Does a windmilling propeller create more drag than a stopped propeller in an engine out scenario?

Question

For this question lets assume that we are talking about a fixed pitch propeller and not a variable pitch propeller. I do understand that variable pitch props should be feathered during an engine failure.

The main rationale for this question is this statement that I found on the Pilots of America forum:

I've been doing some research on something I hard from my old chief CFI a couple years back, but I'm not finding much on it. He is the only person I've heard this from, and any CFI, DPE, ect. I've talked about this to since have not heard of it. That being that when you have an engine failure more than several thousand ft. with a non-full-feathering prop, that you should pitch up and decrease airspeed, sometimes to an intentional stall, to stop the rotation of the prop/engine. (Of course, only after you have attempted to restart.) The reason being that you will see a greater gain in glide distance by reducing the drag created by forcing thee air to turn over the engine. He claimed that while you would most likely be off of Vl/d for a short amount of time, thus not gliding in the most efficient manner, that is outweighed by the decrease in drag. (As I said, do this only with several thousand feet and plenty of time.) He claimed that in the case of something such as a 172, doing this procedure at, say 10,000ft, you would gain several more miles of glide distance.

So what I am asking for is if indeed a windmilling prop has more drag than a stopped prop and if so would it be beneficial to follow the advice in the pilot forum?

Answer 1

I would not follow the advice in the forum. While the reasoning that a windmilling prop does create more drag is sound, I have seen no empirical evidence that says how much it actually translates to in Feet Per Minute. The only studies I have seen have been inconclusive on the subject, and say there's a number of factors that you can't really control in that situation. The prop isn't that big compared to the airplane itself, so if a spinning one has 30% more drag (just a guess, as I said I've seen no actual numbers) it isn't going to make you plummet out of the sky.

What I would recommend is following the engine failure procedure in the POH of your particular aircraft. The advice to slow the airplane to try and get the prop to stop has a number of flaws:

• Distraction from critical tasks: Slowing the airplane until the prop stops is taking your focus away from finding a field to land in, ensuring your passengers are briefed and strapped in, making a mayday call and attempting to restart the engine. You'll be busy enough!
• Low airspeed danger: Slowing enough to stop the prop may require a very low airspeed, and you could have a stall/spin excursion, which is far more dangerous than a glide landing
• Drag curve: flying very slow to get the prop to stop will put you below best glide speed, which is where you get your best glide ratio. Below best glide speed you start to develop a high rate of descent, so by the time you get the prop stopped you have lost hundreds of extra feet. You could lose more altitude than you would potentially save, and waste valuable time doing it
• The chances are good that you couldn't change the prop behavior anyway: if your crankshaft has broken the prop's going to spin no matter what you do, if you've thrown a rod it's going to stop and you couldn't get it to windmill if you flew at mach 1

Doing some math puts this into perspective. Say an airplane's engine-out rate of descent with a spinning prop is 1000, and with a still prop it's 900 fpm. In a glide from 3000ft that buys you an extra 20 seconds, but it probably takes a lot more than 20 seconds to slow enough to stop the prop and then regain a stable airspeed.

Flying very slow to stop the prop is a potentially dangerous distraction, just set best glide speed and if the prop stops its a bonus.

Answer 2

The answer: it depends. Generally, a stopped propeller will be stalled, and so creating little drag. A windmilling propeller, conversely, usually will not be stalled, will be extracting energy from the airstream, and using that to turn the engine (which typically takes a moderate amount of energy), which will increase sink rate.

However, should you stop the propeller? That's a much harder question. Relevant questions are: how slow must you go to stop the propeller? Can you stop the propeller? If it requires dropping below minimum sink speed for an extended period, it is probably a bad idea, as sink increases rapidly with decreasing speed (you're on "the backside of the power curve" any time you're flying below minimum sink speed).

On some antique aircraft, it is or was reputed to be a requirement in some clubs that pilots be able to stop the engine and prop and restart the engine in flight before being granted pilot privileges for, say, the club Tiger Moth (these are generally hand-started). It is widely claimed that stopping the prop does extend the glide noticeably. The Tiger Moth also has a large, slow-turning wooden prop, and a fairly low stall speed.

As for whether I would try this in an emergency? Almost certainly not; if it's the POH, then sure... but I've never seen it in the POH of any single engine aircraft I've ever flown. And if I'm flying engine out, I've probably got worse things to worry about (unless I'm flying my more-usual gliders).

Answer 3

A windmilling propeller creates much more drag, by an order of magnitude at least. The obvious example is an autogyro or helicopter in autorotation, which drops like a stone if the rotor stopped.

