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Vne discovered (Electric Flying Wing)

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Old 10-03-2009, 07:48 PM
  #1  
Mike Connor
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Default Vne discovered (Electric Flying Wing)

Things were looking good and I was getting ready to start the camera for the first full throttle pass after some slow speed flight test. About 2 seconds after full throttle the plane exploded in flight. I have seen mid airs that had less confetti. The front half of the airframe was in tact from about 2 inches behind the front spar forward. It was a fairly clean break from wing tip to wing tip. The back half of the airframe was confetti that took minutes to reach the ground. No large pieces of the rear half of the plane were found. The servos, receiver, battery and ESC were found by themselves in different locations.

The airplane was in a shallow dive and there was no sound evidence of flutter. At this point I am not sure why it exploded like that. Built too light or maybe an incident that happened while static testing the motor a week ago. I wasn't going to tell this story but last week I was static testing the motor and at 33,000 rpm the prop sucked a blanket into it. It was enough stress to bend the motor shaft and now I wonder if it may have stressed the airframe in a way I could not tell (until now). The motor was replaced.

The video shows the launch and the first medium speed pass that Doppler says is just under 100 mph. The ESC data log shows the 6x4 prop around 27,000 rpm at the time. The rpm was around 33.000 when it exploded. The data logger is a great tool

I will build another one with some structure mods. It may weigh an ounce or two more. One servo had stripped gears but the rest of the equipment seems to be okay.


First 15 seconds of flight.
http://www.members.cox.net/fastrc/firstandlast.mpg
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Old 10-03-2009, 08:24 PM
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Default RE: Vne discovered (Electric Flying Wing)

Experimenting is fun, this is a much better ending for any plane than collecting dust in your rafters.

I had a funFly plane flutter to death, it took about 1/2 second and no more wing.
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Old 10-03-2009, 09:22 PM
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Default RE: Vne discovered (Electric Flying Wing)

Mike, I'll bet the blanket didn't help any.....of all things! That's a bizarre sounding failure.

Where did you end up putting the CG?
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Old 10-03-2009, 09:59 PM
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Default RE: Vne discovered (Electric Flying Wing)

use real monokote and shrink the bejeesus out of it, or just sheet the wing, and zero slop in linkages......
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Old 10-03-2009, 11:01 PM
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Mike Connor
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Default RE: Vne discovered (Electric Flying Wing)

cp, When that fancy excel CG program put the CG on my other delta 1 inch behind where it should be, I moved the CG forward 3/4" on this wing and it seems just right. About 1/4" behind where your method put it. [X(] I have never seen a failure like this. No flutter, just a loud pop like a midair. I heard a guy in the pits say "that has got to be the coolest thing I ever saw". I think my next one will have hardwood inner ribs. This one broke wing tip to wing tip midway between the spars. Plus keep the blankets away.

lfinney, It was fully sheeted and the real MonoKote was tight. See pic before top sheeting.

I knew there was some risk because of the 3/8" thick airfoil and 14 oz weight. 100 mph and down to very slow, including stalls, were great. It was almost standing still before it stalled with a little wing dip but easy recovery.
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Old 10-03-2009, 11:03 PM
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Default RE: Vne discovered (Electric Flying Wing)

Ouch!

Sorry to hear the airframe came apart on you...

Sounded pretty nice at cruise.
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Old 10-03-2009, 11:06 PM
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Default RE: Vne discovered (Electric Flying Wing)

Mike,
Was the failure near a seam in the sheathing?
Sounds like you were having some fun with it, before it broke.
edit to add:
How was the power for launch?
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Old 10-03-2009, 11:09 PM
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Default RE: Vne discovered (Electric Flying Wing)

maybe try some .5 ounce carbon veil on the inside of the sheeting, light and strong..., i think may you had very high frequency flutter
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Old 10-03-2009, 11:11 PM
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Default RE: Vne discovered (Electric Flying Wing)

I like the veil idea!
Lots of strength very little weight when laminated with dope.
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Old 10-03-2009, 11:17 PM
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Mike Connor
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Default RE: Vne discovered (Electric Flying Wing)


ORIGINAL: iron eagel

Mike,
Was the failure near a seam in the sheathing?
Sounds like you were having some fun with it, before it broke.
edit to add:
How was the power for launch?
Paul,
It seems like part of it was along a seam in the sheeting but that may be because of the incident and not the cause (maybe). I would think that the ribs so close together plus the MonoKote would keep the skin down.

