Drag: Windmilling vs Dead Prop
 

Lots of good discussions and arguing on various sites over this. But, oddly, not much good, distilled data/info.

Scenario: two single-engine prop planes. Engine quit and seizes in one. Engines quits but prop rotates in the other. Everything else equal. Which has more drag?

Without generating more argument and opinion (yeah... like that will happen), can you post a link to a definitive source for an answer? And that will explain it without formulas that take up 7 pages. I'd like something that can be understood by a History major.

Well it probably isn't exactly what you're looking for, but we secure the failed engine in 2+ engine for a reason. Check out Jeppesen Multi-Engine Ch. 3 Section B and Aerodynamics for Naval Aviators P. 376. A windmilling prop is like a barn door out there when I comes to parasite drag.


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No such thing as a free lunch, and a prop is an airfoil just like a wing...

A windmilling prop is a essentially...a windmill. It's extracting energy from the slipstream and turning it into compression heat in the pistons and internal moving parts friction in the motor. The slipstream energy being consumed by the turning (but dead) motor has to come from somewhere, and that somewhere is the kinetic energy of the airplane. You can trade altitude (potential energy) for kinetic energy to maintain airspeed, and that would be second-nature for a pilot. So net result is higher descent rate.

A non-rotating prop creates some drag, but it's mainly form drag since the prop blade must have significant flow over the airfoil to transfer energy to or from the slipstream. The blade is stalled, and stalled airfoils do not transfer much energy or generate much lift. A prop blade rotating at speed with airflow along the chord is designed to be very efficient at energy transfer but only if the air is flowing the right direction.

More in depth than that will require some math...

I understand what you're stating. I've also read some places where they show/believe the difference is negligible... and in some cases, opposite of what you say, depending on prop length and chord.
Again, I don't know the answer. But I'm hoping someone knows of a link to a solid source.

It takes energy to turn the prop, crankshaft, pistons (fighting compression in the cylinders!), valves, fuel pump, generator belt, vacuum pump. Energy is not free, it has to come from somewhere. If the prop is spinning, all this other crap is spinning too. The AOA on this slowly spinning prop is probably significantly increased due to this load, greatly increasing the drag. Remember, AOA on a prop is relative to the blade's chord line and flight path, which is a function of it spinning around in a circle relative to it's airfoil chord line-generally a lot faster mph than the airplane is moving forward, and the airplane's path (lots of people think it's just the airplane's flight path).

The difference isn't negligible at all. Anyone who has done any multi engine training in a propeller equipped multi engine aircraft understands it immediately, and it's quite visible in the performance of the aircraft. In light twins, it can mean the difference between maintaining altitude and a descent without the ability to maintain altitude. It also make a significant difference in controllability and minimum controllable airspeeds.

A windmilling propeller is responsible for more drag than the equivalent flat plate area of the prop disc. That sounds counterintuitive, but is true. If one doesn't believe the ability of a propeller to cause drag in flight, fly an airplane that can do beta in flight and see what happens. I used to fly a single engine airplane that was designed with a radial engine, but which had been converted to turbines (PZL Dromader), and the installation was rigged such that retarding to idle in flight would slow the aircraft so quickly it would throw one forward in the shoulder harness. One could follow through with the stick briskly until the vertical to prevent a loss of control or stall. It was dramatic.

Windmilling vs. stopped (or feathered) also makes a dramatic difference. Rudder input required in a multi engine prop aircraft is lessened substantially upon feathering (or stoppage of the prop, such as a seizure). The windmilling propeller absorbs considerable energy, driving gearing, accessories, etc, when the engine is no longer driving the propeller.

Prior to a windmilling state, when the engine is imparting energy to the propeller and the propeller is doing work, the combination produces some degree of measurable thrust. In a windmilling state, the propeller isn't receiving torque from the engine any longer, but is still receiving energy from a source, which is the slipstream, and the process works in reverse. Whereas with the engine driving the propeller, that amount of torque and energy went into moving airflow and creating thrust, the slipstream energy now becomes all drag as it drives the prop and that drag is imparted to the prop, shank, driveshaft or crankshaft, and ultimately engine mount, wing and airframe. In effect, the slipstream becomes the engine driving the propeller, and works against the airplane instead of for it.

