Is prop center true thrust center if airflow partially blocked?

[Brian Jackson]

Veering from an ongoing topic here, a thought occurred to me (perhaps incorrectly) that called into question something I had always taken for granted. If a pusher prop were in clean air above its center and in turbulent air below its center because of blockage, would the center of its thrust still be at the center of its hub? This may be a silly academic question but I'd like to better understand why my thinking may or may not be correct. I my mind there would be a net pitching moment imposed on the airframe regardless of Newton's equal and opposite forces that tend to counteract.

 

[Vance]

The Predator had a large oil cooler, a muffler and the engine sump that I felt was blocking the airflow to the propeller.

I wanted to get a feel for how it affected the thrust so I used a particularly smoky highway flare to get a feel for the airflow around the propeller.

I tied her down and ran her up to cruise power (2,150 rpm) and used the burning flare for smoke trails.

I did the test in a 15kt wind.

I found that I could block the flow as close as a foot in front of the propeller without affecting the outflow noticeably.

I turned her to where I had a quartering wind and that affected the outflow a lot making the thrust come off at about half the angle of the inflow.

I have not done further tests on The Predator and this does not take into account turbulence from preceding objects.

I have done inlet tests a lot and found when something is sucking air in the inflow will go around a pretty sharp corner where the airspeed is low.

When I reduced the size of the round muffler and laid the oil cooler down it was noticeably quieter at 60kts indicated air speed particularly from the ground.

 

[Smack]

Brian,
Theoretically, I think you're thinking is correct; different in-flows should produce different outflows (thrust).
Practically, I think it is complicated; perhaps location of obstruction, size of obstruction, % blockage on what % of circle segment, rigidity of prop blades, direction of in-flow (amount of redirection required) if not symmetric around prop, density... I'll agree that anything upstream of the prop (I forget how many diameters is the affect) affects that prop/thrust. Makes sense that if the upstream is not symmetric, there should be a moment created.
The in-flow direction brings to mind "P-factor"; seems THAT concept would support your ponderings.
Brian

 

[Brian Jackson]

Thank you, gentlemen, for the input. This all stemmed from designing the ship to be true CLT. I'm staring at the diagrams and the math works, but realized I had always made the assumption that center of prop hub would always be center of thrust at all airspeeds. Though I doubt it's worth considering on my modest single-place build, it did get me wondering if this thrust center shift (if any) is something that is taken into consideration on other, larger CLT designs.

 

[XXavier]

Due to the obstructions, the thrust distribution may not be regular across the prop disk, and there may be areas of that disk that produce more thrust than others, but all the thrust produced by the disk, irregular or not, is transmitted to the aircraft through the hub. The center of that hub has to be considered as the center of thrust. There's no other source of thrust apart from the prop, and the prop is physically connected to the gyro by the hub...
The irregularities in the thrust distribution on the prop disk may produce a moment that will be transmitted to the gyro, but that's a different thing...

 

[giro5]

Brian, make the problem go away - design a tractor style. Merry Christmas to all. John R.

 

[Doug Riley]

XXavier, if the prop applies a moment to the airframe, that's not a different thing -- it's exactly what Brian is asking about.

In physics lingo, a force (with zero moment) located at a given point on an object is exactly equivalent to a force PLUS a couple (what we mean when we use the word "moment" here) located elsewhere on the object. So...

If your prop's thrust force is centered on the hub, then there is no moment about the hub and all is ideal. If OTOH your prop's thrust is mostly at the top of the disk (i.e. the bottom half receives trashy inflow), then you can say either (1) the thrust is 300 lb. at the hub and there's also a moment about the hub of (say) 75 foot-lb; or (2) the thrust is 300 lb., centered 3 inches above the hub. Two ways of saying the same thing.

Yes, the "off-centered-ness" of the thrust is felt by the hub as a moment from the structural viewpoint, but, from the viewpoint of the prop's effect on the behavior of the airframe in space, the situation is the same either way you choose to describe it.

A similar word game applies to pitching moment/center of pressure shift in airfoils. If you fly a Rogallo hang glider (but, Jesus, don't), you worry about center-of-pressure shift and the glider's tendency to dive as it goes faster. You don't talk about moments as such. OTOH, if you fly a rigid wing (or rotorblades), you think in terms of a fixed aerodynamic center plus a pitching moment about that center that increases with airspeed. Different ways of talking about the same thing.

 

[XXavier]

Yes, you're right. The effect on the gyro is the same...

 

[SandL]

how about … hang the gyro from the head, then standing at the rear, simply push a prop tips forward … (or pull it on a spring balance) try it once with a blade at the bottom and again at the top, measure how much the nose rises or falls. with differing amounts of force. , maybe you could try assuming that you get 10% less thrust from a bottom blade . so try 10% less force on the blade and test the difference … by doing several force tests you may be able to build a graph to put into a calculation

 

[Brian Jackson]

how about … hang the gyro from the head, then standing at the rear, simply push a prop tips forward … (or pull it on a spring balance) try it once with a blade at the bottom and again at the top, measure how much the nose rises or falls. with differing amounts of force. , maybe you could try assuming that you get 10% less thrust from a bottom blade . so try 10% less force on the blade and test the difference … by doing several force tests you may be able to build a graph to put into a calculation

That seems like a very simple method of witnessing a real-world effect. Thank you for the tip.