Captain Beaker

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

JohnBurke

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."

JohnBurke

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.

sailingfun

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.

Captain Beaker

With respect John it is!

Page 149 of naval aviators, figure 2.19:,



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.

JohnBurke

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.

Captain Beaker

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



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.

JohnBurke

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.

Captain Beaker

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.

badflaps

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