Feathered vs Windmilling Prop

[PFGiardino]

N18040, I had specified "piston" because of exactly what you pointed out.

However, after reading what TGrayson has to say, I have new thoughts on the relationship between the engine drag and the windmilling prop. These thoughts are still in development, so do not take them as fact.

If an engine fails, the propeller will gradually lose RPM until it reaches a windmilling equilibrium. To bring the propeller from its powered RPM to a dead stop would take tremendous energy (specifically, the brake horsepower at which the engine was operating). Have you ever pulled the mixture in a single, pitched up, and waited for the prop to stop? I haven't gotten far enough to let it stop, but can tell you it would require a slow relative wind.

In other words, we aren't starting to windmill a propeller from zero RPM, which would require the use of forward energy to turn the blades and crank the engine (aka drag). We're simply letting the propeller slow down until it reaches that windmilling point, and since the crank/pistons were already turning, that windmilling will far impart less resistance than the zero-to-windmill scenario (static vs moving coefficients of friction). That resistance will involve much less drag than the effects of the aerodynamic events around the prop.

 

[sharkey]

I could see engine resistance adding some drag in a recip, but how would you explain that for a turbine such as a PT-6. It has very little resistance to rotation, so if that was the only thing giving more drag, then there would be a very small change in drag between having the prop windmilling and feathered.

You need to get away from the though of the engine's resistance adding to the drag the prop is producing. If anything it would reduce the drag of the windmilling prop by slowing its windmilling RPM. It's still a propeller but the AoA of the relative wind to the blade has reversed, hence the "negative thrust" The faster it spins the greater the negative thrust it produces just like the thrust produced by any fixed pitch prop.

 

[sharkey]

Have you ever pulled the mixture in a single, pitched up, and waited for the prop to stop? I haven't gotten far enough to let it stop, but can tell you it would require a slow relative wind.

Have you really pulled your mixture in a single and tried to slow your prop in flight?

 

[Crism]

You need to get away from the though of the engine's resistance adding to the drag the prop is producing. If anything it would reduce the drag of the windmilling prop by slowing its windmilling RPM. It's still a propeller but the AoA of the relative wind to the blade has reversed, hence the "negative thrust" The faster it spins the greater the negative thrust it produces just like the thrust produced by any fixed pitch prop.

At this point I'm just having a hard time picturing this. I'm at the point where I'm asking the question: If a prop gets below a certain RPM does it create reverse thrust? I'm guessing I'm missing the forward velocity of the airplane in this equation (otherwise the airplane would roll backwards on the ground when you added power).

I need an image of what the prop's pitch looks like when its windmilling compared to when it's producing thrust at whatever speed the airplane's going straight and level.

 

[sharkey]

At this point I'm just having a hard time picturing this. I'm at the point where I'm asking the question: If a prop gets below a certain RPM does it create reverse thrust? I'm guessing I'm missing the forward velocity of the airplane in this equation (otherwise the airplane would roll backwards on the ground when you added power).

I need an image of what the prop's pitch looks like when its windmilling compared to when it's producing thrust at whatever speed the airplane's going straight and level.

Below a certain RPM the AoA could go negative, that RPM would vary with the pitch of the prop and airspeed. It's the airplane's forward velocity that makes the AoA go negative. Did I help to obfuscate the question here?

 

[Crism]

Below a certain RPM the AoA could go negative, that RPM would vary with the pitch of the prop and airspeed. It's the airplane's forward velocity that makes the AoA go negative. Did I help to obfuscate the question here?

So lets relate this to something I've seen before. An approach to a landing in a Seminole.

You're approaching at...say...120kts. Vyse is 88. You need to reduce speed so you bring both throttles back further and further towards idle. Eventually you hit idle.

Now you don't obviously notice this when you're flying but are your propellers producing negative/reverse thrust at that point? Possibly known as the "chop and drop"? (Throttles back, induces reverse/negative thrust, slows the airplane down rapidly, reduces lift and downwash over the wings, drop occurs).

Because you have the forward speed of the airplane, this would be the reason for it. I'm sure there's some sort of fomula somewhere to describe this and the amount.

Same way goes in saying this: you're flying straight and level at 150kts, you bring the throttle or mixture back on ONE of the engines to idle/cutoff, the propeller is creating reverse/negative thrust. I know that part is true. We've already established that.

