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jindoguy(at)gmail.com
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 Posted: Wed Feb 22, 2006 8:19 am Post subject: Observations on flutter by an amateur |
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I am not an engineer so take these observations with all the authority they
deserve and remember the price you paid for them.
As concerns flight control surfaces these general rules seem to apply.
1. the faster the airflow the lighter the control surface should be in
proportion to the wing to which it is attached
2. all control surfaces should be balanced
3. never modify a control surface without re-balancing it
4. never fly a control surface that is over its design spec weight limit
5. do not allow slop to creep into your control system
6. keep your hands and feet on the controls at all times
Some anecdotal evidence in favor of the above
A 1-26 sailplane pilot, while heading home on a long final glide in smooth
air, took his feet from the rudder pedals. Within seconds a growing
vibration could be felt in the tail. Returning his feet to the pedals
silenced the vibration. Upon landing a gross distortion of the tail truss
was found, as evidenced by wrinkles in the formerly taut cloth covering. The
cause was determined to be flutter of the rudder brought about by slack in
the rudder cables and the pilot removing his feet from the pedals. The pilot
was acting as a damper on the system and kept the flutter in check
throughout the normal range of the 1-26's airspeed.
On the LongEZ and Varieze designs of Burt Rutan, the weight of the control
surfaces and their balance about the hinge centerline are called out at both
plus and minus limits. This is done both in the plans for the aircraft and
at least one Mandatory Ground notification from the factory. Both these
designs are composite construction so there can be great variability in the
weight of the ailerons and elevators. Builders are instructed to destroy
surfaces that are too heavy or too far out of balance and make new if the
surface cannot be brought into conformance with the weight and balance
spec's. Both designs are fast, clean aircraft that gain speed quickly in a
dive. The NTSB has recorded one crash due to flutter of a Varieze where the
elevator had fluttered and ripped the canard from the airframe. Post crash
examination showed the elevator was both too heavy and out of balance.
So how does this apply to a Kolb? All Kolb control surfaces can trace their
design lineage back to that original ultralight that Homer started selling
in the late 70's. It was very light and not meant to go much faster than 60
mph with its little one lung two stroke engine. My Mark III, 46 Hotel, is
over twice as heavy as that original aircraft and has three times as much
horsepower. There are Mark III's with 40 to 70 per cent more than 46 Hotel's
65 hp from the 582 its Rotax . I have seen aileron flutter on my aircraft
and others have documented their experience, too.
Do what you want to do when you build your aircraft. Add trim tabs and
systems as you will. Balance your control surfaces or slather them with
bondo to make them nice and smooth (Think it hasn't been done?) Please be
aware that you are working in a dangerous area and act responsibly. Or not.
--
Rick Girard
"Pining for a home on the Range"
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JetPilot
Joined: 10 Jan 2006
Posts: 1246
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| Posted: Wed Feb 22, 2006 8:33 am Post subject: Re: Observations on flutter by an amateur |
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Thanks for the observations Rick ! The more I read the more it become obvious how important it is to balance control surfaces. Kolb MK-III's now come with an aileron balancing standard, and I am also adding a counter balance to my rudder.
The only thing I would take issue on is the requirement to keep hands and feet on the controls at all times. The design of the aircraft needs to be so that it will fly without the pilot holding on to prevent flutter. That is my limit, I would not enjoy flying an aircraft where I could not take my feet or hands off the controls for fear of flutter. I will balance my surfaces so that they will not flutter hands off.
As far as trim tabs, I have abondoned my plan to put them on my MKIII Xtra. As much as I would love aerodynamic trims, they require lots of testing to acheive the right design, weight, mounting etc.... And the results can be fatal if a mistake is made. I will live with stock bungee trim system for now, cumbersome as it may be.
Michael A. Bigelow
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_________________
"NO FEAR" - If you have no fear you did not go as fast as you could have !!!
Kolb MK-III Xtra, 912-S |
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