AA5BY

With no current project on my boards... I'm thinking of doing some test to see if it can be determined where prop induced yaw force is manifest. Thoughts are to build a mock plane test stand that will rotate on a vertical axis so as to respond to yaw forces.

On it, several fore section and tail fin profiles could be tested.

I'll provide pics of the test bed and possibly videos of test to illustrate any thing observed.

jester_s1

Neat project, but there's a problem with your method. Left yaw (I suppose you're talking about on takeoff?) happens while the plane is in motion. Landing gear drag is likely a factor in it, and likely the effects of spiral slipstream as well.

David Bathe

Very exciting, certainly worth going for.

AA5BY

Yeah... no doubt there are limitations. My test will be primarily to determine if there are yaw forces induced on the fore section and whether they are affected by vertical symmetry of the fore section.

The plan is to use a Ryobi conversion engine.

AA5BY

Actually, my thoughts are not so much about takeoff yaw but rather the yaw seen when going vertical. It gets greater as the plane slows so it seems to me that slip stream forces might in fact deter a static condition that exist. My desire it to determine if there is a static yaw force on the fore section and the affects of forward symmetry. Tested will be a symmetrical fore section, one with a high thrust line and finally one with a low thrust line.

My theory is that a symmetrical fore section does not suffer a fore section yaw force but that an asymmetrical one does.

David Bathe

The yaw you see on take off is the same yaw you see in vertical.

da Rock

How are you going to cull out the force from gyroscopic precession from the force from p-effect? How would you know how much force was from which?

AA5BY

I'm not sure there would be any P-effect given no forward movement and without such, there is no angle of attack or variance between ascending or descending prop blade. I confess little understanding of gyroscopic precession but think it is something like a 90 degree reaction to some movement.

To be honest, if some other force yaws the fore section beyond variances in pressure from one side to the other, I doubt my test could distinguish such. If however, changing the thrust center line from high, center, and low produces differing yaw effects, I'd think that would be significant.

To restate my theory, I'm thinking that the prop is an air compressor producing a high pressure zone immediately aft of the prop. If the pressures should have variances between the sides of the fore section, then the normal migration force from higher to lower will yaw the fore section.

pimmnz

Think of the prop blades as wings. It is not a pump, the air pressure around the blades is lowered, compared to ambient, it's just that it is lowered more on the front side than the rear, and the thrust (lift) is the result of the pressure difference. Moving air has a lower pressure than the stationary air around the moving mass. The 'yaw' you will find will be the result of gyro precession, once the prop is turning there will be little 'yaw' as the gyro (prop) will tend to resist motion. There will be a considerable reaction when you try to change the position of the prop disc, and always at 90 degrees from where the force is applied, in the direction of rotation, think like riding a bike, and turning. If you try to turn the handlebar, you will depart the machine, but if you just lean the gyros (wheels)...You will have one other force to deal with too, and that will be torque, the reaction on the engine mounting to the the force required to turn the prop. The combination of the two could lead to some interesting gyrations(!)...
Evan, WB #12.

KnifeEdge540

Is this only seen in Propeller propelled aircraft? Also, do pusher prop type airplanes experience any relalated force?

da Rock

There is also the yaw produced by p-effect produced by the prop when it encounters airflow that is not perpendicular to the blades rotation. The most talked about result of p-effect is when the aircraft pitch is nose up on take off. The blades encounter a greater AOA on one side of the disc and less AOA on the other. In fact, whenever the prop disc encounters any airflow that isn't perfectly parallel to the propshaft there will be pitch and/or yaw forces.

David Bathe

I don't know if P effect actually plays any real part on our models.
95% of which are over powered (power to weight) than a full size.
Any direction we tend to point our planes from 1/2-1/1 throttle (slow moving scale types excluded) the direction of airflow tends to be 90deg to the prop. I've whitness the P-effect in full sized in a a slow gradual climb but it wasn't a "dramatic" event.
I don't think we maintain that angle of attact long enough, or at a diminished power to weight ratio for it to be relevent.
Could be wrong. Just was always told to ignore it as an event by full size pilots that fly RC.

da Rock

The p-effect happens whenever there is a skid. Our models skid in every direction, probably 100% of the time. When they accentuate the skid, for example in a loop, is when we see the most effect. Anyone want to cry foul, that a loop isn't a skid? Yeah, it is for every part of the model that is fore and aft of the center of rotation of the model doing the loop. It just so happens the skid isn't left/right, it's inside/outside.

Any time we steer our planes, they skid along the axis of the turn. It's something you can't avoid when you steer through a medium like air and control direction the way we do. And if you think about it, our models that have right or down thrust built in really are skidding that prop even when we consider them flying straight and true. Can anyone say for sure those right/down thrust solutions work because of where the prop is pointing, or do they work because of where the resultant forces (including p-effect) are pointing.

David Bathe

I wouldn't know.

AA5BY

Evan... I'm unsure that there is a reduced compared to ambient pressure immediately aft of the prop. If reduced exist ahead, then it seems to me that the energy shift would produce higher than ambient aft because I do view a prop as an air compressor though I'm not saying that I'm entrenched to any way of viewing things as I freely confess no formal physics education.

I haven't started building the test stand yet but have been collecting thought about how to approach it. My work load will be heavy for a few weeks but a rain out day at work could provide the time and energy for construction.

AA5BY

Good question on pusher props. If my theory is correct, if there is no symmetrical difference of airframe aft of the prop, then just as I've noted on a symmetrical fore section Yak, there will be no yaw produced by pressure variances of the aft of prop zone. P-factor of course could be at play as with a forward prop.

