Propeller effects on stability

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addaon

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Can someone walk me through the propeller effects on stability?

1) I know there's a major destabilizing effect for a tractor, stabilizing for a pusher; is this due to the turning of the free stream air to align with the propeller axis with change in angle of attack? We've discussed numbers here recently, and I know the WWII fighters tended to lose about 5% stability. I'd expect this effect to decrease with decreasing engine power, and decrease with distance to the center of gravity, but haven't found estimation techniques.

2) I'd imagine that the "masking" effect of the propeller slip stream over tails, etc, would have a destabilizing effect (as the impact of an angle of attack change is less visible to the surface, causing less of a change in lift on the tail). This should be destabilizing for centerline tractors and pushers, and potentially stabilizing for wing-mounted twins (wings in propwash, stab not in propwash)... right? How does this compare to the first effect?

3) Any other effects I should think about?
 

addaon

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To give myself a partial answer, I just found this thesis (warning, large PDF), which discusses exactly these issues. I'm in too much of a habit of just searching LARC, theses I often miss. Hopefully this (plus its references) will get some nice equations onto the table.
 

addaon

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And continuing to talk to myself, I just rediscovered Raymer pp 483-487. As usual, exactly what I was looking for, but couldn't remember where I had seen it. Thanks Raymer!
 

topspeed100

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Thanks addaon..I always tought the prop at front increases the stability. Proplem in the rear prop comes out in the engine out situation if you have sacrifised something like the moment arm length. That is why no one survived the engine out in BD-5A ( short wing version ) during take off....and very few in B models.
 

Norman

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3) Any other effects I should think about?
From paragraphs 1 and 2 it sounds like you already understood this better that I and that thesis will probably have the theory down pretty well but I'll add my $0.02 anyway.

I visualize the directional stability effects of the propulsion as a virtual empennage with the aerodynamic centers of the fins on the prop disk or, in the case of a jet, at the intake. The size of the virtual empenage is dependent on the thrust. So with a propeller the imaginary fins are largest at stall speed and shrink as you go faster but with a jet the size of the imaginary surfaces are fairly constant. Then of course you have to figure in that the normal force on those surfaces increases as airspeed squared.

My guess is that the stability contribution of a prop (either + or -) decreases with speed but a jet has an increasing stability contribution with speed.

Have you noticed that the BWB designs have their intakes as far aft as possible?:think:

I'll download that thesis now
 

Aircar

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Almost any textbook on stability and control will have the answers -eg Perkins and Hage ,Dickinson etc --the derivation can be done by blade element theory taking into account the yaw or pitch angle of the prop but it is more complex since the inflow angles are altered according to where on the prop disc the effective angle of attack is greatest hence thrust .

The easiest way to determine the sense of propeller normal forces is to note that the flow OUT of the propeller tends to be more parallel to the axis (normal to the disc) and the change in momentum from the change of direction gives the direction of the force --this makes it stabilizing for aft of the CG and destabilizig in front (same goes for twins and the lateral offset is also moment arm ) --for magnitude the generic formula is given in "power effects" in P&H p 231 - 245 --it is not simple.

Also note that for control surfaces or fixed tail in front of a propeller the propeller reduces their effect by 'unturning' the flow change due to tail operation --the effects of dynamic pressure and corkscrewing propwash are separate from the propeller normal forces or control interactions
 

addaon

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Ben, these questions are with regards to a different design; I trust the 701 quite a bit, but that's a "practice building" project, not a design project.
 

addaon

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Nope, feedback's never a problem! It's just that the flying wing design I've been working on for the past four or five years is stability critical, and I'm trying to get a better visualization of the tradeoffs of something like the Verhees Delta prop placement (much higher scrubbing drag, lower empty weight because of moment impact on needed ballast, less stability change with power, etc).
 

Matt G.

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I started a similar thread to this a couple weeks ago. The information in Raymer is pretty minimal and just a small part of the big picture, in my opinion. As Aircar says, Perkins and Hage is a big help, but unfortunately, the charts in there are for a specific WWII-era Hamilton Standard prop, and are not universal. I haven't had a chance to read through the entire document you posted, but it looks similar to information contained in NACA WR L-217 and L-116. One of those two has a more general method that can be applied to any propeller, but it requires a lot more information about the prop than what is readily available on the internet. Might have to contact a propeller manufacturer and see what you can get. I have several books with varying degrees of information on the subject, and I still don't have a complete picture in my mind of how to account for everything that needs to be accounted for.
 
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