Not mass balanced & cable failure

[Steve Jeff]

Hi everyone here!

I'm a LSA and PPL piIot, working on my CPL, and I have some questions about the aeroelastic effects and flutter phenomenon. I would like to talk a little about aerodynamic flutter onset speed and flight control malfunction. It is known that freeplay, worn-out control rods or slop in flight control cables might induce flutter.

What I'm interested in is how critical flutter speed is affected by those problem. I'm wondering especially about a cable control failure where the surface would be disconnected and freefloating. There are small light sport aircrafts and even some FAR 23 standard certified aircrafts which don't have mass-balanced surfaces, especially ailerons which I guess would be more prone to flutter. How critical flutter speed lowers in a situation like that (and how prone to violently flutter are these ailerons in an emergency disconnected sitation)? Is there a linear drop in flutter speed? Can it reach even lower speeds in the normal operating envelope e.g. lower than Vno or Va? Normally, assuming no malfunction, flutter speed is at least 10% above Vne or Vdf, which is dive test speed used during flight testing. How do you think things might change?

The same about the others control surfaces e.g. a broken trim tab linkage.

I really appreciate your help. Thank you very much!

[pacerpilot]

The information your looking for has been compiled in great detail by NASA. The affect of worn parts on flutter is a function of how the component can withstand the flutter, not inducing it because of looseness. As for flutter at low speeds, I've never heard of any occuring. Also, don't confuse flutter with "flop". After you view some of the videos you'll understand more about it and see that we're not typically flying within a range that is anywhere near a problem.

[Steve Jeff]

The information your looking for has been compiled in great detail by NASA. The affect of worn parts on flutter is a function of how the component can withstand the flutter, not inducing it because of looseness.
As for flutter at low speeds, I've never heard of any occuring. Also, don't confuse flutter with "flop". After you view some of the videos you'll understand more about it and see that we're not typically flying within a range that is anywhere near a problem.

So, how do you figure out those accidents where a disconnected surface triggered flutter?

Also, don't confuse flutter with "flop". After you view some of the videos you'll understand more about it and see that we're not typically flying within a range that is anywhere near a problem.

It is known that at LSA speeds flutter might be less of an issue than at higher speeds, but what you said sounds like we are immune to flutter at LSA speeds and that's really not true. I would really appreciate to detail your statements a bit.

[WLIU]

Are you most interested in theory or practice?

In practice, the first step in dealing with a control system problem is to slow down. Get the airspeed down to landing approach speeds seems to work best. That reduces the forces on the control surfaces and system components. Lower airspeed also generally, but not always, means lower energy and lower frequency inputs to any component prone to flutter.

Typically only one thing breaks. So your next task is figuring out how to fly home with what you have left.

Example - With one elevator cable corroded or shot away (know of both instances), you can use trim to put the remaining cable in some tension so that you can carefully use the stick or wheel normally.

Aileron and rudder control failures are challenging as trim on those are much less common. But the good news is that one of these controls can compensate for a problem with the other if you can quickly learn to fly the airplane a little crooked. That said, the landing may be a little messy.

So rule #1 is slow down. Then move the control stick to see what works.

One interesting exercise is to try flying around without using one control. For instance, try not using the elevator control but instead flying the airplane down final using the elevator trim and the throttle. Plan a long, stable, final. Make small changes in power and trim.

In the aerobatics and skydiving world we get to explore some of these challenges. One pilot that I know was flying an Unlimited category aerobatic competition program at practice altitude when he broke his control stick off. Recovered the airplane using trim and carefully landed OK. A skydiver driver that I know had a parachute deploy into his tail, jamming the rudder and elevator. Got everyone out of the airplane, used the ailerons to point the airplane at a big open field, and used his parachute.

Best of luck,

Wes
N78PS

[Steve Jeff]

I'm interested in the practical outcome of a cable failure. I know those things, practiced them all, simulating them as close as possible, what I wonder regarding a cable failure is flutter, especially on an aileron which is not mass-balanced. That's why I'm asking. When you encounter flutter, the recovery seems easy: retard throttle and slow down as fast as you can, also you might try to load the surface, but I wouldn't think you have time to do that on such a destructive flutter onset the aircraft might be pieces in seconds.

[WLIU]

Many airplanes have rudders and elevators that are not mass balanced. My observation and limited experience is that because those surfaces are short and stiff they are very unlikely to flutter in a typical light plane, whether or not all of the cables are connected.

Ailerons are a different story. All of the ailerons that I have seen are mass balanced. Wings are longer and more potentially elastic structures. I think that I recall that Kitfox tried not having mass balance and ran into problems. So I will suggest that in the event that an aileron cable fails, the probability of running into a flutter issue is small if you identify the problem and slow down. And if the aileron does act like it wants to flutter, the appropriate action is to slow down.

With all of that said, we are talking about extremely rare events for your typical general aviation operation. I would not over-think this. You are more likely to be struck by lightning assuming that you maintain your equipment to expected aviation standards and do a real pre-flight inspection of the aircraft.

Hope this info helps. I will suggest that you should walk down the flightline of your airport, and walk into the maintenance shops, and look at how the different makes and models of airplanes are put together. Books are great but actually looking and touching fills in the blanks that books pass over. Each machine that you look at is the physical implementation of an engineer's solutions to the problems that you are thinking about. And each different make and model shows the thinking of different engineers or teams of engineers. You will see some great ideas, some OK ideas, and some bad ideas. All educational.

Best of luck,

Wes
N78PS

[Steve Jeff]

All of the ailerons that I have seen are mass balanced

Many LSA are not. Also, FAR 23, American Champion series (Scout, Citabria etc.) as an example.

[WLIU]

The balance weight is hidden in the leading edge of the aileron.

Wes
N78PS

[Steve Jeff]

Right. There are different designs on different aircrafts, some are outside you can see them, some are inside at the leading edge, and some are nowhere, cause they are not mass-balanced and thereto I referred.

[WLIU]

You should find that if you remove an aileron from the aircraft that do not have aileron balance weights listed as a discrete part, that the aileron assembly is at least 80% balanced. My memory is faulty but I recall that 100% balance is not required to avoid flutter in low performance aircraft. I am not where I can dig out the reference right now.

The point though is that if an aileron cable fails, the wing does not fall apart immediately. Control surfaces are light enough that they will only droop a little against the air flow. And you get to learn how to use the remaining controls to fly the airplane.

At this moment I do not recall ever hearing of an aileron control system failure. I have heard of them being connected backwards. Some of those pilots figured it out and some crashed on takeoff.

Fly safe,

Wes
N78PS