A stopped propeller experiences a certain amount of flat plate drag area or form drag, but much, much less than a windmilling prop.

This. Think about it for a bit.

Also, difference between putting your car in gear and putting it in neutral while coasting down a hill :)

I believe the correct answer is it depends, if both aircraft have fixed pitch cruising props say set at 18 degrees, like something like a 172, I'd say the correct answer is there ain't much difference...

if both aircraft are constant speed singles a wind milling prop set to low rpm, resulting in blade angle angle of 30 degrees the wind milling prop, would probably glide a little further than other aircraft with a stuck prop which has moved to flat pitch.

A propeller being dragged through the air at high rpm that results in flat pitch will result in far more drag by far.

I think the pitch that makes the difference is about 15 degrees.

This tallies with what I have observed in flight. The resistance movement of engine/gearbox is a bit of a red herring, and doesn't stand up to scrutiny.

I can't find anything that explains this in simple terms however there a some papers on this, but they are not easy reads.

A stationary prop creates incidental form drag.

A turning prop extracts energy from the slipstream. Its definitely going to create more drag. How much drag? Try turning your car motor at 120 rpm by hand and see how much energy that takes. OBTW, the turning prop still has its form drag in addition to the slipstream load. Rotation does not make the form drag vanish.

that's a red herring, the amount of torque to turn the engine over at say 120rpm could just as easily be applied to an engine that is stuck solid.

Page 149 of naval aviators, figure 2.19 has a good diagram that shows a wind milling propeller can produce substantially less drag than a stationary unfeathered one. Note that this is for a very course prop. The converse applies for a fine pitch prop.

This is perhaps the something 'definitive' the OP requested.

https://www.faa.gov/regulations_poli.../00-80t-80.pdf

You don't understand the text or the diagram very well at all, and apparently didn't read the text.

You don't understand drag or the energy absorbed by using the slipstream to turn a propeller, either, and no, it's nothing like "applying torque" to a frozen engine.

Go back and read Page 148 of your referenced text, Aerodynamics for Naval Aviators. It specifically discusses the necessity for feathering a propeller, and states, among other things:

It states that the parasite drag of a feathered propeller is "a relatively small contribution to the airplane total drag." More importantly, it goes on to state:

"At smaller blade angles near the flat pitch position, the drag added by the propeller is very large. At these small blade angles, the propeller windmilling at high RPM can create such a tremendous drag that the airplane may be uncontrollable."

It's for this reason that an overspeed propeller in aircraft such as the P-3 and C-130 pitchlock and the reason that the inability to feather an engine is a big problem, and also the reason for negative torque sensing to relieve drag even on an operative engine when in a windmilling state at low power settings. Do you understand this?

ANA, same paragraph, goes on to state "The propeller windmilling at high speed in the low range of blade angles can produce an increase in parasite drag which may be as great as the parasite drag of the basic airplane." That's not a red herring. Read it again.

Moreover, the same paragraph continues to state: "Thus, a propeller windmilling at high speed and small blade angle can produce an effective drag coefficient of the disc area which compares to that of a parachute canopy." Still a red herring?

Regarding aircraft control, the same paragraph states: "The drag and yawing moment caused by loss of power at high engine propeller speed is considerable and the transient yawing displacement of the aircraft may produce critical loads for the vertical tail. For this reason, automatic feathering may be a necessity rather than a luxury."

The correct answer is that a windmilling propeller produces more drag than a stopped propeller. You don't seem to understand the question.

Are you attempting to say that an aircraft with a fixed pitch "cruise "prop" produces the same drag as the same aircraft with the propeller stopped? Feathering isn't possible in a fixed pitch propeller, but stoppage of the prop is.