As the video showed, launch was not a problem. I had much more thrust then weight with a 6x4..
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Old 10-03-2009, 11:29 PM
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Default RE: Vne discovered (Electric Flying Wing)


ORIGINAL: lfinney

maybe try some .5 ounce carbon veil on the inside of the sheeting, light and strong..., i think may you had very high frequency flutter
That's a thought. I will do something to stiffen it up front to back. If there was flutter I did not hear, I think it was in the wing and not the control surfaces. It must have been violent because one servo arm was broken off and the other one was missing the metal clevis. Everything fell into soft dirt and tall grass with no additional apparent damage.
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Old 10-04-2009, 12:11 AM
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Default RE: Vne discovered (Electric Flying Wing)

Mike,
I am glad that you finally got her airborne,and very sorry that you found VNE on it.
The launch looked good but looking and flying are two different things, all I saw was a bit of torque after release.
During the fly by it looked as if it was on rails nice and steady...
How did you like flying an electric?
By the description and resulting damage I think that the wing itself failed and was very violent, by using a mid air to compare it to, really illustrates that.
I think you got going fast enough that you literally sucked the back of the wing off of the airplane. It could also have been turbulent airflow along either along fuselage near the root, or the winglets.


A few things you may want to consider, besides using a little carbon fiber, and I know the first thing will take you aback a bit...
I would taper the wing from the root to the tip, and use carbon fiber along the top and bottom of the spars.
I understand that it only a 3/8" thick airfoil and the actual taper would only be 3/16" but that actually will make it stiffer span wise.
Now regarding the ribs (this won't seem as outlandish)...
Try laminating them with the carbon fiber veil. I have done this and it does make the ribs much stronger.
One last thing that may be an issue is the translation from wing to winglets (vertical fins) causing turbulence either at base or trailing edge.

Now all of this is trying to come up with possible ideas given the rapid catastrophic failure and confetti.
Where the front part of the plane was intact, I do not think it was engine shredding the airframe.
Any guess as to how fast it was going when it "broke"? And I do mean guess. I wonder if it was aerodynamically or structurally induced...
To bad you do not have a slow motion replay of it coming apart, it could give you some clues as to where it moved first, or when parts started to leave.
Still to heard a guy in the pits say "that has got to be the coolest thing I ever saw", had to point out you were pushing the envelope. as well as take the sting out a bit...
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Old 10-04-2009, 12:39 AM
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Default RE: Vne discovered (Electric Flying Wing)

On the next one I'd use thicker balsa ribs with maybe 4 key ribs made from spruce, bass or poplar.

That's it, it just didn't have a good enough backbone. Did the wing seem plenty strong spanwise? If not you could cap the main spar with CF strips

I don't think a slightly thicker airfoil would slow it, but I don't really know.

I'd be leery of changing much more than that, every additional idea brings a weight penalty. This thing looks like it should kick some butt!
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Old 10-04-2009, 12:49 AM
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Default RE: Vne discovered (Electric Flying Wing)

Paul, If I would have gotten it on video it would have been worth while. I was about 1 second away from starting the video whe it poped loud. I was not the pilot on this flight. I launched the plane and then took over the camera but never made it. The motor was still mounted with no damage to the front except for a broken prop. Because of the decent and 33,000+ rpm I suspect easily over 120 mph even though it had not been at that rpm long. These discussions along with some thought are givig me ideas to consider on my next one. The unique shap was what I liked and do not wantn to give that up. If NASA/Boeing can make it work, so can I.
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Old 10-04-2009, 12:49 AM
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"This thing looks like it should kick some butt!"

