Could wing height position (through changing pitch) affect propeller airflow?

[WrathofAtlantis]

I am talking about low wing single engine WWII fighter types.

In particular, I am interested in accelerated propeller airflow through the transition/curvature of the prop slipstream spiral distribution around the wing, from being partly below, towards mainly above the wing. This as the wing's position lowers, relative to the bottom half of the prop, compared to the airflow direction in a turn. Could the slipstream spiral "switch" to being mostly above the wing?

It seems odd that something behind the prop could affect, asymmetrically, the airflow through the prop... Or, at least, the ability of the prop to generate thrust evenly, throughout its surface.

WoA

 

[FactChecker]

I am talking about low wing single engine WWII fighter types.

In particular, I am interested in accelerated propeller airflow through the transition/curvature of the prop slipstream spiral distribution around the wing, from being partly below, towards mainly above the wing. This as the wing's position lowers, relative to the bottom half of the prop, compared to the airflow direction in a turn. Could the slipstream spiral "switch" to being mostly above the wing?

Yes, it might

It seems odd that something behind the prop could affect, asymmetrically, the airflow through the prop... Or, at least, the ability of the prop to generate thrust evenly, throughout its surface.

The short answer is, yes, the changed position can change the airflow. Everything is inter-related. Pressures and flows behind the prop must come from somewhere before the prop. The flow from the prop must fit in with the other airflow around the wing. So changes in airflow behind will almost certainly change things before.

 

[WrathofAtlantis]

That is fascinating...

Thank you very much for your clear answer. It is much appreciated.

Is there any kind of consensus as to the nature of [above vs below wing] flow distribution changes, in the prop's slipstream spiral in a turn, on low wing single engine monoplanes? My focus would be on how nose length would affect this at sustained speed turn angles of attack, so more modest angles of attack like 7-8 degrees. (Since a longer nose raises the position of the prop -relative to the turn airflow- more, for a given AoA)

WoA

 

[FactChecker]

Sorry. I'm afraid that I am not knowledgeable enough to help much. I hope that someone with more aerodynamics expertise can answer your questions.

 

[Ralph Rotten]

Sounds like you are talking about variable pitch props, which are still a thing in aviation.
But I don't quite understand your question about changing the airflow.
Essentially you can feather a prop, either for high altitude flight or during an engine failure.
You can also turn the prop so far that it actually works as a thrust reverser.

 

[WrathofAtlantis]

What I mean is that, if the nose is lifted 7-8 degrees, in relation to the turn's airflow, then the propeller is lifted somewhat higher above a low set of wings, in relation to that airflow.

This could mean a large part of the prop spiral airflow might be diverted above a low-set wing.

This would induce a tightly-radiused profile curvature to part of the spiral airflow (perhaps accentuated by the -presumably- obstructing boundary layers around the wing), which curvature might accelerate the air behind the prop, in an uneven fashion, creating asymmetrical loads on the prop, as the nose is raised into the turn.

On a 10 foot diameter prop, at 200 mph and a 1000 feet turn diameter (about 3 Gs), the inside edge of the prop is 1% slower than the outside edge. I will assume this roughly equates to 1% more inside prop half thrust (from the slower inside half incoming air).

Accelerated air on the outside half of the prop (due to the spiral airflow behind the prop curving -thus accelerating- from straight to curving above the wing) would do the exact opposite...

If the accelerated air behind the outside prop half equated a 6% overall load loss, and this 6% loss of thrust, being asymmetrical (due to only the bottom of the air spiral being involved in shifting above the wing), I wonder if this lost load might not get partially transferred as an increased load to the unaffected inside prop half (from the overall deceleration), perhaps equal to half that 6% value, which would mean a 3% inner turn prop half load increase, plus the aforementioned 1% inner half load increase from the turn curvature itself.

This would amount to a + 4% load on the inside turn half, and a minus 6% load on the outside turn half, with a total load imbalance nearing 10% in total.

On a (typical?) 3000 pounds maximum prop load, at low speed and high power, that would be plus 120 pounds on the inside half, and minus 180 pounds on the outside half: Total imbalance: 300 pounds.

I wonder if those figures are at all realistic, as 300 pounds is beginning to be significant...

WoA

 

[Ralph Rotten]

The prop wash is centered on the fuselage (single-engine) so even though a steep angle of attack will cause turbulent air to flow over the wings, it is mostly caught up in the fuselage airflow. At most it would only compromise a small portion of the wing's lift.

Have you ever tried Xplane simulator?
You can do some amazing experiments in that software.
I once built a flying brick...just to see if it would fly. It did fly, but as you may have guessed, it flew like a brick.
With Xplane you can build anything: planes, rockets, VTOL, orbital, trans-orbital, fly on Earth or Mars, rocket cars...the possibilities are endless, and it will output specific data if you need it to.

My latest Xplane project is a Martian lander. Essentially it's a high-tech version of Lunar Lander.
Mars is $@#! hard to land on.

