Less than 100 % Static Balance

Working on my drawings for the wing and flaperon, and I discovered that there is no way the static balance weight shown on the original plans will 100 % balance my flaperon. One of the other builders said that it was not 100% balance but for flutter protection.

Any thoughts/comments? Is this sometimes done like this? The difference is significant, as in 5 in/lb about the hinge vs 40 in/lb about the hinge.

 

Flutter generally needs 100% balance, so partial balance for flutter prevention seems a bit odd. On the other hand, a proven design is a proven design, however it was arrived at.

 

100% balance is rare. Cessna's, Piper's, Beach, none of those have controls balanced 100%

 

Going all the way back to ANC-12 (Published in 1948):-

For airplanes with limit diving speeds below 260kn (300mph) equivalent airspeed, complete static balance is not often necessary. The allowable amount of unbalance is often determined by comparison with previous aircraft or by calculations in accordance with ANC-12(3)

So total balance is certainly not a requirement for slower aircraft. I'd aim to get the underbalance close to the original design otherwise you are in uncharted territory.

 

A full span ultralight flaperon might be torsionally floppy, because of length, and need some tuning?
Most ultralights have no control weights at all, as far as I know.

 

If it's a bit floppy, then the ideal balance for flutter prevention is less than 100%. This is probably a really good time to stick to the plans...

 

As pointed out earlier, it is actually quite common to have less than full mass balance. It is not an all-or-nothing proposition.

Also, it is not unheard of for increasing the mass balance to actually reduce the flutter margin instead of increasing it. At issue is that adding mass at the hinge line can make the wing itself more susceptible to flutter even as it inhibits flutter of the control surface.

Somewhere on the interwebz is an article about the complications encountered while mass balancing the ailerons on the early Loudenslager acro machines. Interesting reading.

--Bob K.

 

This 1955 paper has some empirical criterion. If I recall, you may have to know some vibration frequencies:

http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA955270

 

After this SS-1(aka "Squirrel") elevator fluttered I went 100% without hesitation...there's something about recently surviving a flutter event that wants one wanta go whole hawg, right or wrong. So far it's OK but the PIC is wary...there may be some flight condition out there that wasn't tested. Dunno, but assume so. In the past I've been bad about pushing the limits but not on this thing anymore. The redline is sacred.

Somebody talk about the difference in control surface flutter and control surface flapping.

 

Flutter can occur in more than one mode. For example wings can twist about the elastic center or simply flap up and down like a bird. Especially with long floppy wings like a sailplane. There was photos of a glider during flutter testing (in Soaring magazine decades ago) a Libelle, I think. The wings were obviously contorted with several waves just like a stretched slinky shaken from one end. I think it fluttered in three different modes.

 

Well being that the original is wood with tow for reinforcement, (that has got to be fun to calculate) and mine is aluminum I am sort of more than a little off grid. I am trying to maintain the exact weight/profile, but the stiffness, stress, detail structure is different. I estimate the weight will be a little more too. The plan is to stick to the given mass balance location given the answers here......thanks all.

 

I hope I do not have problems, and I hope I do not have to do a vibration analysis, but it is nice to know where to look. Thanks

 

Quote from a discussion on the T-18 forum: "Keep in mind that the purpose of balancing the ailerons is to change the aileron's effective cg in relation to the center of pressure and the hinge line so that a gust causes the resultant forces to try to translate the aileron up or down at the hinge line (hard to explain without drawings) instead of rotating. The net result being to prevent the onset of flutter."

John Thorp, in another conversation, responding to the question: “To what extent should we balance the aileron – 80%, 90%, or 100%?”, John’s answer was “80% is a good ball park figure.” As related factors may combine to cause a 100% balanced aileron to go "divergent" (i.e. - flutter)

Another Technical Reference is NACA TN-609

 

Flutter is the aero-elastic coupling of two vibration modes - they, in a simple sense, feed each other. Divergence happens in a single mode. They are calculated much differently. You can have flutter at speeds much lower than the divergence speed.

 

Here's the 1980s draft rewrite of the 1955 Report 45. It has some interesting comments at the end:

https://www.scribd.com/document/244996641/Flutter-Prevention-Criteria

 

yes it is for the Aluminum Dragon, Best L/D reported to 25-30 mph. The other builder said and now I remember,...."Flutter at speed" and I think would be referring to dive speed, which I believe is close to or at Vne "never excede" speed, in smooth air, which is 60 mph.

It will be interesting to see if this "micro lift" thing works out, pick a speed at which to have zero loss, and fly around.

 

Minimum would be careful flight testing at small speed increments. Preferably with 'plan B' strapped to your back.

 

Remember that you need to test to Vd, not just to Vne, and Vne is defined as 0.9 * Vd, so 90% of whatever you test to.