About Propellers Other Products Articles Bookshop Gallery Links Contact Supercool

Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


The Prop Doctor is all a-flutter


Wow, what a busy month it's been; I better prescribe myself a pick-up. Ah, that's better, good ol' VB. There is almost too much to report. My new F5B (that's a weird R/C electric class using folding props and 1600 watt motors) prop threw a blade on launch. The poor guy holding it copped blood blisters under every fingernail as a result: hope he doesn't sue. My new Unlimited scale 23X30 prop hit over 220 MPH: landed with one blade ready to drop off. Perhaps I should write about prop failure modes.

I would too, except somebody suggested my super-thin F2A prop was too thin and likely to flutter. Hell, he could be right: somebody has to be the bunny, I don't own a $900 FAI motor to do these tests. You, dear readers, are my unwitting test pilots. And, like Uncle Sam, I reckon test pilots are expendable. So what is the story with flutter? Well, if you fly F2A and haven't had prop flutter, you are just not trying. Tilleys' law states that "the thinner the prop tips, the faster the airplane". Hell, he's a big tough guy, I'm not going to argue with that! And my props are very thin at the tips, you can depend on that, too. Such props are very likely to flutter while they are run up on the deck. The phenomenon is "stall flutter": you'll know if you've got it, the noise is like banging a Scotsman around in a 44-gallon drum. But in F2A, the RPM on the deck are usually low, as the motor does not get on pipe until the model is airborne, a condition where stall flutter is unlikely to occur. Still, you can get flutter when airborne due to vibrations in the prop, and vibrations coupled into the prop from the engine. These latter are unpredictable, you just gotta get out there and fly. However, there is an interesting equation for flutter due to propeller properties alone. The maximum flutter speed Vf is given by:

Vf = K * c/R * SQR((t/c) ^ 3 * G / (p * (Xcg-.25)) 

Here, K is a constant dependent on blade plan-form (taper, tip rake etc.), c is mean chord, R is prop radius, G is shear modulus of prop material, p is air density, and Xcg is the X abcissa of the c/g position expressed as a fraction of the chord.

Now don't be put off by this equation: it's quite useless to you, so I won't be testing you. However, it says some very interesting things.

* If you increase the chord, you raise the flutter speed
* If you thicken the prop, you raise the flutter speed
* If you toughen up the material, you raise the flutter speed
* If you mass balance the prop, you raise the flutter speed.

Huh, what was that last one? That last term (Xcg-.25) is very interesting indeed. It says that if the c/g of the propeller airfoil is at the quarter chord line, then the prop won't flutter! Hey, that's radical! This is why helicopter rotor blades have a very heavy leading edge: they are made so that they balance at the quarter chord position. With such narrow chords, rotors would have a very low flutter speed if it were not for this feature. The same applies to wings, when the condition is referred to as "mass balance". Some wings, and many control surfaces, are designed with this in mind. 

Indeed, my light, hollow, carbon 32X12 aerobatic props are moulded with massive fibreglass leading edges, rather than with spars. With the mass balance condition met, very little material is required in the propeller skin.
Unfortunately, F2A props are too thin to make hollow. Xcg for F2Aairfoils lies at about .4 (40% of chord back from the leading edge), so they are not mass balanced: hence the concern about flutter. So what to do?

We can't increase the chord, since then the prop is then excess load for the engine.

We can increase t/c, but not at the tip. The thickness-to-chord ratio up to 3/4 radius can be as much as 13%, but this must drop to 4% at the extreme tip if thrust losses due to shock-waves are to be avoided. So what is left? The thing we haven't said is that the flutter is primarily due to torsion: that is, twisting of the blade (not bending). The parameter G, in the equation above, really refers to how stiff is the propeller material. In the case of the carbon fibre F2A prop, stiffness depends heavily on the carbon-to-resin ratio, and the orientation of the carbon fibres. Careful lay-up to minimise resin content is essential.

Diagonal-weave carbon cloth, laid in the prop surface, front and rear, places the fibres directly in the torsion field. This greatly increases torsional strength, at the expense of bending stiffness. This is the same principle as geodetic structure in wings (Vickers-Armstrong Wellington bomber, for example). 
That leaves parameter K, the blade plan-form parameter. If you make a prop with swept forward tips, it will flutter horribly: that is, it is flutter divergent. If, on the other hand, the tips are raked back, the effect is to dampen out the flutter.
So if you are mad enough to try Supercool Kerr03 F2A, you will find it is really very thin, and very bendy. But just try twisting it. It also has the raked anti-flutter tip. The main worry with this prop is that it is thin all the way to the root. It could fail in torque or centrifugal modes, but who knows without running it?

That leaves F2ACW001 F2A prop. It is the same as Kerr03, but thickens up considerably inboard from the 80% radius. This, plus its dedicated counterweight feature make it the prop of choice at this time. At least until you bunnies out there get hopping!

To return to stall flutter for a moment: if you get flutter on the ground with one of my C/F props, just ignore it and release the model anyway. The flutter will cease when the airplane gains some speed. The props are very tough and will not fail due to this flutter.

You may be aware that aluminium props are banned from use on models. Aluminium is not so tough. To finish, I quote from Den Hartog (1934, Mechanical Vibrations).

"Since the introduction of aluminium-alloy propellers in airplanes, a number of fatigue failures have occurred. Some of these were noted in time to avoid 
failure, being seen in the form of cracks, but in other cases either an entire 
blade or the tip of a blade has blown away in mid-air. The fact that these 
failure were unmistakably due to fatigue makes it certain that they were caused by vibration." 

 Back to Top