Balancing

[Mat]

If your plane is balanced in the middel where the fuseilage is but the is a mass at one wingtip and the mass is distributed differntly in the other wing could that effect it in stunts in flight.
Would the acceleration while rolling be different on each side?

[Mike Taylor]

No. It won't matter.

Balance is a matter of weight. Weight is a matter of mass x acceleration, and the earth's gravitaional acceleration (32' per Second) is already accounted for in the balancing. Doubleing of halving the acceleration would make no difference...

[vintage1]

Can make an internal diference. A wing with a lump of lead at the tip and another one with a larger lump of lead in the middle will 'balance' as far as static weight goes, but as you start to roll it, the forces in the two wings structures will be very different.

[Mike Taylor]

Vintage,

If that is correct, how can a single balded prop on a speed model work? It has a long light side and a lump of lead inside the spinner to balance it, rotates at very high speed, and yet stays in balance.

[Mat]

It's probly the centrifical force at a spcefic rpm give an equal pull on both sides.

[BMatthews]

Vintage,

If that is correct, how can a single balded prop on a speed model work? It has a long light side and a lump of lead inside the spinner to balance it, rotates at very high speed, and yet stays in balance.

It may be in static balance but from what I've heard talking to folks that use them is they shake like a paint mixer thanks to the off center thrust of the single blade. But they make the model go faster so they don't care.

I've also used single bladers on rubber models for 3 of my old timers. None of them worked smoothly through the whole motor run. The angled back balance weight is supposed to offset the thrust but in operation there's one or two rev speeds where it all matches and the rest of the time the model wobbles and the landing gear shakes like it has palsy.

.... I hate them with a passion....

[slipstick]

Vintage,

If that is correct, how can a single balded prop on a speed model work? It has a long light side and a lump of lead inside the spinner to balance it, rotates at very high speed, and yet stays in balance.
It's relatively easy to balance a prop like that for a fixed rotational speed. As with most out-of-balance problems you mainly get into trouble during acceleration not when operating in steady state at the design speed.

OTOH single blade speed props have been know to shake themselves, and/or the model, into many pieces if the engine tune goes too far off and they are asked to rotate at the wrong speed :(.

Steve

[Purdue Aero Man]

Vintage,

If that is correct, how can a single balded prop on a speed model work? It has a long light side and a lump of lead inside the spinner to balance it, rotates at very high speed, and yet stays in balance.

Vintage is correct. Assuming you model the lump of lead at a wingtip as a point mass (or something failry small), the rotational interia of that mass is I = mR^2. So on one wing you have a mass of 2m that is fairly central to the half span, yet on the other wing half you have a mass of 1m that is located at or near the tip, resulting in a roational interia that can be up to twice as great on the second wing half. A huge difference in wing stress when you're performing rolls? Perhaps not. fo an aerobatic airplane, quite possibly. Remember, a lot of things work well in theory, but people die out there every day.

[Mike Taylor]

OK, let's look at another example - a centrifuge. These have a long, light arm and a short heavy-weight counter balance. These work at any speed.

In the example of the single-bladed prop, any wobble or such is the result of the un-equal thrust loads on either side (more on the prop side, none on the no-prop side) or the prop not being totally balanced, or not being balanced in regards to the for and aft dimensions.

Another example - Tires on cars. Without wheel weights, new tires are out of balance due to heavy spots, primarily in the tread area - their 'wing tip'. The wheels are spun or in really primitive set-ups balanced on a teetering point with a bubble-level. Expensive balancing machine measure both the inside and outside so weights can be properly placed. My SS goes to a good tire shop where they balance my wheels correctly. There is no shake anywhere from 0 to 155 mph where the computer start dropping spark to the cylinders as a speed limiter. If there were a 'critical rpm' they would shake somewhere in bewtween those two extremes, probably rather violently, but they don't. BTW, it is an SS Camaro and, yes, it does get to go that fast...

[Mat]

Wouldnt the prop thrust not be in porportion with the rpm because, depending on a number of factors, the prop might just start chopping up the air or the airfoil pulls the prop differntly and the drag would change.

[BMatthews]

Wouldnt the prop thrust not be in porportion with the rpm because, depending on a number of factors, the prop might just start chopping up the air or the airfoil pulls the prop differntly and the drag would change.

.....er... howzzat again?

Props work best with the less blades they have. I don't have it in front of me but it's a proven theory. Frank Zaic did a big write up in one of his yearbooks about it with math, numbers and everything. But while technically a single blade prop is best the reality is that the troubles with balancing the weights and thrust make it more trouble than it's worth. Two blades is the best you're going to get for general use.

So why all the multiblade stuff on so many high speed warbirds and the like? Simple.... landing gear limitations. You can't make a two blader big enough and geared enough to avoid the mach number problems without requiring stilt like landing gear that will be far too long to house in the plane. Note how as engines got more powerful the number of blades went up to match while the diameter only went up a little.

[vintage1]

What I said was not that the prop as a wole would be out of balance (or wing) but that the internal stresses on each side would be different.

In the case of the single bladed prop, its easy to see that the tensile stresses halfway up one blade are eniterly absent from the other :D

If you get a thin bit of piano wire and stick a blob of clay on one end. and balance it at its balance point and then wobble it up and down, its in balance yes, but all the flexure will be on the long floppy end.

I have no idea what differemnce that all makes to a plane, but assymetry about the roll axis that is detectable by a simple test like that cannot but degrade performance in a precision aerobat.