The windmilling RPM of the propeller depends upon it's blade angle, but also upon the airspeed. The higher the airspeed, the faster the windmilling RPM. You seem tied to the concept of a fixed pitch Cessna 172, which is a slow airplane anyway. If you don't understand the relationship, see what the RPM of the 172 is in a dive at idle at Vne vs. a descent at Vx or Vy for the same airplane; the faster the airflow through the windmilling propeller disc, the faster the RPM, and the greater the drag rise.

Your 172 at idle on the ground can achieve a prop RPM as low as 600-900 RPM, while in flight may seldom be seen below 1200-1500 RPM, even at low speeds such as final approach. Why? Airspeed. At higher speeds, there will be higher RPM's and there will be a greater drag rise from the windmilling propeller.

The drag on a windmilling propeller will always be higher than a feathered or stopped propeller and more than one type of drag exists. It's not just aerodynamic drag at the propeller blade, induced drag, but also increased parasitic drag as well as the drag of moving the propeller against the resistance of the engine. On that account, you don't seem to understand how significant the internal drag is in the engine, or that much of the operating power of the engine itself is used to overcome its own internal resistance. Once the engine is no longer driving the propeller and the slipstream is, all that internal resistance and the energy absorbed by the slipstream performing the work of moving the engine and propeller (and accessories) is all drag.

Altering the blade angle will make a slight difference, if you can alter the blade angle, but remember the discussion is about a windmilling propeller vs. a feathered propeller (or stationary prop for your fixed ptich that can't be feathered). As you move off the low pitch stops, you're moving toward a feathered position. If you're saying that a propeller closer to the feathered position produces less drag than one on the low pitch stops, you're correct (given a constant airspeed value), but this belies your point. In fact, you're making the opposite point; a windmilling propeller produces more drag. The amount of drag depends upon blade angle and RPM and airspeed (RPM in a windmilling state being a function of airspeed, for a given blade angle)...but a windmilling propeller will always produce more drag.

If you move away from your 172 to a higher performance aircraft operating at higher speeds (to which end Aerodynamics for Naval Aviators was addressed), the drag rise can be so significant as to render the aircraft uncontrollable...that's the difference between feathered and windmilling. You'll also recall above that ANA points out that the drag rise can be high enough to cause structural failure of the aircraft, particularly the vertical stabilizer and attach points.

That's pretty damn significant, and hardly a red herring.

Next consider where many constant speed propellers go when oil pressure is lost (not uncommon in an engine failure, especially an engine failure in which oil is lost...I've had a number of those). Many propeller installations revert to the low pitch stops under spring and/or nitrogen pressure, and begin to act as a fixed pitch propeller; RPM varies with power setting and airspeed.

There's a reason that turpopropeller engines fail to the feathered position, or are supposed to and move to a higher pitch, reducing windmilling drag until the propeller can be or is feathered by pilot action or by natural consequence. Until that time, regardless of whether on the low pitch stops or at an intermediate position, the windmilling propeller will still produce considerably more drag than the feathered prop.

In what aircraft and under what circumstances have you observed this?

It's absolutely not a "red herring," and your assertion is supported by no documentation. In fact, it's not supported by your link to ANA, either. I can tell you that I've flown turboprop installations (as noted previously) that produced such significant drag at an idle windmilling state that I was physically thrown forward hard in my shoulder straps and had to follow the stick through forward to nearly the vertical to keep from stalling and experiencing a control loss, due to the rapid drag rise. As ANA noted, it was very much like throwing out a parachute, and breathtakingly dramatic.

If you've ever gone from a windmilling descent to a feathered descent then you'e experienced the significant reduction in drag from the feathered prop, vs. the windmilling state. There's really no way around that.

Many flying Vans RV's have looked at this question since the aircraft has such a poor glide ratio. Quite a few have actually shut the motor off and ran tests. The aircraft glides far better with the prop stopped then windmilling. Probably on the order of 20 to 25%. This is a fairly small aircraft often with a big prop. If the prop is composite it's fairly easy to get it stopped. If it's metal it can be more difficult since there is no ability to feather. Course verses fine pitch also makes a difference but not nearly the effect stopping the prop has.

