8178

My Feedback: (17)

If right thrust is so important to correct the spiral airflow impact any ideas why the P-51, the best propeller driven aircraft ever, has zero right thrust?

aspectratio

Mesae,

Thank you for acknowledging my Bernoulli idea. And thank you for the contribution. I had not thought of it before I read this thread.

You are not the first person to accuse me of being stubborn, and probably not the last. There is no question that you have more stick than me, full scale and otherwise. I consider myself an advanced intermediate pilot. On the other hand, many years ago I received college credit for four semesters of math beyond college algebra, three physics courses, and some mechanical engineering, and some philosophy. Just last December I changed a bicycle tire, and I had to play with the gyroscopic effects one more time. But, sometimes, I cannot help but think in terms of force vectors, vector algebra, moments, and angular momentum.

This forum seems to be working well. I do not want try to add any new technical terms if possible. I prefer the thought experiments, and the testing of a hypothesis with an experiment.

Philosophically, I am a firm believer in questioning one's beliefs. [sm=idea.gif] Uh, even that one.

I appreciate the contributions.

[sm=thumbup.gif]

aspectratio

steck79,

I have been thinking about this whole subject, and now I am beginning to wonder if there there might be a reason to believe that there should be a fast turning slipstream vortex, somewhere behind the airplane. Maybe the wind sock on your antenna is a clue is a clue to something after all.

Can you find anymore evidence that the air is rotating there?

britbrat

"The best propeller-driven aircraft ever", is a fairly wild & irrelevant statement.

However --------- with fighters, stability was (& is) a secondary consideration to agility. Right thrust can inhibit maneuverability, or introduce tracking problems at varying power settings. It was generally not part of the package in fighter stability fixes.

Most WWII fighters were a dangerous handfull at high AOA &/or at low speeds with sudden power application. The Luftwaffe lost more Bf 109's in take-off accidents than they did to enemy action.

The Griffon-powered Spitfires were completely unmanageable with sudden full throttle on take-off roll.

The Corsair was another killer with injudicious throttle application during take-off, or a missed approach.

8178

My Feedback: (17)

Do you know of any full scale aircraft with right thrust? I have a friend with a Yak 52 and I do not think it has right thrust.

britbrat

I don't

Oryx

What most WWII fighters do have is an asymmetry in the vertical tail assembly. In the case of the P-51, the leading edge of the vertical tail is offset about 1 degree to the left if I remember correctly. Most WWII fighters compensate with an offset to one side or the other like the P-51 example. The direction of offset obviously depends on the prop direction. An alternative method is employed for example in the case of the Messerschmitt Bf-109: the asymmetry is built in by using a cambered vertical tail where one side of the vertical fin is thicker than the other side. You can pick up any high quality drawing (such as those by Arthur Bentley) if you want to confirm these offsets/asymmetries.

The fin offsets on WWII aircraft have the same purpose as the side thrust offsets of our models. Of course, the asymmetry is not enough to compensate for all the effects when you suddenly throttle up, or when you rotate the aircraft, so you still need to use a lot of rudder during take-off and landing on the fullsize aircraft. However, what the offsets do achieve is that the rudder end up approximately center when the aircraft is at cruise speed with a cruise throttle setting.

Of course irrelevant to the discussion, but that is a pretty wild statement

Oryx

Just for clarification: with offset I mean an angular offset, equivalent to incidence on the horizontal tail.

mesae

The Extra 300L is one. There are many others. This picture is from an Extra 300L maintenance manual, from Extra's website. There does not appear to be any fin offset. I shrank this drawing for uploading--it's much clearer natively.

Also, aspectratio, it's perfectly all right to look at this in terms of force vectors. That's the only way it makes sense anyway. Pardon the shocking crudeness of the drawing; I'm not at home near my new copy of TurboCAD Pro 11.2.

stek79

aspectratio,
I understand your point... me too actually I'm thinking that, but I remember that little windsock... I will try the experiment when I'll try my friend airplane

mesae

What would cause the rotation to accelerate?

stek79

spiraling slipstream? At least the name suggest that

sky5jump

The slip stream elongates with increase in speed and becomes less of a factor. when flying slow the stream will have the greatest effect. Use to say to students think of it as a slinkee being strached out.
Also most small general avaiation aircraft that I know about have an off set engine.

sky5jump

Looking at the first statement here. Right thrust was not intended just to combat spiraling slipstream. You also have P factor. When the plane is in a climb attitude the downward thrust on the blade has more pitch then the side traveling up. ( Yuou will notice a change in pitch if you look at a prop from the side then raise it to a climb position) This will cause the plane to pull to the left.
Also you have a oppisite reaction to the twisting of the prop. (most turn to the right from the cockpit) This wants to twist the plane to the left, in effect lifting the right wing and wanting to turn the aircraft left.
An with you tail dragger , you must remember Gyroscopic pression. When you take a spinning disc (The prop) and move it from its plane of raotation you will get a resultant force 90 degrees from the point of roatation. Meaning you have a tail dragger on the ground and on roll out you raise the tail. Applied force is to the top of the roatation Lets call it 12 o:clock. As this force is applied there, your re****ant force will be 90 degrees in the dirrection of rotation. Witch will be on the right side of the prop and cause the airplane to want to turn left. Same effect when you do a landing flare.

