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Old Dec 12, 2014, 09:12 AM
Defies Logic & Common Sense
speedactyl's Avatar
Philippines, NCR, Makati City
Joined Jun 2012
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VNE of your plane?

How do you know an VNE of a particular design?
Or do you risk pushing too far and the wings rip right off.

It a question been trying to figure out since we been pushing the flight env of our scratch builds.
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Old Dec 12, 2014, 09:49 AM
treefinder
springer's Avatar
SE MI
Joined Oct 2004
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I suspect for most of our foamies, you find it when the wings rip off. It would take a lot of instrumentation to capture stresses in airframe that might be plotted and compared to stresses in some static failure test. Of course the instrumentation weight and balance will affect the foamie orders of magnitude more than instrumenting a full size. Then we also have the actual airspeed vs pilot percieved airspeed/groundspeed issue to confound the problem.

Use rcspeedo or an Airspeed data telemetry setup and fly high speed passes til it breaks. It's only foam after all.
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Old Dec 12, 2014, 01:59 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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The major stress on the wings is when you change direction suddenly, ('G' load). Many model with weak wing structures have folded the wings pulling out of a loop when not going particularly fast.

If flying in a straight line, a control surface can fail, and is more likely to happen before the wings fail.

Unless the control surfaces are well installed and slop free, they can start vibrating/oscillating at high speed, to the point where the hinges or material can fail.

All models are different, even material strengths vary, so it's impossible to calculate the VNE, you just push and push until something fails, and then rebuild it, (or the next version) stronger.


Addition: Some of the dynamic soaring models, (gliders), are pushing over 450mph.
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Last edited by eflightray; Dec 12, 2014 at 02:01 PM. Reason: Addition
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Old Dec 12, 2014, 03:45 PM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
Joined Feb 2010
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Control surface flutter is mostly what I have seen when V has been overdone, though an ARF glider I have once flapped its wings in a most alarming way as I was diving out of a strong thermal. Not for long though. I eased-up quick time.
I have a Corflute plane that would flap its elevator in a high-speed dive. Simply rigging a scale-like bracing wire (thread actually) linking the top of the v-stab, ends of the h-stab and the bottom of the fuselage cured the problem. Now the plane does not have a VNE.
Thing is, we have no way of knowing what V is, unless we have rather high-end telemetry.
So it's all a matter of keen observation and fast reactions or the walk of shame.
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Old Dec 12, 2014, 08:17 PM
Defies Logic & Common Sense
speedactyl's Avatar
Philippines, NCR, Makati City
Joined Jun 2012
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Thanks guys . Very interesting insights . I do get odd flutter on some designs . Especially on dives and high Gs

funny my friends sometimes says my build insnt strong enough. but I couldn't reason you just went way pass the VNE its design For.

they have an have a habit of putting bigger motors on small planes I guess.
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Last edited by speedactyl; Dec 12, 2014 at 09:27 PM.
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Old Dec 13, 2014, 05:13 AM
Registered User
New Zealand, Canterbury, Rolleston
Joined Oct 2011
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Quote:
Originally Posted by speedactyl View Post
How do you know an VNE of a particular design?
Or do you risk pushing too far and the wings rip right off.

It a question been trying to figure out since we been pushing the flight env of our scratch builds.
Only happens in a dive and a spin/roll helps too. Do you want a list of broken firewalls too, which happens when the fuselage, now without wings, hits the ground. OK, sometimes only one side breaks off. and makes an even better spin. ....

Surprising how long it takes for the broken off wing to come down - flutter, flutter, flutter.....

All this means I don't know what the VNE for each of my designs is. Generally it means strengthen the wing spar and try again.
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Old Dec 15, 2014, 01:42 AM
Defies Logic & Common Sense
speedactyl's Avatar
Philippines, NCR, Makati City
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any way to design a plane that would reach terminal Velocity before it hits VNE
that way you wont be going too fast to break anything.

also 2nd to that is G loading.
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Old Dec 15, 2014, 08:28 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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Copy someone else's design and build, they have done all the work for you.

What actually is it you want to do with a model ?, just go as fast as possible ?, pylon racing ?, aerobatics ?, ?, ?.
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Old Dec 15, 2014, 09:34 AM
Defies Logic & Common Sense
speedactyl's Avatar
Philippines, NCR, Makati City
Joined Jun 2012
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RC buddy told me to build a small plane 800mm then wants a powerful 2200KV 3s motor in it.
later saying the frame cant handle the forces.....Duh??

just need info how make my next airframe more sensible.
wish I did Appeal to him to go back to the original setup.I just went Challenge accepted.
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Old Dec 15, 2014, 11:19 AM
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South of France
Joined Jan 2008
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It's hard to answer to your question Speedactyl :
it depends of the shape of the moving parts, surfaces and a lot of other parameters like :

* Do you use carbon fiber / packing tape / fiber glass or wood reinforcements ?
* How do you plan to actuate the moving parts ?
* What kind of flight do you want to perform ? (flying straight as fast as possible, or performing high speed aerobatics and so pulling "high G" manoeuvers ?)

Even in real life aircraft design, the engeneers have to TEST :

For exemple a video of a DG-1000 glider wing :
test rupture d'aile DG1000 (2 min 0 sec)


They bend it until it breaks and record the force needed to break it.
With this results, the engeneers are able to calculate the maximum G force the glider wing can handle.

(The same is done with Airliners wings.)

After that, there is some wind tunnel experiments, then the test pilots perform some "in flight" experiments :

Here a flutter test that will allow to indicate a VNE and allows to avoid this flutter effect that can be VERY dangerous :
Aircraft Flutter (0 min 24 sec)


Have a nice day !
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Old Dec 15, 2014, 10:18 PM
Slipping the Surly Bonds
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Attica, MI
Joined Dec 2006
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I have been flying an 800mm foamie with a 3000Kv 755 watt motor on 4s and have not had any failures yet. Though I do inspect after every flight. Pushing up near 120 mph on it. My biggest concern is for the elevon hinges, then the horn attachment, and then robust pushrods ending with metal gear servos.

Ken
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Old Dec 15, 2014, 11:55 PM
Defies Logic & Common Sense
speedactyl's Avatar
Philippines, NCR, Makati City
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Pylon Seagle (4 min 16 sec)

this one is 900mm with the typical dihedral Vtail setup .
still uses the 9g plastic servos.

not so much for our EPP eagle. when was the last time a plane of this design became a pylon racer?
until that crazy spiral roll dive ruin everything. good thing the plane is still intact as its EPP usual on grass
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Old Yesterday, 12:03 AM
20 years on the Internet
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United States, AZ, Phoenix
Joined May 2012
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Ripping the wings off is half the fun of building something.

NS
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Old Yesterday, 08:52 AM
Canadian Bacon
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Kingston, Canada
Joined Jun 2004
13,170 Posts
Funniest thing I ever saw was with one of my Zagis where in a shallow dive, the tips would not be moving but the centre section would be waving up and down about 8 ins. The wing should have broke but didn't. I wonder where Tic is. Havn't seen our Zagi fan for a while.

Gord
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Old Yesterday, 10:54 AM
Defies Logic & Common Sense
speedactyl's Avatar
Philippines, NCR, Makati City
Joined Jun 2012
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EPP's natural behavior on such loading.

funny i have one plane build out of EVA that does same on the wing tips but that was on stall
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