With respect John it is!

Page 149 of naval aviators, figure 2.19:,

http://www.airlinepilotforums.com/me...72-feather.png

Apologies for the poor pic you can download the pdf here https://www.faa.gov/regulations_poli.../00-80t-80.pdf it's page 167 on the pdf or pg 149 on the original

In this diagram the cut off point is a pitch of about 20 degrees. The 15 degrees that I cited in my original post is from a scientific paper from NACA from the 1930s.

The diagram clears shows a course pitch prop at 30 degrees wind milling with about half the drag coefficient of the same 30 degree pitch prop stationary... (30degrees is about full course on bonanza for example)

In my original post I was clear to state that a wind milling prop in fine pitch (e.g. <15 degrees), such you might find a multi-engine aircraft will provide substantially more drag than a stationary one.

The fine pitch setting on turbo props are typically very low blade angles, from the diagram I cited 5 degrees will produce a drag coefficient for a wind milling prop of about 3 times higher for the same stationary prop.

To be absolutely clear 'a fine pitch wind milling prop' with produce significantly higher drag than the same fine pitch prop when stationary, that is not the case for course pitch prop.

No captain, it's not.

You didn't read your material very well, because it undermined your effort; it said the exact opposite of what you attempted to say, as noted above.

A coarse prop will produce less windmilling drag than when feathered or stationary, but will still produce substantially more drag windmilling than when stationary.

The point your'e attempting to make is contradictory, and perhaps you don't understand this.

You're saying that a fine pitch windmilling prop produces more drag than a course pitch windmilling prop. This is true. The closer the blade angle moves to feathered, the less drag there is. The fact that there is a difference in drag, however, demonstrates quite clearly that a windmilling propeller has more drag than a stationary propeller.

Older aircraft using hydromatic propeller installations could use a feather pump to drive the propeller to feather. If the magnetic holding coil for the feather button didn't release at feather, the pump would keep running and drive it back out of feather, too (had that happen more than a few times). The procedure, then, was to manually hold in the button and manually pull it out when satisfied with the propeller status, which required watching the propeller.

One advantage of the hydromatic feathering system and pump was the ability to schedule blade angle by running the blade to a desired point.

There is a notable feel and change in rudder pressure between fine and coarse propeller positions, and feathered; with windmilling producing substantial drag regardless of propeller blade angle.

The greater the propeller blade angle (the more coarse), and the lower the RPM, and the lower the airspeed, the lower the drag of the windmilling prop disc.

I can tell you that on the direct-control B24, an outboard engine out takes about 70 lbs of rudder pressure with the other three engines running and making power, even at lower power settings such as 30" Hg. Any change in rudder required is very noticeable. Having spent hours in that state (in fact, having flown airplanes across the country in that state), with plenty of time to experiment with blade angle, and windmilling effects, I can tell you that any amount of windmilling propeller will produce drag. The value varies with blade angle, airspeed, and RPM, but it's certainly there.

To backtrack, you previously stated that any drag wrought by the engine, geartrain, accessories, etc, is a "red herring." Although untrue and although nothing was offered to support that view other than an erroneous statement that it wouldn't matter if the engine was "stuck solid," you didn't address the importance of RPM in the matter. Aerodynanmics for Naval Aviators did. The prop disc absorbs more energy (and creates more drag) at higher airspeeds when the propeller is driven to a higher speed and more energy is applied from the slipstream to the airplane (as opposed to the engine to the propeller to the airflow. The slipstream is doing work on the airplane, vs. the engine doing work on the airflow. Instead of producing thrust, the airplane is receiving or creating drag. It certainly does require energy to perform the work of moving that propeller; the flatter the pitch the greater the resistance, the greater the drag the more work is done moving it and the more drag is created and the more energy imparted against the airframe.

