Is there any aerodynamic reason for the long noses I see on stunt planes, is this just a way to achieve balance, or are we all just in the habit of making a good mechanical fit of the engine and tank without (horrors!) notching the leading edge of the wing?

Put another way, if I've got a heavy engine, is there any reason _not_ to shorten the nose, as an alternative to building it long and having to put weight in the tail?

I have always thought that the nose length issue was related more to moving the flywheel of the prop. I know that if I change the flywheel characteristic of the prop by going say from a EW to a W I can feel the difference. The issue then would seem to be mechanical and not aerodynamic.

The flywheel effect is a moment, not a force.  Where it happens on a rigid airplane doesn't matter.  

If the flywheel efferct remains a constant and the distance to the center of lift changes there is no effect? An honest question about levers and such because this is your wheelhouse. Teachable moment (doh)  and all that. Thanks.

If I'm reading your wording right, yes.

The flywheel effect, by itself, is a pure torque.  Torques act the same on a rigid body in free space regardless of where they are applied.

If there are any other effects from the prop (I think there's some lift to be had from a prop that isn't at right angle to the line of flight) then that'll show up differently as its lever arm changes.  But I have no clue how significant these effects may be (or even their direction) -- hence my question.

Gyroscopic effect = precession.

A 'pure' gyro has no damping -- damping would have to come from aerodynamic forces on the prop.

Maybe I should make some planes with different nose moments and give them a try.

Were your long nose racers balanced with long tails?  Long tails would tend to give damping on the corners, regardless of any propeller effect.

The flywheel effect, by itself, is a pure torque.  Torques act the same on a rigid body in free space regardless of where they are applied.

   Thank you!  Exactly correct, but so rarely grasped by stunt fliers.

     Brett

Deal with prop effects with a bit of down thrust. You still need enough room to mount the engine, and a large enough tank to get through the pattern. When you take away, or diminish the prop effects with thrust line adjustments, you're left with physical weight, and moment adjustments, needed to balance the plane fore and aft.

The terms "nose moment" and "tail moment" seem to be peculiar to stunt.  I remember hearing them as a kid and figuring I'd learn their significance in school.  I never did.  The standard definition of "moment" in physics (and engineering) is torque.  I figure that somebody a long time ago was making a list of stunt plane parameters and picked some lengths he could measure and named them "moments".  As to from where to where the "nose moment" is defined, you'd have to ask the guy who defined it.  I'm pretty sure it has no significance.  Tail length does have an aerodynamic significance, and is usually measured from the quarter (mean aerodynamic) chord of the wing to that of the tail.  I don't know if that's the same as "tail moment".

Gee Howard,

I didn't know Brett's company name was classified?  Guess I better not use it anymore...... 

Jim Pollock   

This is kinda interesting, because: a) if there were an aerodynamic effect, however small, Brett would be the one who could feel it, and 2) Brett has no doubt seen the data on it.  A guy at Brett's company, with whom Brett probably worked, wrote about a zillion papers on aerodynamic characteristics of airplane and rocket noses. 

    I doubt it was anyone I worked with - I have so far managed to avoid having to deal with AIR in my professional life.

    I am not sure who you would be referring to, and I haven't read many useful papers (aside from the VERY interesting but not stunt-related study about aerospikes and burning aerospikes on Trident missiles. )

      Brett

I could be wrong here, but I would venture to say that long nose moments are less difficult to pitch up or down when the are propped with less circ. Shorter nose moments seem to gain some stability from large props and in fact seem to fly off of the prop so to speak.

Practically, as John put so well, the significance of the nose moment for our purposes is simply a function of accommodating engine and tank for a 6 minute or so run.

Short nose big prop designs seem to turn differently...looks like the tail slides back and locks on the desired attitude and kind of pivots/ turns on the prop arc more so than the wing. Somewhat an illusion but you can see and feel the difference at the handle.

Tourque is a force  as in Lbs / in .

Moment a DISTANCE  . as in In.


So we CAN have Lb / in / in .

Or levarage  . Applied Moment .

Prop AND crankshaft are BOTH gyroscopes , and all the other little spinning bits , including the big end .

Wottl happen if we put a chap with a gyroscope in one oNASA's centrefuges . Better not've eaten ? !

Geez guys... my old man explained what a "moment" was when I was nine. In short..as Howard said it is simply a distance. That distance in relation to other distances on either side of a fulcrum is why they call it a "nose moment" or "tail moment" using the CG as the fulcrum or pitch axis I believe. If I am wrong I am sure someone will let me know.

Now, let's really go crazy and introduce the term 'couple.'