Drag is a function of blade area for a stopped rotor and a function of disc area for a windmilling one. The difference is most pronounced for large rotors with few blades and almost nonexistent for turbofans. This answer gives some numbers:

If we compute the lift component per area of a vertically autorotating rotor, it is comparable to a coefficient of 1.1 to 1.2 referenced to the rotor area. According to this source, a flat plate has a drag coefficient of 1.28 and a parachute has one of 1.4. So in vertical descent, the auto-rotating rotor is almost as good as a parachute of the same area.

If you are high enough when the engine stops, it's a good idea to slow down a little so that the propeller stops windmilling. Once it has stopped, you can accelerate back to normal. The propeller will usually not start windmilling again, as it's drag, and thus available power, is now much lower.

Answer 4

The drag is significantly more for a windmilling propeller.

Both aerodynamic drag and energy lost in the engine contribute. I estimate you will sink at least 200 FPM faster if you let the prop windmill.

For aerodynamic drag, it's impossible to convert this into a feet per minute estimate because it varies so much with the design of the propeller and the overall drag of the airplane. But you can see here: Aerodynamics for Naval Aviators, 1965 (page 149 in the page numbers, or 167 in the PDF) that the drag can increase significantly.

A typical fixed-pitch prop has a pitch of around 15 degrees (more details at bottom). A constant-speed prop which has lost oil pressure, and is not automatically feathering, is probably more like 5 degrees. (Feathering props are used on multi-engine planes to reduce drag if one engine fails. Non-feathering props are used on single-engine planes so if the pitch control fails but the engine still works, you don't lose all your power). Overall, the actual propeller parasitic (simple aerodynamic) drag increases by up to a factor of 3. Not orders of magnitude, but it's significant. Since there's no way for me to estimate how much of the total drag is due to the propeller, all I can say is that this is probably noticeable. However, if you have a cruise prop, or an adjustable prop set to high pitch, it's possible that it works out close to even, as above 22 degrees, the windmilling prop actually has less drag.

But then there's the extra factor of the drag created by the engine, which is probably much more significant. It's possible to come up with a reasonable ballpark figure. Estimation and highschool physics are required.

An airplane without power loses potential energy, in the form of altitude, to drag. Since the airplane's speed doesn't change, its kinetic energy doesn't either and only potential energy need be considered. We calculate how quickly energy is drained out of the airplane by the spinning engine.

Work is the amount of energy that is transferred from one place to another, and power is the amount of work over time. The formula for work (in a rotational system, such as an engine) is torque * theta, where theta is the total angular distance rotated. Power (watts) is expressed in joules per second, although here I'll figure power in joules/minute because our other time units are also in minutes. The joule, of course, is the unit of both work and energy.

Assume an airplane weighing 1000kg flying at 1500 meters AGL. Its potential energy is:

1000 kg * 1500 meters * 9.8 (gravity) = 14,700,000 J (14.7 mJ)

Assuming a propeller windmilling at 200 RPM, the angular velocity is:

2pi radians / revolution * 200 revolutions / minute = ~1260 radians / minute

I estimate the torque, given in newton-meters, is somewhere between 50 and 500 Nm, tending toward the high side. 50 is from my personal experience turning a propeller at slow speeds by hand, it's about that much force; but in the air I think this is extremely optimistic. 500 is a higher estimate, justified as follows.

Single-engine planes such as the Cessna 172 frequently have a 180HP engine. The propeller normally has enough air-grabbing ability to transfer that 180HP into the air at engine redline of ~2700 RPM.

Converting HP to joules / minute (1 watt = 1 joule/second):

180 HP * (746 watts / HP) * (60 seconds / minute) = 8057 kJ / minute

Assuming the propeller efficiency is relatively constant with RPM, you can convert propeller power delivery capability linearly with RPM:

8057 kJ / minute * (200 RPM / 2700 RPM) = 596 kJ / minute

So the propeller should be able to transfer about 600 kJ / minute back into the engine. This is in the ballpark of my estimate of 500. However, as not all the normal engine power goes into the propeller (due to mechanical losses and engine powered accessories) the estimate of 500 seems to be pretty close, and I'm sticking with it for sake of simpler math. This estimate is pretty seat of the pants - lots of error sources, like variations in propeller efficiency with RPM and being driven in reverse - are ignored. But if the propeller is less efficient, it dissipates more power - so even my high estimate might be too low.