AA5BY

I'd be interested to hear any flight reports detailing yaw when going vertical. Does it exist and what is the forward configuration of the thrust line? Does the fore section have vertical symmetry around the thrust line or is the thrust line high with an asymmetrical vertical fore section? I'd greatly enjoy hearing from anyone with a large WW-I model having a low engine thrust line or any model with a cowled upright engine displaying a low thrust line fore section.

Another example from my hanger is my Ultra Stick Lite. It has nearly a symmetrical form with the engine centered on the firewall and the belly pan angled from the firewall to the gear whereas the top section is horizontal. The engine also has a slight asymmetry with the jug inverted. The plane yields only a very small amount of left yaw that is corrected with the slightest touch of right rudder.

I confess to possibly barking up the wrong tree with this theory but it simply makes sense of my experiential observations.

pimmnz

Da Rock, in this experiment, with the engine on a test stand, there will be little place for 'P' effect. As for pushers, they exhibit the same reactions to props as tractors. And David is right, model airplane props, being small and high revving have little 'P' effect anyway. AA5BY, if there was a higher pressure behind the prop, then the airflow through the prop would be from the back to the front, yes? Air flows from high pressure to low pressure, always has. The proof that the air pressure around the area of influence of the prop is lower than ambient is that the air all round the prop is drawn towards the thing. Otherwise it would just churn away and the air would all flow away from the thing, and there would be no thrust. Sometimes the obvious might not be...
Evan, WB #12.

AA5BY

I greatly enjoyed your comments about skidding. As noted previous, I've a fare amount of time with sailboats, which have a few things in common with airplanes. One of them of course is yaw. Sailboats perform around a balance center and if there is a force that causes an imbalance (almost always to some degree) then the boat is skidding. The skid has to be corrected by the rudder at a cost of drag and those who race sailboats are quite serious about the effort to hold boat balance and limit skidding. There is also the side wind force that plays a serious part in skidding, though sailors call that leeway.

In the hunt to deal with both boat imbalance skidding and leeway, very creative things are done. Just one is to the use of asymmetric foils. One of the best examples of this is the Hobie 16 catamaran which has no dagger boards or fins to resist leeway. It uses asymmetrical amas (hulls) with the foils of the hull shapes having flat vertical outward form and curved foils on inward sides. When the boat heels under the wind velocity, the deeper leeward ama lifts the boat to weather and thus deters the leeward drift. It is very effective and Hobie 16's are witches going to weather without any dagger board or keel. The cost of course of having opposing lifting foils is extra drag when the boat is level in the water going downwind.

Another interesting sailboat issue is that when a sailboat (mono or single hull) heels, it causes an asymmetrical form beneath the water line. Historically almost always these forms yielded leeward lift (undesirable). Modern racing hull shapes have been designed that actually produce a lifting to weather asymmetrical shape though the greatest effort has been to use asymmetrical shaped dagger boards on each side of the ship and have only one immersed to lift the boat to weather (windward).

Not sure if any of this is interesting on this forum... and I'll leave it there.

AA5BY

Guess I'll have to sleep on this a bit to get my head around it. I think I understand that flow and pressure are two differing things coming off a prop. A low pitched prop creating more flow and a high pitched creating more pressure. If the pitch were taken to its limits where flow stalled there would be higher pressure aft but no flow and thus no thrust so I can see that it is not pressure that produces thrust, but rather flow. Yet, I struggle to think that the zone aft of the prop has less than ambient pressure.

Accepting that doesn't kill my theory that an asymmetrical fore section creates a variance of pressures each side of the fore section that causes yaw.

charlie111

I know with models and real aircraft that right rudder is needed on takeoff.It is also needed when recovering from a stall.When your airspeed is low.The (P) Factor comes into play.The air goes around the fuselage and hits the Vertical stabilizer on the lef side turning the aircraft to the left.A toe brake is needed until you reach the airspeed needed for control surfaces to work.Thats just the way it is.Don't ask me for the calcunations or Formulas.I can't help you there?

pimmnz

Forget the flow and pressure thing, the prop is a wing, the lift (thrust) is generated exactly the same way as any wing, from the pressure difference between the bottom and top of the blade. The air behind the blade is moving, and as the total energy of this mass is the same as the ambient mass and proportional to the volume, pressure and temp, then the energy of the motion has to come from somewhere, and the pressure drops and temp rises, to balance the equation. The reason air is continually drawn into the prop disc, is quite simply that the pressure of the air around the blades is lowered, and the air mass all round the prop pushes into the disc, you can check this yourself with a paper streamer held in front of the prop, or near the tips, and you can see the flow directions of the air mass around the props. As for the yaw, pushers do this too, so the fuselage thing can be neglected. Perhaps a better avenue for research is the effects of torque on the airframe, as a dynamic force which the airframe is constantly trying to balance.
Evan, WB #12.

AA5BY

My theory is far less about takeoff yaw, which to my way of thinking will certainly include P-factor induced yaw. The question is more about when a plane is going straight up. I can't envision P-factor being at play without an angle of attack. If that is true, then what causes left yaw on only some planes when going vertical? Why do some yaw grossly and others not at all?

AA5BY

Do pushers do it going straight up? Just trying to clarify we are talking the same thing. I can certainly understand a pusher yawing from P-factor during a positive angle of attack climb. Again, I'm talking about yaw when going vertical.

pimmnz

'P' factor, if measurable on models, only exists when the free stream direction is not normal to the prop. The situation is transient at best, then only for a few seconds. Why do models yaw offline vertically? Low airspeed and lots of torque, low airspeed means that the model cannot generate the opposing forces that balance the torque. Start you experiments there. And yes, pushers and tractors react the same.