I just need a picture of the blade angle and AOA relationship to make this work. I've got a friend of mine who understands this whole thing and he's going to work on getting a picture.

 

[shdw]

At this point I'm just having a hard time picturing this. I'm at the point where I'm asking the question: If a prop gets below a certain RPM does it create reverse thrust? I'm guessing I'm missing the forward velocity of the airplane in this equation (otherwise the airplane would roll backwards on the ground when you added power).

I need an image of what the prop's pitch looks like when its windmilling compared to when it's producing thrust at whatever speed the airplane's going straight and level.

First, I think some of you missed this statement above:

Internal engine drag, since it makes the equilibrium RPM smaller, actually works to reduce drag, rather than increase it.

Are you familiar with encountering a downdraft in an aircraft? Do you know that when you encounter a downdraft the effective AOA of the wing decreases? The stronger the downdraft or the slower your airspeed, the larger the decrease in AOA.

Apply this to what you know about a propeller being a wing.

Consider the prop RPM as airspeed from our downdraft discussion and the aircrafts forward speed presents a constant down draft on the propeller. As the RPM decreases (the airspeed in our comparison) the effective AOA of the prop will decrease.

Hopefully this makes sense and didn't violate any physical laws.

 

[matman]

TGrayson, thank you for your correction. It seems I've taken another unverified former CFI bit of "wisdom" for granted with out doing my dilligence. That's good info, but I'm still working though the mental picture of the force acting in the same direction of rotation.

OP... listen to him.

The instructor was not wrong. The drag from an un feathered prop is comparable to sticking a piece of plywood out of the window. When it is feathered it is completely inline with the relative wind, making it streamline.

 

[sharkey]

So lets relate this to something I've seen before. An approach to a landing in a Seminole.

You're approaching at...say...120kts. Vyse is 88. You need to reduce speed so you bring both throttles back further and further towards idle. Eventually you hit idle.

Now you don't obviously notice this when you're flying but are your propellers producing negative/reverse thrust at that point? Possibly known as the "chop and drop"? (Throttles back, induces reverse/negative thrust, slows the airplane down rapidly, reduces lift and downwash over the wings, drop occurs).

Because you have the forward speed of the airplane, this would be the reason for it. I'm sure there's some sort of fomula somewhere to describe this and the amount.

Same way goes in saying this: you're flying straight and level at 150kts, you bring the throttle or mixture back on ONE of the engines to idle/cutoff, the propeller is creating reverse/negative thrust. I know that part is true. We've already established that.

I just need a picture of the blade angle and AOA relationship to make this work. I've got a friend of mine who understands this whole thing and he's going to work on getting a picture.

I wanted to give some real world examples but I wasn't sure if the prop would actually make reverse thrust. If it were to, though, these two situations would be likely scenarios and I believe it does. But I'm not positive.

Hopefully your friend can find a picture or you can get your hands on a prop model to help visualize. It's like tying your shoe; way harder to talk through it than to show someone.

 

[tgrayson]

I need an image of what the prop's pitch looks like when its windmilling compared to when it's producing thrust at whatever speed the airplane's going straight and level.

Here's one that I posted in the thread two years ago. It's not quite what you asked for, but it's close. It shows, hopefully, how the rotation of the blade and the forward velocity combine to determine the AoA on the blade. The length of the vector arrows is proportional to the velocity of that particular component.

 

[tgrayson]

You're approaching at...say...120kts. Vyse is 88. You need to reduce speed so you bring both throttles back further and further towards idle. Eventually you hit idle. Now you don't obviously notice this when you're flying but are your propellers producing negative/reverse thrust at that point?

This is generally regarded as true (for whatever that's worth); however, it would depend on what the idle setting is. It's certainly possible that idle is set high enough to provide some positive thrust at some airspeed, but whether this is likely, I don't know.

 

[SHayes]

Hmmm, verbatim cut and paste from the Rotorcraft Flying Handbook.

And that is a problem why?????

 

[chuckles1225]

I just need a picture of the blade angle and AOA relationship to make this work.