Back to the engine failure. The power dissipated by the engine is therefore:

1260 * (50 to 500 or you pick) = 63 kJ to 630 kJ per minute

A Cessna 152 has a sink rate of 725 fpm when flying at best glide with stopped propeller (best glide speeds are normally specified with stopped prop); The Cessna 172 is closer to the mass I am using, and has similar glide performance, so I'll just use that same number. This is an estimated calculation, anyway. At 725 fpm descent rate (221 meters per minute) it normally takes 6.78 minutes to bleed off that 1500 meters of altitude you started with. Dividing potential energy by time:

14,700,000 / 6.78 = 2.168 MJ/min energy loss

At the low estimate (probably reasonable only for an engine that suffered total compression loss but no other damage), losing an additional 63 kJ/min only increases your sink rate by

(2.168 + .063) / 2.168 = 1.029

about 3%, or from 725 fpm to 746 fpm. You might not even notice this on the instruments, although if you snag on a power line at the last second as a result, you'll certainly notice that. However, at the high estimate, losing an additional 630 kJ/min would increase your sink rate by:

(2.168 + .630) / 2.168 = 1.29

29%, or from 725 fpm to 935 fpm. That's very significant. And this does not even include the extra aerodynamic drag from the spinning prop. It is only due to the energy lost in the engine.

So, in conclusion: If power fails, stop the prop.

There is, however, one final point. If you lost power because of a mechanical failure, it's very possible that the propeller will stop all on its own due to whatever damage caused the power loss. However, running out of fuel is the most common cause of in-flight power loss. If you run out of fuel, the prop will probably keep spinning unless you stop it yourself.

• Pitch angle in degrees is different from the way pitch is normally described, which is measured in inches, where something like 76"x60" would be typical. You can calculate pitch angle based on the propeller measurements in inches using the formula for helix angle. If you do, remember propeller pitch is specified at 75% of the diameter of the blade, rather than 100% as in the math textbooks).

Answer 5

This paper provided by Erbureth provides empirical evidence to answer the question. The short answer is that sometimes a stopped propeller provided less drag and sometimes a windmilling one provided less drag. This actually makes a lot of sense to me as the drag should be related to the location of the stagnation pressure line on the prop.

A few observations from other answers:

• One of the things I always hear when this question is asked is that a rotating propeller turns into a disk and has more area. This is clearly wrong as the area of a propeller blade is always the same whether it is rotating or not.
• When a prop is feathered the prop stops windmilling and the drag is considerably less. This create the perception that the stopped propeller has much less drag but in reality it is due to the fact that the feathered prop has much less form drag. The fact that the prop is stopped is not the main reason.

Answer 6

The argument that the windmilling prop is slowing the plane down by extracting energy to turn a dead engine ignores the fact that a frozen prop tries to turn the whole plane. The control inputs to keep the plane level are probably very close, and the turning airfoil may actually be slightly less draggy, so what is going on here?

It is well known that props are airfoils, and turbulence created by one blade affects the other. In the windmilling case, the turbulence created by the propellor has a greater affect on that other airfoil, the wing.

This is mostly likely the major factor in loss of gliding distance. Important to realize the air that the plane is gliding through has no kinetic energy to extract, it's all in the kinetic and potential energy of the plane. But the windmilling prop does lower the efficiency of the wing in using the planes energy by creating more turbulence.

Answer 7

I have tested this multiple times in a C152. Starting at 10,000 feet and cutting off the fuel to the engine -applying nose up to hold altitude as long as possible (thereby slowing my forward speed) until stopping the propeller. Then gliding with the propeller stopped for quite a distance, passing over multiple airports in the process. A windmilling propeller left me short by a significant distance.

I know I was weird in my younger days, but I was flying!

Answer 8

It depends on the blade angle

Here's a picture from a very old report:

This is of course specific to the particular rotor and the flow condition at which it was tested, so the magnitude of the numbers on the y axis will be very different for different situations, but the general picture is likely similar for most propellers:

1. If the prop is feathered, that's always best (duh, we knew that).
2. Low blade angles are worst (also clear)
3. If it is at very low blade angles, then having it locked is best
4. In between: a spinning prop is actually better!

Number 3 and 4 deserve some explanation:

High blade angles

This one is fairly obvious: In completely feathered state (and assuming the blades had no twist), there is no force on the blades except a bit of drag caused by the prop airfoils at zero incidence. Of course the blades are twisted, so they are pushing air around circumferentially a little.

If the blade angle is reduced a bit on a windmilling rotor, things start moving: Imagine for a moment a feathered rotor with no engine to slow it down. If it's perfectly feathered, it won't move. If the blade angle is then reduced just a bit, the blades all start pushing air in clockwise direction, which sets the rotor in motion in anti-clockwise direction, which reduces the amount by which the air is turned again, until there is an equilibrium speed, at which the outer part of each blade is actually moving so fast that it pushes some air anti-clockwise (i.e. takes it along), while the slower-moving inner part is still pushing it clockwise. All turning angles are really small, though, and so are the resulting forces.

By comparison, a stuck rotor does not move and starts creating downwind swirl, that is: It adds some clockwise or motion to the air that's coming in, and that creates drag, of course, in the same way that a wing creating lift causes drag.