 

[WrathofAtlantis]

The prop wash is centered on the fuselage (single-engine) so even though a steep angle of attack will cause turbulent air to flow over the wings, it is mostly caught up in the fuselage airflow. At most it would only compromise a small portion of the wing's lift.

That could be, but my main interest, for now, is in the extent of the asymmetry of the prop's thrust during turns.

A serious load asymmetry, even 10%, by itself, could have major implications.

Also, ideally, this prop asymmetry would have to be evenly divided between a loss on the outer turn prop half (accelerated prop spiral airflow), and a load gain through air deceleration on the inside prop half.

The reason I would want both is that, first of all, it makes the total prop imbalance greater by combining more modest (and thus more plausible) values, and second, it would justify the combined use of much reduced power, with the strange use of full coarse prop pitch at very low speeds, to increase the sustained turn rate. (See account below, of which I know of several very similar)

Lower power would reduce the inside turn disc half overload, while a coarser prop pitch would compensate the lost outer turn disc load, matching better with the acceleration of the prop's outer airflow shifting above the wing.

It is multiple accounts like those below, plus many other disparate clues, like the observation of large turn ability gains with shorter noses, but especially extra power being detrimental to sustained low-speed turn rates, that motivate my line of inquiry:

Hanseman (505 sq.) combat report, 24 May 1944 (Merlin P-51):

"Dogfight at 500 ft. (with a second higher aircraft, after climbing from 130 ft., having closed to 50 ft. on a landing wheel down 109G)"--"At first he began turning inside me. Then he stopped cutting me off as I cut throttle, dropped 20 degrees of flaps and increased prop pitch. Every time I got close to the edge of the airdrome they opened fire with light AA guns."--"Gradually I worked the Me-109G away from the field and commenced to turn inside of him as I reduced throttle settings."When the enemy decreased power, I used to throttle back even more. 250kmh seemed to be the optimal speed to turn (sustained speed turns with 1944 Me-109G, 160 mph, an aircraft which had a 405 mph top level speed...)"
- Kyösti Karhila

Donald Caldwell wrote of the FW 190 D-9’s operational debut in his "The JG 26 War Diary Volume Two 1943-1945" (pages 388 – 399): "The pilot’s opinions of the “long-nosed Dora”, or Dora-9, as it was variously nicknamed, were mixed. The new airplane lacked the high turn rate and incredible rate of roll of its close-coupled radial-engined predecessor."

1946 US evaluation of FW-190D-9: "1-The FW-190D-9, although well armored and equipped to carry heavy armament, appears to be much less desirable from a handling standpoint than other models of the FW-190 using the BMW 14 cylinder radial engine."

The last two quotes concern a lightened long nose (from heavier short nose) conversion of the same airframe type.

WoA

 

[Ralph Rotten]

"That could be, but my main interest, for now, is in the extent of the asymmetry of the prop's thrust during turns."

Still not sure what you mean by asymetry. Are you referring to the yawing effect of the prop during high engine performance? That is normally solved with a little rudder pressure?

One thing to consider is that the wing profiles used in most modern planes are publicly available. They were the result of NACA's early studies into wing types. In...32...I think, the Govt ran a comprehensive study to determine the best wing shapes for different speeds & altitudes. Many of these NACA originals are still in use. They are also available in Xplane (for both the wings and props) so you can really go crazy with different configurations. The Xplane builder is klazy-cool. It's like a laboratory for flying. Burt Rutan used Xplane when he built his Spaceship1 simulator, and the software was so accurate that they actually made changes to the control system based on data harvested from the sim.

Image from my twitter account, pulled from screenshot, Xplanes 10.

 

[WrathofAtlantis]

"That could be, but my main interest, for now, is in the extent of the asymmetry of the prop's thrust during turns."

Still not sure what you mean by asymetry. Are you referring to the yawing effect of the prop during high engine performance? That is normally solved with a little rudder pressure?

No, what you describe would be the P-factor, which is mostly a horizontal left yaw (horizontal in relation to the airframe).

The P-factor is also supposed to have a "left turn-promoting" horizontal (to the airframe) load asymmetry on the prop (so not just a lateral yaw, or side-pushing, force, but also a longitunal force). But in a 70° left bank, this would tend to pull the nose downward, and would only help a left turn by a modest amount.

I am talking about an uneven vertical (to the airframe) pitch load asymmetry, or rather, a nearly vertical pitch-resisting load, in relation to the airframe, when the airframe is sustaining a 70° bank at 3Gs. So, to the horizon, this uneven anti-pitch load I am talking about would be nearly horizontal in a 3G turn.

WoA

 

[Ralph Rotten]

It is indeed easier to turn in one direction than the other with a powerful prop engine. Coordinated turns require rudder, and prop-yaw requires one pedal to counter, so in one direction you only need standard rudder during your turn, and the other you need rudder+1. Even a Cessna 152 requires a little right-rudder during climb-out.