A fine pitch stopped propeller has greater drag due to greater flat plate area than a feathered stationary propeller. A coarse pitch stationary propeller blade or prop assembly has less drag than a fine pitch stationary prop, but more parasitic drag than a feathered prop. If in motion, it's not just parasitic drag, but induced drag, and significant energy required to overcome internal engine and gearbox resistance, too.

Do you understand the concept of a helicopter rotorblade in autorotation? Aerodynamics for Naval Aviators makes a comparison. An autogyro or helicopter in an autorotative state, airflow upward through the rotor disc, is able to descend or fly without falling precisely because of the amount of drag imparted to the disc. Lowering collective allows fastest rotation, lowest blade angle; pulling collective increases blade angle, slows rotation, reduces rotor energy, briefly slows descent rate, then increases it substantially. RPM slows, blade angle changes. It's a dynamic, inter-related arrangement.

The bottom line for the question at the outset of the thread is that a windmilling propeller produces more drag than a stopped prop, or a feathered prop. In some aircraft and propeller installations, not all, blade angle may be altered from fine to coarse and drag reduced, but drag will always be greater windmilling than stopped. Drag windmilling can be greater than an equivalent solid plywood disc. That's a lot of drag, and given that the propeller is small with little flat-plate area, it's not merely the angle of the prop blade (parasitic drag) that's responsible for the drag rise when windmilling. It's the energy absorbed from the slipstream and is a function of rotational speed, airspeed and blade angle.

Blade angle has two aspects here. One is the amount of flat-plate area or exposed two dimensional area as seen from the front of the aircraft looking aft; a fine pitch prop has more flat plate area and more parasitic drag than a feathered one; a coarse prop an intermediate value between the two. This is only part of the problem. If the propeller is stopped, there is a difference between feathered and fine pitch, but it's not really that significant. Prop blades aren't very large. The real drag doesn't begin until the slipstream starts driving the propeller in a windmilling state.

If you want to experience a difference in the drag imparted through gearboxes, engines, and accessories, note the difference in a turboshaft installation and a free turbine prop installation (such as the difference between a TPE-331 and a PT6A. The TPE-331 windmilling produces substantially more drag, as there's a lot more resistance to turning that propeller, and a lot more drag when windmilling. The PT6 has gearbox drag, but no engine drag and not nearly the resistance in windmilling, and consequently less drag. It's no red herring.

With a coarse blade angle in a windmilling state, the propeller won't be driven to as high an RPM for a given airspeed, and consequently there is less drag. It's not simply the flat plate area due to blade angle. Remember that there are more factors than one; blade angle as it relates to the RPM achieved for a given airspeed is what's important, not simply blade angle.

On some airplanes, the blade angle in a windmilling no-power state will not be changeable, short of feathering, and some can't be feathered. In some aircraft the propeller will move toward feather or will attempt to feather on its own, reducing blade angle and reducing windmilling RPM, It's the reduction in RPM, far more than the raw issue of blade angle, that reduces the drag on a windmilling prop disc.

Not much point is providing further references if you ignore the one already provided :p

https://s27.postimg.org/4m08r566r/feather.png

Here it is again with a better pic. Take two props both with a 30 degree blade angle, one stationary, one wind milling, the wind milling prop has a drag coefficient 50% less!

The drag created by a propeller is a function of the local airspeed over the blade and the relative angle of attack of the blade.

When a propeller windmills the relative angle of attack is reversed, flat pitches typically results in a high angle of attack and high local airspeed, i.e the propeller does a lot of work, and drag on the aircraft is very high.

The engine/gearbox is a red herring because it is not the cause. A free wheeling propeller with no engine/gearbox slowing it down and a very flat pitch will result in a very high drag coefficient, many multiples higher than a stationary one of the same pitch. A feathered propeller will have the far less drag than both these two scenarios.

John with respect I am not arguing with the rest of your post.
https://postimg.org/image/criapaufj/

Wrong again.