A windmilling rotor with an engine to turn is going to be somewhere in between those two cases, depending on how much torque the engine needs to be turned, and how much torque the prop is able to produce.

In this state, slowing down the prop is actually not helping!

Reducing blade angle

If we decrease the blade angle from this point, the locked propeller starts producing increasing amounts of swirl, which increases its drag about linearly with blade angle (like the lift on a wing!). Eventually, flow on parts of the blade start separating, and the force no longer increases (exactly like increasing angle of attack on a completely stalled wing also doesn't really do much). In the meantime, a windmilling prop whose blade angle is successively reduced finds increasingly high rotation speeds at which the torque from aerodynamic forces which drive it and the ones which slow it down are in equilibrium with the torque needed to turn the engine. In the extreme case of a free-spinning prop, this will still align the blades roughly with the inflow, and cause a comparatively small axial force, since it's still just pushing some flow clockwise and some anti-clockwise. However, since there is an engine slowing things down, and higher rotation speeds require higher torque, and since the prop is spinning ever faster (higher stagnation pressure), the drag it produces does increase. Roughly quadratic with the blade angle (like the drag of a wing at increasing incidence!)

Propellers at low blade angles

And now we reach the point where the spinning rotor actually starts turning air so far that another mechanism comes into play:

When a helicopter pilot auto-rotates, they set the collective to low blade angles, and the graph above agrees that they should. The reason: the lower the blade angle, the larger the "driving" region of the blades, the more torque they develop, and the higher the aerodynamic force on the outer part of the blades, which eventually does not just add some anti-clockwise momentum to the flow, but actually turns it against the inflow direction (see Wikipedia for some details). Effectively, the outer parts of the blades generate "lift" (that is: a resulting aerodynamic force) which has a significant component against the direction of flight: The blade profile is almost aligned with the disk plane, the rear-facing side of the blades is the suction side, and the flow on the blade is turned by almost 90°

Compared to that, the flow over the blades of a locked propeller simply separates. That leaves a big old turbulent wake behind and causes a good deal of drag, but this is way less effective at slowing down the aircraft than keeping much of the flow attached and turning it as hard as possible.

However, it should not become quite as bad as a helicopter rotor in autorotation, because the helicopter pilot is using the collective to set the blade angle such that they get close to the highest sustainable (!) force. I.e. whatever RPM gets allows them to produce the largest possible force at which the rotor can maintain its own rotation. If they relax a little, the rotor accelerates but the helicopter does, too. If they pull too much, the helicopter slows down, and shortly so does the rotor, and then the force reduces. It would be a great coincidence if the fixed propeller on any given aircraft was installed at precisely the "optimum" pitch for autorotation. Actually, that's probably something which any propeller designer tries to stay away from.

What it means:

A windmilling fixed-pitch propeller can indeed be much worse than a non-rotating one. However, that depends a lot on the blade pitch angle (and the blade twist, because blade angle varies along the blade...), on whether it is being slowed down by the engine, and by how much...

If the blade angle is reasonably high, then trying to slow the prop down is making things worse. If blade angle is low enough to produce autorotation-like effects, then slowing it down might help (but might also be dangerous, for reasons given by others here).

How to tell what a given aircraft prefers

I don't fly aircraft, so I cannot comment on flight procedure etc., and I fully expect that proper test pilots have something to say about how easy it may be to get yourself killed (or not), but in general: For any given aircraft, the ideal glide procedure is found by turning the engine in question off and finding whatever airspeed provides the largest sustained glide number -- that is: you'd need to hold some airspeed in question until the propeller speed has settled and determine the glide angle, then repeat that for a slightly smaller airspeed, etc...

Done!

Answer 9

A windmilling prop is allowing the slipstream to push it around. A stopped one is fighting the slipstream - pushing/deflecting it into a spiral. The reaction from that is what tries to turn the plane, as Robert DiGiovanni says. And it is doing work, because it's applying a force to the air, which moves as a result of it.

So I'd expect less drag from windmilling. Its like freewheeling on a bike versus having a locked-up wheel.

I'm assuming same AoA in both cases, i.e that stopped does not imply feathered. Some people appear to have assumed the opposite, to which I'm curious: how would you feather a fixed pitch prop, which is what is stated in OP.

Answer 10

A stopped propeller is stalled and so creates a small amount of torque to "spin" the aircraft. It's drag is small as the surface blade area is small. A windmilling propeller will not stall, so the energy extracted could be much more. It depends if the prop is free-wheeling or actually turning the engine.

Answer 11

I would spend as long as possible trying to get the engine going again while looking for a place to land. Keeping the engine turning will maximise the chances of a restart while you try different fuel selections, mixture settings and magneto switch positions.