Well said!
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Old 10-04-2009, 01:09 AM
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Default RE: Vne discovered (Electric Flying Wing)


ORIGINAL: combatpigg

On the next one I'd use thicker balsa ribs with maybe 4 key ribs made from spruce, bass or poplar.

That's it, it just didn't have a good enough backbone. Did the wing seem plenty strong spanwise? If not you could cap the main spar with CF strips

I don't think a slightly thicker airfoil would slow it, but I don't really know.

I'd be leery of changing much more than that, every additional idea brings a weight penalty. This thing looks like it should kick some butt!
Yes, I plan to use a couple of hard wood ribs to stiffen it up front to back and laminate cross grain 1/32 balsa on top of the center section should help with out much weight. Hope I can get away with the same airfoil as it should be low drag. Spanwise seemed stiff connsidering.
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Old 10-04-2009, 01:11 AM
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Default RE: Vne discovered (Electric Flying Wing)

ORIGINAL: Mike Connor

Paul, If I would have gotten it on video it would have been worth while. I was about 1 second away from starting the video whe it poped loud. I was not the pilot on this flight. I launched the plane and then took over the camera but never made it. The motor was still mounted with no damage to the front except for a broken prop. Because of the decent and 33,000+ rpm I suspect easily over 120 mph even though it had not been at that rpm long. These discussions along with some thought are givig me ideas to consider on my next one. The unique shap was what I liked and do not wantn to give that up. If NASA/Boeing can make it work, so can I.
I am pretty sure it was at least 120...

Mike in the aerodynamics forum there is a thread about "real scale flight" one of the guys involved in the discussion put up links and some data display from a simulator he was using.
It gave you very interesting wind tunnel type of results as far as aerodynamics, just something I thought you may want to look at... The software looks like something NASA would use and was developed by modelers in Germany, for use by fellow modelers. kinda neat!

People before NASA and Boeing made flying wings work, Jack N's was pretty neat I always thought; and I am sure that you will get this one to work!!!
Combining the wing and a delta is the most logical step for performance aircraft from my point of view, add in the BWB concept and you can start to take it to the limit.


CP's football is going to an interesting airplane as well.

Remember to make an omelet you have to break some eggs...

edit to add: A old photo from years ago...
Edit to add:
No that is not me in the picture,Just a photo of a new BWB someone was working on some time ago...
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Old 10-04-2009, 01:16 AM
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Default RE: Vne discovered (Electric Flying Wing)


ORIGINAL: iron eagel
Remember to make an omelet you have to break some eggs...
I may break another egg before it is over. Glad the wood is only about $20-$25 and a few days to build.
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Old 10-04-2009, 02:15 AM
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Default RE: Vne discovered (Electric Flying Wing)

At least the wood is cheap...

I was trying to get a plane ready to maiden tomorrow...
But I just had to give up, before I screwed any thing else up...

By the way the carbon fiber veil I use weighs in at 2.oz per square yard, not much weight at all and strong when combined with balsa. I have used it with balsa sheathing and it comes out as light or lighter than MonoKoat. One drawback is you must have an external antenna, plus is that it gives you a real strong stressed skin structure. Yet another big drawback is working with the dope but, most of us know how to deal with that.
Regardless, you have one nice design going, I can't wait to see were you go with it!
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Old 10-04-2009, 10:22 AM
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Default RE: Vne discovered (Electric Flying Wing)


ORIGINAL: iron eagel

...By the way the carbon fiber veil I use weighs in at 2.oz per square yard, not much weight at all and strong when combined with balsa. ... One drawback is you must have an external antenna, ...
Do you mean .2 oz? My 2.4 antenna is taped 90* on top of the wing. There may be a problem with the radio signal from the bottom.
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Old 10-04-2009, 11:55 AM
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Default RE: Vne discovered (Electric Flying Wing)

Yes, I meant .2 oz per yard...
It was late, and I had just broken the horizontal stab loose from the fuselage of a plane I had hoped to maiden today, so I was a bit distracted...