Not only did I NOT ignore it, I quoted it, and it said exactly the opposite of what you've attempted to say. You really have no idea what you're talking about, and while you continue to post references, you don't understand what you're posting.

You really can't.

It's a very straight forward diagram, you've made no mention of it, that I can see.

C'est la vie

if the original poster still cares, maybe it answers his question and gives him some insight.

Seems to me that a non feathered windmilling prop in a DC-7 was a whole lot of rudder.

No question about that! My posts were just rebutting a common misconception about the reason why. That misconception being that high drag comes from the engine/gearbox, it doesn't it comes from the propeller, and it's very significant while wind milling in fine pitch.

In a light twin I've noticed 500fpm difference between closed throttle fine pitch and feathered. Remember that a prop needs to be unstalled with sufficient tip speed in order create massive drag, which is the situation on a typical engine out. If you can't feather then try to get it stopped if you need the glide.

Total agreement, everything including the improvement gained by stopping the engine IF feathering fails AND the prop wind milling in FINE. Caveat though is that stopping the engine with prop wind milling in FINE may not be safe and practical, even with power reduction on the remaining engine the respective VMC and Vs for the configuration may make this unwise.

Your assertion agrees with the data from the graph I posted and the engineering papers I so far found on the topic.

If the propeller was fitted with a sprag clutch, such that the propeller could windmill freely your assertion would still be correct, because it is the propeller that causes the drag, not the gearbox/engine.

Dangerous, foolish, and incorrect assertions on the part of the new guy.

I strongly recommend that any students reading disregard; his counsel could just get you killed.

Captain Beaker,
Can you provide some real world examples where your predictions would apply?
Because while the underlying physics might be sound, its rather academic, as it dosen't take into account the real world system interactions that makes your situation a transient state at best.

Makes a huge difference on a CE-208. You have to feather that thing, the glide distance chart requires it.

The graph shows 3 things:

1. Engine/Gearbox is not the cause of the drag, e.g. compare the scenario at say 30 degree pitch.

2. Flat pitch and wind milling creates very significant drag, the drag coefficient increases massively as the blade moves towards 0 degrees. This is entirely consistent with the need to feather in an aircraft cable of fine pitch. It is also consistent with safety devices needed to ensure blade angle remains outside the 'beta range' while in flight.

3. It's consistent with my own observations and the flight manual, for an aircraft fitted with a fixed pitch prop of approx 18degrees found in a C-172. i.e. wind-milling or stationary does not seem to have a very noticeable effect. It's consistent with the need to set low rpm on a constant speed single.

All of this is entirely consistent with every flight manual I have come across.

The original poster asked an interesting academic question, so maybe I provide some thoughtful input on that academic discussion. I haven't proposed anything contradictory to what you expect to find in the flight manual.

Couple of thoughts, just for fun :D

How come the propeller creates so much drag in the positive beta range, even when driven by the substantial torque of a turbine in flight idle?

While a helicopter is in auto-rotation, the blades free wheel because it is fitted with a sprag clutch. If we consider the blades to be a large windmilling prop rotated 90 degrees wouldn't the resistance from the engine be advantageous? ;)

Probably best I get my coat.:)

(Sigh).

Coarse vs. Course.

Sprague vs. Sprag.

A little knowledge is a dangerous thing.



http://i3.kym-cdn.com/photos/images/...40/656/f26.jpg

True, but you'd need to be very near stall to get a fixed-pitch prop to stop on a typical light ASEL. If the motor is damaged that might make it easier to stop.

Once stopped, you may be able to pitch for best glide without the prop spinning again, thereby enjoying reduced drag and increased glide range.

Would I try it? Maybe under some hypothetical situation where I was going to be gliding for a long time and needed every bit of range. You're going to waste some energy slowing the plane so you'd need a lot of altitude to get an ROI. Would I recommend it? No.

This is just another thing to be aware of. If I have altitude and need the performance/ glide, I'm stopping the prop. If there's a good place to land under me then Ill enjoy the extra drag. A Senneca crashed just short of its destination because the dead engine was turning.

