engine and the airspeed reduces. It gets a bit less noisy as well.

[anecdote: check captain checking new pilot on a Heron arrives in the cruise. Check Captain reaches over and pulls all four mixtures out to the same point - as a demonstration of "efficient" engine management. Pilot under check thinks something is wrong - so pushes mixtures back in - and with the surge of power realises that the Check Captain had shut down two engines without realising it!

[True story, that.]

When any engine (turbine or piston) are running and by definition of "running" (generating positive torque, regardless of rpm) said engine transmits that torque to the motors output shaft or in this case the prop.

Depending on the amount of torque being generated (regardless of rpm) the engine will continue to accelerate (as a result of torque being produced) until something limits that maximum rotational speed, in this case a a prop governor increases blade pitch thus providing an increase in thrust output and the resistance to rotational torque hence limiting rotational rpm.

A prop governor is ONLY designed to respond to POSITIVE TORQUE from the motor (turbine or piston) a governor RESPONDS to torque not rpm (You can't have rpm without torque). RPM is simply a byproduct of said torque and rpms are only used to determine where a governor is operating at in respect to applied torque.

If you want to debate how a governor functions, then yes internal pressures that regulate the hydraulic forces sent to a prop hub are a result of the governors intenal pumps rpm and therfore the resulting pressures generated by that hydraulic pump.

In a NEGATIVE TORQUE SITUATION the prop governor will respond counter to POSITIVE TORQUE and the prop and windmilling engine will reduce it's rotational rpm relative to the speed of the flow of air accross the props blades.

This is a critical aspect to the design of a prop because if the prop were designed to increase the blades angle in a negative torque situation, the engine and prop would overspeed and result in a catastrpphic runaway situation.

There are inertial devices that prevent a prop from going into a negative blade pitch in flight (Feather). These arm when an over-ride is activated in the prop control system thus allowing the prop to continue past a flat pitch angle to a feathered blade position and are moved their by way of a piston and spring that act against the blades internal positioning arms.

Prop governors work as a result of torque output. Torque output determines the rpm and the limits of said rpm are a result of a torque limit set by the pilot thru the governor.

RPM's are a result of torque, torque is not the result of rpms...its of them laws of physics kinda things ya know.

If the engine is not running (producing positive torque), the RPM drops due to the governor commanding finer pitch as a result of the negative torque condition, this decreases the angle of attack and the prop rpms are reduced.

Google article:
Google Image Result for http://www.hariguchi.org/flying/info/figs/csp3.jpg

Perhaps you should read your own link!
(emphasis mine)

Not correct, I'm afraid. The governor is there to govern the RPM - and that is what it does. reduce power on a fixed pitch prop, and the RPM drops. Reduce power on a constant-speed prop, and the RPM remains the same (the hint is in the name!).

Reduce power on a constant-speed prop, and the RPM remains the same (the hint is in the name!).

I am still concerned if folks don't include the "within the control range" caveat.

A BMEP gauge is the answer!

G'day ;)

It is not surprising that you cannot convince a non-aircrew colleague, because, your information is incorrect....in so far as the following engines are concerned:
Pratt&Whitney R2000, R2800CB16, R4360...CurtisWright R3350 turbocompound series.

Yes,the RPM remains the same (in your selected scenario), the oil pressure (usually) remains OK, however, the manifold pressure and BMEP are most definitely reduced when the fuel flow ceases due to fuel exhaustion.
And yes,I've flown all of the above types on a variety of aircraft, IE: DC4, DC6, DC7, 1649 Constellation, Stratocruiser (B377).

Thanks everyone for your inputs - even those who have answered questions that weren't asked !!!!!
(Checkboard obviously understands)
411A........Are those engines fitted with superchargers or turbo chargers ? (I did specify a "Supercharged" engine). Obviously if there's no exhaust the manifold pressure will drop to atmospheric pressure.
Not many British engines had BMEP indication.

RTFQ....

But that is not what he was asking....

And to refer to my link...


His question is "if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm"

The answer is:

No, the engine is producing a negative torque (windmilling) by virtue of the air passing accross the blades. A Governor is only designed to respond to POSITIVE TORQUE to achieve a selected rpm, since the torque is now a negative the result is a reduced blade angle to the flattest blade pitch position (Mechanical stop).

The question is not how a governor works, but how a prop responds in respect to the available torque with an engine failure.

And again, the answer is that the rpms will decrease as a result of negative torque and the governors response to that negative torque is reducing blade pitch resulting in lower rotational rpm from the flatter pitch and reduced blade angle. And again this is why the prop will need to be feathered since the drap rise with a flatter pitch will exceed the residual thrust available from the other engine and you WILL lose altitude ( loss of airspeed then lift, remember that equilibrium thing from your private pilot days?)

All have superchargers, some two speed.
One engine I specified has both a supercharger and a General Electric turbosupercharger (R4360) and another (the CurtisWright R3350 turbocompound engine) has both a two speed supercharger and three power recovery turbines (PRT's).
IF you want to learn more about these large complicated engines, visit AEHS Home and become a member.

The governor doesn't care where the torque comes from, it cares only about the RPM. In the situation of a windmilling prop, the net torque remains "positive" in the sense that you mean it, with a negative torque from the engine offset by a positive torque from the air forces on the prop.

The mechanism of the governor is the same: the blade pitch adjusts until the net torque manages to drive the prop at the set-point RPM. If the engine starts ingesting pure air, the torque from it will reduce, and (in principle) the pitch will become finer and finer until it is being driven at the set-point RPM by the air forces, or it hits the fine stop, in which case the prop has to slow down.

I find it hard to believe that the other engine on a twin will continue to produce enough power that sufficient airspeed will be maintained so as to windmill the prop on the dead one at a cruise RPM. But if you had 6 or 8 engines, I can quite believe that one could fail and the aircraft would settle at a lower speed with one prop, in effect, in reverse.

What I don't get is the significance of the supercharger in the original Q. Doesn't the same effect occur on a normally aspirated engine with a wide open throttle?

Largest supercharged piston I've flown is the Hercules 14 cyl. sleeve valve radial.
IIRC, to re-start in the air: RPM lever out of feather and press feathering button had it windmilling merrily away. Lot of push in that Q formula.
(There were some other things to actually get it to run - fuel and ignition would probably help :) )

Thanks bookworm. You're absolutely right about the operation of the propeller control unit (CSU)The significance of the supercharger is that if the engine rpm is maintained and the supercharger is engine driven (as compared to a being turbo-charged), the boost (manifold pressure) and charge temperature will be maintained.
So you've got a dead engine - causing drag - that maintains the same RPM, Boost, Charge temperature and oil pressure as the functioning engines.

If you were totally distracted at the time of the failure - setting up an attack on a submarine or dealing with another emergency (an electrical fire in the fuselage), the failure could be missed until the aircraft has slowed down.......which I agree on a twin would be pretty quickly but with another three engines it takes a bit longer.

411A..............my information is NOT incorrect, I've had it demonstrated to me, seen it and done it. Have you any experience of multi-engined British piston engined aircraft ?

Sorry, it is incorrect, in so far as the American piston engines that I described.
I've flown 'em all...not just seen 'demonstrated'.:rolleyes:

Negative...the Brits were very good with turbopropellor designs.
Pistons?
Second fiddle, by far, compared to the American designs.