Not mass balanced & cable failure

[SteveJeff4567]

I found that many certified LSA aircrafts and some standard FAR 23 certified don't have mass-balanced surfaces. I'm interested in how they are designed to show freedom from flutter, especially control surface flutter and if these aircrafts can handle a disconnected freefloating surface emergency (due to cable failure, for example).

If they are not prone to develop flutter in normal operation, how things may change if, for example, the aileron cable fails?

 

[Sparweb]

I believe that the limited speed of LSA's is the biggest factor in avoiding flutter (and may have something to do with how the 120 knot speed limit was determined when the FAA set out the rules).

As for FAR 23 types, can you offer an example? I can't think of one without mass-balanced ailerons/stab/rudder right now. I admit that I've only flown 172's and DA-20's, and my work on FAR 23 types includes only a handful of twin-engine models.

I remember getting into a "advanced ultralight" (Canadian equivalent to LSA) and accompanying the pilot on the walk-around. THAT was an eye-opener on what is and is not important when you can't fly faster than 80 miles per hour.


STF

 

[SteveJeff4567]

You are right, the limited speed of LSA is a big factor to avoid flutter in normal operation, so it may not be necessary to have mass-balanced surfaces. In normal operation, the aircraft was flight tested for flutter and also in normal service there were no flutter occurences.

However, the question is, how things change in a cable failure in these aircrafts? Even if it's a very rare occurence, from time to time cables may fail. I'm wondering especially about an aileron cable failure, when the aileron is disconnected and freefloating, how prone is to flutter? Is this scenario, is it at risk to (violently) flutter?

As about FAR 23 aircraft, for example, American Champion series (Scout, Citabria..) have no mass-balanced surfaces, especially ailerons.

Also, about the "advanced ultralight" (Canadian equivalent to LSA), one of the airplane I use to fly was designed and certified on these standards and I can say that the speed limit is not that low as you said 80 mph. The cruise speed is 110 mph and the max cruise speed, Vno, is about 120 mph and in other LSAs it may be even a bit higher.

 

[Sparweb]

I haven't thought much about the subject since school, but there are several cases of "stick-fixed" stability, "stick-free" stability, control authority, and so on, to be evaluated in a comprehensive aircraft design. The flutter problem can be studied within these boundaries, because (I think) the information derived from the stability/control analysis will inform the flutter analysis. Then again, if it's an ultralight or LSA, the designer may rely on tests alone. Since flutter is a dynamic scenario, and aeroelasticity is hardly a first-year science subject, all aircraft are subject to tests to investigate flutter tendencies, no matter what category they're being certified in. In the area of flutter (or aerodynamics), I know much less about the evaluation and certification of the whole aircraft than I do about modifications to aircraft, when antennas/doors/science instruments etc are attached to the exterior. These are invariably tested in flight. Perhaps someone who has more familiarity with aircraft control certification practices will be able to confirm or correct what I've written.

The ultralights I was in simply could not go any faster. It was physically not possible with a 80 hp engine. It wasn't rules limiting the speed, it was Thrust/Drag ratio. Diving didn't help much.

In your research, have you figured out if the LSA's on the market tend to be just "book limited" to 120 knots, or similarly limited by power and drag, so they can't go faster even if you try?


STF

 

[SteveJeff4567]

They can go faster but usually it's not a good idea to fly close to red line on the airspeed indicator. Two of the aircraft I use to fly have a pusblished never exceed speed of 130 kts and the other one aircraft 146 kts. In a high dive with full power you can achieve these speeds.

 

[WKTaylor]

I have seen elevator and rudder surfaces which were aerodynamically balanced. The surface over-hangs the end of the stab and protrudes forward. The forward section provides counter forces for the trailing section... and usually forms the tip.

NOTE. a large-enough overhang with heavy mass [forward] might be a sneaky way to add booth aero and mass balance.

My Dad built a Thorp T-18 as I was growing-up. One thing I never understood was why the stabilator and ailerons required heavy lead masses for balance, but the rudder had NO added mass for balance. I finally asked the designer John Thorp. He smiled and said it was aero and mass balanced: look at the design! I reviewed the drawings and finally realized the surface aft of the hinge line was all made from O-too-thin-0.016 skins, ribs and bent stiffener-angles. The spar, nose-ribs and the forward [nose] skins were all made from thick [0.062?] sheet metal and protruded quite-a-bit ahead of the hinge-line. This design not only provided a rigid/strong rudder: it was elegantly balanced “by cleaver design”.



Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[berkshire]

Certain Sailplanes do not have mass balanced control surfaces, and from time to time there have been recorded instances of these aircraft taking off with one control rod disconnected.
There was an incident with the Glasflugel Libelle sailplane, where after a pilot took off and landed again quite quickly with one aileron disconnected,( he experienced the wing flapping up and down with gust loads while the aileron was free floating), the manufacturer later issued a service bulletin calling for the surfaces to be balanced with lead strips along the leading edge of the control so that if somebody else did it, it would be a little safer. I also owned and flew a Dutch built sailplane that flew for close to 15 years with no problems with a redline of 146kts with unbalanced ailerons ,however the Dutch government issued a service bulletin requiring Mass balance weights be added, following an incident with one machine in 1985.
B.E.

 

[SteveJeff4567]

where after a pilot took off and landed again quite quickly with one aileron disconnected,( he experienced the wing flapping up and down with gust loads while the aileron was free floating)

Well, it got fluter, right? Thanks God it was not fatal.

 

[berkshire]

It was not divergent and did not increase in amplitude.
B.E.