1 - It is important to note the context of the reference material - the graph was used to illustrate the substantial drag of a windmilling prop on a failed engine. The fact that it shows the drag curves cross over in the higher pitch angles is interesting, but without knowing the other variables - RPM, airspeed, prop dimensions and propshaft torque - it is meaningless, and very misleading - a change in any of these variables will totally change the graph.
To illustrate - note the stopped prop line represents infinte torque/friction from a stopped engine. Now lets reduce the friction so that the prop barely rotates - say 1 RPM. What would the incurred drag now be? I can tell you it will not be the windmilling drag line on the graph.
This is the interaction I was talking about earlier - the engine/gearbox friction dosen't CAUSE the drag - but is one of the inputs that will most definately affect the drag by changing the windmilling RPM at varying airspeeds. If you have an engineering background we can continue this discussion on how these higher order effects stack up.

I dont think anyone has issues with point 2 - you are pretty much reiterating the consensus.

3 - Please elaborate on your testing methods and results.

And on your 'fun' thoughts
Idle beta 'creates so much drag', but guess what happens when you go deeper into beta?

You missed the whole point of autorotation - its not done to increase drag to reduce the ROD, but to build up rotar momentum so there will be enough energy built up by the time you get to the ground to flare out for a survivable landing.

No need to get your coat - but try to consider your audience before speaking and modify tone accordingly.

I posted the graph from ANA, because the text was brought up. I hadn't read it for a long time but sure enough it had the info to show that drag is not created by the engine/gearbox.

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS report no 464 which was written in the 1930's answers this in much more detail. It provides graphs for different blade angles with respect to velocity etc. It was mainly concerned with solutions for dive bombers of that period... I have another paper somewhere else.

I appreciate you understand the drag is not created by the engine, and as you point out reduced rotation caused by friction will have an effect, but there is a CUT OVER point. Precisely what that Is I don't know, thought the data seems to suggests it is somewhere around 20 degrees.

I apologise if I ruffled some feathers:D

I need to make a trip with Captain Bunsen Honeydew:eek: over the next few days but would be happy to discuss the data properly and learn something.

The text was quoted because you introduced it; Aerodynamics for Naval Aviators was your reference, and contradicts nearly everything you've attempted to say.

The graph you posted, from ANA, does NOT suggest anything whatsoever about mechanical drag, and does NOT make the point that you believe it makes. What it does do is show that you don't understand the information.

My mistake, in my very first post I had in mind the NACA data, your correct it was actually myself that introduced ANA in subsequent post. Must have been all the ad hominems that caused the confusion :mad:

But what say you about the graph in any case? What is your the explanation for the two scenarios at 30 degree pitch?

FlyingChicken makes a relevant point about other variables, perhaps the graph is not representative when other factors are considered. Fair point, perhaps look for better data...

If you care you can download NACA report 464 from NASA website
and review that data.

Regarding 'fun'
Never been in a helicopter, but I would have thought BOTH the glide and flare were important... Its important in a autogyro. It was just an illustration, perhaps a poor attempt at sarcasm.

Well I would expect the FCU to command additional fuel to the engine?

Thank you for the link to the NACA paper. It is nice to see some actual data. I would be interested to see the other paper you mentioned if you manage to locate it.

My thinking is that the "cutover" pitch you mentioned will vary dependent on prop dimension, relative wind velocity, drag/torque from internal engine friction, and resulting prop RPM. Not sure if there is a way to solve this analytically.

Keep in mind this is an airline pilot forum. The way you are going about analyzing the problem is beyond the variables a pilot can control from the cockpit. Either a prop can be feathered or it is stuck. Either the engine will windmill or it is seized.

With a couple of exceptions, there's no collective or blade angle change in an autogyro, and whereas the autogyro runs in autorotation at all times (and flares by changing rotor disc angle, not by pulling collective (altering all blade angles simultaneously), and whereas the autogryo rotor disc has no connection to a transmission or engine, you can't correlate the autogyro with the helicopter.