Electrically speaking carbon fiber acts like metal to radio frequency, so you must keep the antenna away from it.
I have used styrene tubes to mount the antennas for 2.4 Ghz in a similar fashion as full scale aircraft, problem being in a high speed aircraft this will act as a drag farm for you...
Unless you can embed the antenna in either a wooden fin or strut or part of the airframe not covered with carbon fiber there will be some signal loss. While veil is not as dense as mat or cloth I would imagine that there still is some attenuation experienced with it's use on sheathing. Now I have used it on structural members and not on the shell of a couple of planes with no ill effects here the sheathing was covered with .3 oz fiberglass veil.
I am going to try Combatpigg's method of using CA to apply some glass in the near future, the technique he uses for fiberglass seems like it is very lightweight as well as strong.
As you may have gleaned from my comments I am a big fan of stressed skin construction, but I like to combine it with a lightweight structure as well for high strength.
I have included a few photos to show you how I tapered an airfoil from root to tip and the structure I have used several times now, very similar in some aspects to your SD.
I don't know if you can see how the ribs are notched into both the leading edge and the trailing edge, the structure is made up of balsa stock the same thickness then the taper is sanded into it using a long sanding block with guides at either end to maintain the correct taper.
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Old 10-04-2009, 02:42 PM
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Default RE: Vne discovered (Electric Flying Wing)


With the short 2.4 antennas it would be hard to place them where CF veil would not interfere. You can see in the pic how I did mine and every time you fly over not inverted it could be a problem.

To taper a 3/8" thick (at the root) airfoil that is 42" long would be a choir. My thoughts at this point are that some part of the structure broke loose and there was an instantaneous self destruct. The cause could have been the blanket in the prop, too much flexing front to back because of the light and thin airfoil or a bad glue joint. Most build up wings may be thicker but most of that is just air. My next one will have 3 basswood ribs on each side while keeping the same total # of ribs. Depending on how rigid it is will determine if the skin is laminated with anything. If I can't keep it together while keeping the airframe weight at 16 oz or less, I will move on to another project.
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Old 10-04-2009, 03:12 PM
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Default RE: Vne discovered (Electric Flying Wing)

Airtronics just came out with a 2.4 RX for CF sailplanes that might work.

You may want to run the top and bottom sheeting different ways to gain in torsional stiffness.
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Old 10-04-2009, 03:53 PM
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Default RE: Vne discovered (Electric Flying Wing)

I hear you about tapering the airfoil being a chore...
I done it with smaller spans as well as large spans with a much thicker airfoil. With a thicker airfoil you just down size the ribs as you move out to the wingtip, making the ribs is simple were you only need the root and tip patterns like cutting a foam core and sandwich a stack of balsa blanks between the patterns and shape all the ribs together. When you are dealing with a larger thicker wing it is not all that big of a deal, but all of this does not easy to implement when you are dealing with a airfoil as thin as yours.
The three basswood ribs should make a big difference as far as the front to back rigidity.

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Old 10-04-2009, 04:18 PM
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Default RE: Vne discovered (Electric Flying Wing)

Unless I'm missing something, this frame cuts the ribs into seperate sections where the spars pass through? The spars pass through un-notched, full depth? If so, then a few ribs either side of center should be capped with CF strand or similar before the sheeting goes on.

I think this is what will give the frame a continuous backbone fore and aft, while leaving the spars to pass through whole.

If you ever wanted to try a tapered airfoil with this type of framing, make yourself a long board with 40 grit glued to it. The trick is to use thick riblets, so they can hold up to the rasping and hogging without getting blown out. Even if the underlying work isn't perfect, the sheeting has a way of averaging out the contour.
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