mesae

Right thrust is not used to correct for Gyroscopic Precession, since it works in different directions depending on which way the airplane is rotating. It tries to yaw the airplane left during a pitch down, but it tries to yaw the airplane right during a pitch up. During left yaw, it tries to raise the nose, etc. That is why flat spins tend to work better when spinning left. Indeed, many airplanes that flat spin to the left, cannot flat spin to the right (when upright). The opposite would be true for negative spins. Some so-called 3D planes have enough pitch authoritiy to overcome precesstion in a right positive spin, to raise the nose and go flat.

stek79

Well said msae, and I may add that right thrust is not there to contrast P-factor also, for the same reason: when flying upright, there is a momentum towards the left due to P-factor, and thrust is towards the right: so far, so good. But if we are in inverted flight, P-factor still generates a momentum towards the left, but now also the right thrust become left thrust!

This is true at least in an aerobatic plane, perhaps it could be used to contrast p-factor in non-aerobatic planes.

aspectratio

Mesae and stek79,

As dick Hanson pointed out that there is not much, if any, thrust generated near the hub. It is air moving over the cowl louvers that is “sucking” air into the intake. We are back to the Bernoulli Principal again.

8178's experiment with the fan points to a sort of “hollow tube” effect, but dick Hanson’s pusher props indicated a tight a tight spiral downstream. This got me to wondering. If you look at the spinning prop near the hub, it is acting something like a centrifugal pump. It is rotating and flinging incoming air coming outwards (centrifugal force is not really an accurate term, but lets not get into that, yet). Farther out, the prop is acting like a flying wing with an airfoil and creating “lift,” which is moving air back, as well as rotating it some because of drag. The tips of the propellers are moving at high velocity.

Since props have an airfoil like a wing, there is a pressure differential across the surfaces. The result is, for lack of a better term, a spiraling vortex coming off the prop tips, similar to the way wing tips produce a spiraling vortex. This is more acceleration of the air, but this air is not accelerated around the propeller’s axis of rotation as much as into high velocity eddies that have an axis of rotation perpendicular to the prop’s axis and parallel to the arch of the prop.

My point is that not only is the air accelerated because it is rotating and moving back, but it is also getting whipped up into a froth of high velocity eddies. In other words, the prop is producing all kinds of acceleration in the air The air behind the prop is a virtual “bubble” of high velocity, low pressure air, as compared to the surrounding air. This bubble would be somewhat larger than the prop diameter, and it would exist in front of and behind the prop. The question is, what happens to it?

Most “bubbles” tend to pop, but this is a low pressure “bubble.” The high velocity eddies are going to slow down quickly because of friction. Somewhere behind the propeller, this mass of low pressure air is going to collapse. When that happens, if it has even a small rotation left in it, it is going to gain angular velocity (conservation of angular momentum). It is going to act like water going down the drain in a bath tub. The rotation of the earth is enough to cause a spiraling vortex of water in a bathtub drain. It should not take much rotation in the air to lead to a tight spin as the diameter of the low pressure mass decreases to nothing.

I think it is reasonable to expect that the air ought to tighten up and spin somewhere behind the prop. That is the hypothesis, anyway.

sky5jump

The right thrust is not added in small GA planes just for slip stream or P factor, but to over come the 3 most basic charteristics of an airplane wanting to turn to the left. An of course as you change the angle of climb this will effect all the forces involved
The slip stream does spiral around the aircraft and at slow speeds the slipstream is tight and close together as it slips around the aircraft it will make contact with the virtical stabalizer forcing it to the right and turning the aircraft left. As te aircraft increase speed this spiraling mass of air will elongate out behind the aircraft and have less effect on the virrtical stabalizer

stek79

Also, when the plane is fast there is much more damping force by the fuselage itself, which tends to a stable equilibrium position - i.e. parallel to the airflow velocity vector.

So in GA right thrust is put to overcome P-factor also? I mean, some manual explicitly states that?

mesae

Right thrust is added (if it is added) to get the plane to fly straight at cruise (by far the most common flight condition), regardless of the force(s) that would otherwise make it fly crooked.

mesae

A few problems: The pressure behind the prop is HIGHER than in front, and higher than ambient, until equilibrium is again reached. The amount of thrust generated depends on the pressure rise (delta p) times the area of the propeller disk. The Bernoulli effect is seen at the front of the propeller disk, where the pressure is lowest.

Why would you suspect a decreasing pressure differential to cause more air movement as it dissipates? Decreasing pressure differentials are approaching equilibrium, and momentum change is decreasing.

With the bathtub vortex - you seem to be ignoring the sustained pressure difference set up by the drain itself that sustains the vortex. I see no applicability to the propeller, except that they rotate like tip vortices, which decrease in intensity with time and distance from the energy source - this effect has been measured by NASA, and agrees with theory.

You are invoking magic again.