The helicopter collective must be lowered and the rotor clutch disengaged (manually or automatically, depending on the system), to prevent loss of RPM (which will occur if there's drag on the rotor disc through mechanical means). flare at the bottom end in training will be power and collective, and in an engine out, final flare involves use of the collective.

Rotor blade inertia is crucial in a helicopter as is RPM. Increasing blade angle by pulling collective when in an autorotative state will decrease rotor RPM. This is not an issue with the autogyro, in which use of the cyclic to vary the plane of the rotor disc is the only option, other than adding power through an independent, fixed pitch propeller. (in nearly all cases)

Will do in the next few days, will also post some of the graphs from the NACA report with annotations, for discussion...

Same of those factors are there

I'll take a quick stab, the graph in figure 4, is were all the data is combined into a working graph.

The graph gives negative thrust coefficients (i.e. Drag coefficient) for stationary blades of differing pitch. Not surprisingly the drag coefficient remains constant with variation in speed.

You can work out the drag coefficient from a free wheeling propeller i.e. zero torque at differing pitch, this too is constant. You do this 'by projecting down from the point of zero torque to the appropriate thrust curve' then moving across for the drag coefficient.

To work out the correct solution for a windmilling prop with friction, a friction/torque curve is needed, however the actual result should lie on the respective curves so many quantitative comparisons can still be made.

I will try an illustrate this later... Don't forget to consider how a CSU effects this.

That's true but the problem can be evaluated beyond the pilot/control limitations to further understand what's going on, is this not how this thread started?

This is the data relevant to the OP question:

fixed pitch 17degrees @100mph drag in pounds

stationary at 88° (i.e. feathered) 5.8lbs
free-wheeling at 17° 60.1lbs
stationary at 17° 94.4lbs
dead engine wind-milling at 17° 101.1lbs

Consider the scenario with C172P with an 18degree prop I believe I said there would not be much difference...

Anyway don't take my word for it read the data yourself, I could be a dog, this is after all the internet.;)

Following up from my last post, this is figure 4 from NACA report 464 available from NASA. I have added some annotations.

https://s29.postimg.org/qeiamh85z/drag.png

To work out the negative thrust coefficients (i.e. Drag coefficient) for stationary blades of differing pitch find where the blue line intersects the relevant curve. The blue line represent a stationary blade.

To work out the drag coefficient for a free wheeling propeller start by finding where the red line intersects the relevant curve (red line is zero torque). Then 'by projecting down from the point of zero torque to the appropriate thrust curve', move over to left for the drag coefficient.

To work out the correct solution for a windmilling prop with friction, i.e. one with an engine directly attached a friction/torque curve is needed, however the actual result should lie on the respective curves so many quantitative comparisons can still be made.

Don't forget to consider how a CSU effects this. The CSU will move the blades to a finer pitch, and even at the lowest RPM setting may result in a fine pitch.

What this graph tells us:

1. Energy used to turn over the engine, or the resistance/friction of the engine is the not the cause of the drag. It does have an effect, depending on the blade angle it can make it substantially greater, or a minor difference or in the case of a very flat blade angle (> 7degrees) engine friction can actually reduce the drag. Energy used to turn the engine is a red herring.

2. Fine pitch produce very high drag coefficients, higher than a stationary prop, and far high than a feathered prop.

3. For the blade angles 12,17,22 a reduction in negative torque results in reduction in drag, i.e. less friction is better, this is consistent with flight manual check list for closing the throttle in a piston engine aircraft.

4. Blade angles < 7 degrees, an increase in negative torque results in a reduction in drag, positive torque is required for maximum drag.

Take all this with a grain of salt, but none of this contradicts any flight manual or training manual text, with regard to pilot actions. Feathering is imperative to reducing drag when propeller is capable.

Are you even a pilot? Your posts would suggest not. You appear to be attempting to look up material on the internet and make it fit what you think is correct, but is not.