 

I do not read this section of the forum so often, so sorry for late input ... I will react to Brett’s comment, because it has actually just that what I want react to:

The nose moment makes NO difference aside from the balance and mass properties. The point he was trying to make is that any torques arising from the propellor rotation are the same regardless of the nose moment. It's clear that precession and p-factor are both real things but they have nothing to do with the nose moment.

This is mechanically ok, but if we speak about P-factor, and now I mean that which comes from side wind and which make pitching moment up and which we must balance by down tail moment, that IS depending on nose length ... just imagine, if the prop is exactly at tangent point, then it has zero side wind, but extremely long nose will cause side wind and thus prop will make up moment. However that difference is very small.

OF course the original question - aerodynamic effects - is also no (or at least negligible). Many/most of the important features of stunt plane performance have nothing at all to do with aerodynamics, although that about all we talk about.

Here I will not agree too much … the position of the fuselage to the flying path (or position of LO to the CG) is very important, and side wind can change it, and short/long nose can change proper balance if side areas, which can lead to improper flying properties in the wind … I know that in time when we tried outboard mounted engines and when we had extremely thin and short noses with heavy MVVS 40, we needed to install additional fins on nose, to extend its side area.

What happened to the old "enough room" for the engine and tank thingy like we use to do.

Communication seems to have broken down slightly over differentiating "pitching" vs "pitched." It is while the nose is rotating up or down (pitching) that the precession effect takes place - as Howard said. After it has settled at some pitch angle (i.e. is 'pitched'), there is no precession from pitching, although there is still precession from flying in a circle (yawing) that causes an upward pitching force. However, the other prop effect does take place when there is an aoa. We need to keep these separate.

Igor has a point about the nose side area. The air meets the nose from the inside of the circle when the plane is flying tangent to the circle at its c.g. However, I hadn't realized that the effect was that notiicable, considering the longer tail arm and its side/fin area, along which air passes from the outside.

SK

Duke moved the prop mass back. This would have reduced the polar moment of inertia of the plane, increasing its pitch rate. Moving mass rearward, even when it is increased to maintain the chosen c.g. can still achieve this, although the minimum turn radius will increase. Interestingly,...

If the plane balances with a greater weight on a shorter nose or has a shorter nose and shorter tail to balance it, then its polar moment of inertia can be smaller. This basically means that the plane can be pitched more quickly (higher angular acceleration). The moment of inertia is proportional to the first power of mass and the second power of the radius (distance from point of rotation); so the radius dominates. That is something of value to combat flyers, but usually not needed by stunt folks, whose planes pitch quickly enough to challenge reflexes, but also need some pitch damping to allow smooth recovery without overshooting, or overcorrecting causing point-losing wiggles.

For example, since for concentrated masses, the moment of inertia I = mr²,  we can change only two things: the mass or the radius ('of gyration'). Suppose our engine/prop has a mass of 10 mass units of some sort and its center of mass is located 10 appropriate length units ahead of the plane's center of mass.

So m₁ = 10, and r₁ = 10.  The moment of inertia contribution from the engine/prop, to be countered by the elevator's torque, is I₁ = 10 x 10² = 1000 units.

If we double the engine/prop's weight and re-balance the plane, we will find that it will balance with the engine half the previous distance forward of the chosen c.g.

So m₂ = 20, and r₂ = 5. The moment of inertia contribution from the engine is now I = 20 x 5² = 500 units.

By mounting even an engine/prop combination twice as heavy, we have cut the inertial moment in half. The plane will actually pitch quicker, even though it will require much more lift (higher aoa) for maneuvering.

So, in short, Duke probably had a reason for that. The shorter shaft moved the prop in and probably saved some mass. This would have made for quicker turns. As we've seen above, this can be done even with heavier weights. There is another related consideration - FWIW. For light engines like Fox .35's, adding shaft extensions may reduce the polar moment of inertia by moving the largest mass (engine) back, but it can and usually does move the c.g. forward, rather than back. I think a lot of classic stunters actually got heavier and gained turn radius due to the mistaken use of these devices. The point of c.g. benefit for extensions on classic sized/proportioned planes is somewhere around engine weights of 10 oz. 'but of course, "we don't need no steenkin' math."
SK

Hi Gang,
Nose length or moment arm is bad!
The nose should, as a practical matter, be just long enough to balance the plane and no more.

The prop is a destabilizing influence.
It produces a "sideward" force in the direction that the axis of prop rotation is inclined to the oncoming air.
This can easily amount to a similar effect as having an unintended canard surface of ~3% of the prop disc area placed where the prop is.
That requires either more tail or a more forward CG, and that's without any gyrosopic whatevers going on.

take care,
Dean Pappas