In the Cloud Suck thread Tony pointed out an excellent "Into the Bowels of Darkness" article. In it and other similar discussion on the topic gliders get into trouble because of some pretty low Vne speeds. What is the limiting factor in Vne here?
Glider speeds...
- Thread starter Henning
- Start date
tonycondon
Gastons CRO (Chief Dinner Reservation Officer)
often, flutter.
What if you make the wings and tail from a "solid" honeycomb extrusion?
dell30rb
Final Approach
I don't know much about gliders but i've seen a few youtube videos of flutter... yikes!
I'd think with such long wings the honeycomb is not such a good idea.. they need to be flexible. Probably too much risk the wings could crunch and fail in turbulence with honeycomb wings
I'd think with such long wings the honeycomb is not such a good idea.. they need to be flexible. Probably too much risk the wings could crunch and fail in turbulence with honeycomb wings
gismo
Touchdown! Greaser!
To some extent, the stiffer you make the wings the worse the flutter.
BillTIZ
Final Approach
You may be able to control flutter in the wings, but any slop in the aileron cable or push rod and the control surface can start the flutter. If you feel the buzz in the stick, SLOW DOWN.
We train the "benign spiral" technique for getting down out of cloud suck, or getting trapped near a lennie when you get pushed backwards not it from not flying fast enough.
That technique would not work well in a hang glider from the "cloud suck" OP.
We train the "benign spiral" technique for getting down out of cloud suck, or getting trapped near a lennie when you get pushed backwards not it from not flying fast enough.
That technique would not work well in a hang glider from the "cloud suck" OP.
But the stronger you make it the more resistant to fatal failure mode. You can kink it and it can still hold together to get you to the ground. Remember, the only need for the change required is survival of the event, not necessarily avoidance.
I liked the "If you feel the Gs lessen, you are "righting" (lessening the bank angle of) the plane", that is a very good survival tip to keep in your bag of luck multiplier tricks right there....
ajstoner21
Cleared for Takeoff
I don't know much about gliders, but from an engineering viewpoint, i would guess it would strongly depend on the natural frequency of the wing structure, and the frequency of the aerodynamic forces.
If the frequency of the aerodynamic excitations match the natural frequency of the wing, you'll get resonance.
So, making it stronger, as in "stiffer" may or may not help.
BiffJ
Pre-takeoff checklist
Having spent some time building 15 meter sailplanes some years ago (Zuni II's) I absorbed some knowledge of this issue from the designer and a couple qualified test pilots. Flutter is indeed a problem and the long wings make things tougher for the designer. The long wings create a high amplitude low frequency issue which allows flutter to start at a lower speed. By stiffening up the structure it raises the frequency and usually decreases the amplitude. Honeycomb or foam helps but solid isn't really the answer as that can also just add weight which increases momentum making things harder to stop once they start. Carbon Fiber helps a lot by stiffening the structure but with the long wings and the desire for extreme glide ratios the wings need to be relatively thin and that creates a limit to the stiffness. Flexible wings are great in turbulence for the rider but flexing is not a good thing for flutter. It just softens the ride. Stiff and strong is best for structural strength and flutter. Water ballast helps unload the wing at the fuselage junction by moving the weight out to the wing and also dampens the vibrations that lead to flutter. That usually allows some increase in Vne with water. BillTIZ is 100% right about the control cables/pushrods or other connections. Any spring or slop in them can allow flutter to occur below the normal frequency of the wing. That is one reason that most high performance gliders use pushrods in the aileron circuit rather than cables. Most of the pushrods are supported in more than one place as well to prevent them from flexing.
As for what how to deal with situations where a speed increase is an issue I'd defer to the glider pilots for answers.
Here is an interesting story along those lines.....
We had 3 pilots from Poland who borrowed Zuni's for the 1983 World Soaring Championships in Hobbs New Mexico. One of the guys was very good and did well in the contest despite flying the older design against the new Ventus and ASW(K) types. He told me that between thermals he'd push the stick til things started shaking and hold it there. I mentioned that he was flying at the onset of flutter but he said it was a good ship and could handle it......wonder how many glider racers do that????
Frank
As for what how to deal with situations where a speed increase is an issue I'd defer to the glider pilots for answers.
Here is an interesting story along those lines.....
We had 3 pilots from Poland who borrowed Zuni's for the 1983 World Soaring Championships in Hobbs New Mexico. One of the guys was very good and did well in the contest despite flying the older design against the new Ventus and ASW(K) types. He told me that between thermals he'd push the stick til things started shaking and hold it there. I mentioned that he was flying at the onset of flutter but he said it was a good ship and could handle it......wonder how many glider racers do that????
Frank
May or may not help whether you get a failure I concur, my issue is one of the failure mode of the different materials. Fiberglass and Carbon Fiber whether in epoxy or an ester resin all fail explosively when their limit has been met. Metal has a different failure mode, and a honeycomb extrusion will maintain more structural strength for the weight after a failure of the type flutter will cause than we get with current construction methods.
ajstoner21
Cleared for Takeoff
Thats very true Henning. I didn't quite think about it that way. You are definitely right that some materials have might have a dangerous failure mode compared to others.
By the nature of a glider you can't really design out the risk, but you can try to design in survivability.
COFlyBoy
Line Up and Wait
In what way are you proposing using the honeycomb? As a core in a composite sandwich, the honeycomb doesn't really provide any tensile strength, only compressive. There would still need to be a skin over the core to carry the tensile loads.
No, as an extruded "box core" in the wing to absorb the failure energy before it can damage the shell structure. They are resistant to flex in the longitudinal axis.
tonycondon
Gastons CRO (Chief Dinner Reservation Officer)
i think that 45 degree speed limiting dive brakes are probably a better solution to staying out of a cloud suck situation in a sailplane. and that ability is pretty standard on a semi-modern glider.
BillTIZ
Final Approach
I know a couple of HP aircraft with 90degree flaps that will not accelerate with the nose practically straight down.
tonycondon
Gastons CRO (Chief Dinner Reservation Officer)
early standard class ships (pre 1969ish) have vertical speed limiting dive brakes as well. The BG-12 also has flaps which will keep the speed under redline. In fact, well below redline.
So why do people still get in trouble?
tonycondon
Gastons CRO (Chief Dinner Reservation Officer)
cases of cloud suck problems in sailplanes are few and far between. Kempton's article is the only published account i've seen. I've talked to a few guys who have come close but always managed to stay out of the cloud. Not all gliders have such powerful airbrakes and even if they do there is still the very real potential of overstressing the airframe in a high speed descent. The video was a hang glider. Hang and Para gliders are a whole nuther ball game with limited speed and descent options.