I put together an animated power point that makes visualizing the whole aerodynamic process a lot easier. If you do download it make sure you actually view the slide show and not just look at the slides individually in the editing mode, as they won't make any sense.

http://www.mediafire.com/?ndmdmrgjmzq

 

[shdw]

And that is a problem why?????

Because you represent it here as your own work. Try mentioning where you get it from, unless you yourself wrote it.

 

[SHayes]

Because you represent it here as your own work. Try mentioning where you get it from, unless you yourself wrote it.

HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA.....you are WHACKED!!!!! Did you represent yourself as the president of the US just because you didn't say you weren't?

Nowhere in my post did I take credit for that...it is knowledge. Knowledge to be shared by all, who cares where it came from.

BTW...nice website...did you borrow any of that information from the FARs without saying so????????

 

[PFGiardino]

Have you really pulled your mixture in a single and tried to slow your prop in flight?

I should have been more clear; no. I wouldn't try to stop or slow the prop. Even though the engine would still be able to be cranked over via starter, that's beyond reasonable safety limits and beyond what is necessary for training purposes.

I have, however, shown students than pulling the mixture out has the same practical effect (feel/sound/thrust), as the throttle. During normal flight maneuvers, the RPM doesn't go far lower than a usual idle. It is my experience that it would take drastic measure to stop it (power on stall attitude). I like to quell the "scary red handle" phenomenon. Have you read any of John Deakin's work on Avweb? He writes some very informative articles about engine management techniques and performance, including one article about that very subject.

[The above paragraph concerns single engine, normally aspirated trainers.]

As for the issue at hand, I've really learned a lot. It seems that the Rotor Flying Handbook would be a good read, even for a fixed-wing pilot like myself, as it helps one visualize these topics a bit better.

I won't buy the plywood theory.

 

[PFGiardino]

double post, apologies.

 

[MikeD]

You are correct except that drag is not the only thing being produced by free spinning rotor blades. Lift and more importantly ENERGY is being produced also...unlike a windmilling prop on an airplane.

During vertical autorotation, the rotor disc is divided into three regions—the driven region, the driving region, and the stall region. The size of these regions vary with the blade pitch, rate of descent, and rotor rpm. When changing autorotative rpm, blade pitch, or rate of descent, the size of the regions change in relation to each other.

The driven region, also called the propeller region, is the region at the end of the blades. Normally, it consists of about 30 percent of the radius. It is the driven region that produces the most drag. The overall result is a deceleration in the rotation of the blade.

The driving region, or autorotative region, normally lies between 25 to 70 percent of the blade radius, which produces the forces needed to turn the blades during autorotation. Total aerodynamic force in the driving region is inclined slightly forward of the axis of rotation, producing a continual acceleration force. This inclination supplies thrust, which tends to accelerate the rotation of the blade. Driving region size varies with blade pitch setting, rate of descent, and rotor rpm.

The inner 25 percent of the rotor blade is referred to as the stall region and operates above its maximum angle of attack (stall angle) causing drag which tends to slow rotation of the blade. A constant rotor rpm is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the deceleration forces from the driven and stall regions.

By controlling the size of the driving region, the pilot can adjust autorotative rpm. For example, if the collective pitch is raised, the pitch angle increases in all regions. This causes the point of equilibrium to move inboard along the blade’s span, thus increasing the size of the driven region. The stall region also becomes larger while the driving region becomes smaller. Reducing the size of the driving region causes the acceleration force of the driving region and rpm to decrease.

On the first sentence quoted, you quoted me as something I didn't even say. And the above is correct.

 

[tgrayson]

And the above is correct.

Except for the one sentence he actually authored himself.....a windmilling rotor does NOT create energy. In fact, no process creates energy, it only changes it from one form to another. Rather, a windmilling rotor (and prop) steals energy from the airframe and channels it into the atmosphere.

 

[MikeD]

Except for the one sentence he actually authored himself.....a windmilling rotor does NOT create energy. In fact, no process creates energy, it only changes it from one form to another. Rather, a windmilling rotor (and prop) steals energy from the airframe and channels it into the atmosphere.

Don't get me wrong, I have my own issues with some of the ways the FAA Rotorcraft Handbook presents some concepts. But to be specific, potential is indeed changed to kinetic and back, and the managing the driving region of the rotor system is key to an auto. It's weak wording on their part..