dubd

On Sunday, April 7th, my FEJ 1/7 F-14 crashed. The cause is under investigation, but there was elevator flutter before the plane crashed. I saw the flutter happen and then the plane pitched towards terra forma. I gave elevator input, but the plane did not respond. I am posting this thread to open up a discussion around possible reasons that can cause the crash. I lost quite a bit of time and money with this crash and I hope the outcome can prevent this from happening to current and future owners of this aircraft. Fly Eagle Jet is also looking into what could have caused the crash. There are some really great modelers here, so I also want the community's opinion. Let's try and keep this constructive.

Here's some info to consider:

* I had 2 Electrodynamic 2s A123 packs that were fully charged before the flight.
* I did a preflight check of the control surfaces and did not detect any slop
* The plane has always been flown at 1/2 power or less when straight and level.
* JR 8711 servos were used on the elevator.
* 2 Jetcat P-140rx engines were in the plane.
* The plane has < 25 flights.
* This plane was never flown hard, but I didn't just fly around in circles either. Flights 1-8 were check out flights. Flights 9+ included loops, rolls, and split S's.

There was an extensive build thread of this plane done here: [link=http://www.rcuniverse.com/forum/m_11107361/mpage_1/key_/tm.htm]FEJ F-14 Build[/link]

Video of the crash (best viewed at 1080p):

[youtube]http://www.youtube.com/watch?v=dMZwYHJFDe0[/youtube]

Video run through a stablizer. Courtesy of Jim Brown

[youtube]http://www.youtube.com/watch?v=Unwq7IpNLIQ[/youtube]

jetpilot

My Feedback: (48)

Very docile flying. I expected to see aggresive flying to have elevators fail. You were exiting a pretty large loop and it was hard to judge speed but it didnt look excessive.
Sorry to hear this Dantley!
Hope you get to cause!
Scott

hmjets

Hi
I think is evident,Flutter strip the servo is the probable cause.
Nice plane,sorry for your loss.

dubd

hmjets, if that is the case, I'm trying to understand what caused the flutter.

rhklenke

My Feedback: (24)

A total-destruction crash is a huge kick in the stomach - I'm really sorry that this happened to you. The times it happened to me (twice, unfortunately) it really made me consider quitting, but I've always come out of it, sometime later, being glad I didn't. Here's wishing you a speedy "recovery."

As far as crash analysis is concerned, it curious that it tucked like that. It did look like the stab was fluttering more down than up, which probably caused the nose-over, but I'm assuming you pulled back on the stick and it didn't respond at all? If the "good" stab was deflecting up while the fluttering one was deflecting down you would have though that at a minimum, you would have seen a roll to the left as a result. It looked like it did initially roll left, but the last frame where the tail was visible looked like it was back level. I don't know if that means anything...

The forces of flutter are incredibly destructive. They can strip metal gears, bend titanium, or break carbon pushrods, or tear servos from their mounts. Its hard to say what caused the initial flutter, but once it starts, at least one, if not more of the above effects probably happened. There are so many factors in flutter and its a non-linear phenomenon, its really hard to predict. The usual "preventative measures" like a completely slop free linkage, strong servos, and balanced control surface are not cure-alls and can, depending on the overall system characteristics, sometimes make flutter more likely and/or more destructive.

In this case, since you didn't have flutter before and it suddenly occurred in a area of the flight envelope that you had been in before (i.e., not going too fast, etc.) its likely that the onset was triggered by something changing/breaking...

Bob

JackD

My Feedback: (4)

Scott,
I was there... this is not a fast plane. maybe 150ish????

FalconWings

My Feedback: (57)

Agree with Scott, you did not seem to be pushing it at all.

I can see you did not lose aileron control, did you have a chance to tell if your engines went into failsafe?
This does not seem like an electrical issue.

'Im sorry to tell, but 90% of the flutter issues discussed here on RCU have been due to craftsmanship. Looking at Rafael's thread.....it seems like a big possibility. Did the tail section survive? Can you get to the elevator shafts? Even if they are broken, they may shed some light.

So sorry to hear Dantley!
David

Jack Diaz

My Feedback: (1)

Sorry Dantley.
We know how it feels [:@]. Lucky you were not inside.

It seems to me that the violent up and down oscillation of the elevator tip, destroyed the laminar air flow over those surfaces; thus, they lost lift.
It looked more flapping than rotating around the pivoting axis.

Jack

jet time

My Feedback: (75)

It is a great loss for Dantley. We are all working together to find the cause. HM, we are leaning towards that at this point.
By no means did Dantley "over fly" the airframe. This was all at a moderate speed and that is what is so frustrating here. He is a great pilot and did do all the "preflight" on it. Help is what we are looking for. This is the first loss like this and we don't want any others if there is something we can do.
Thank you

Lowell

dubd

David, my engines did not go into failsafe.
Jack Sr, any theories on what could cause stabs to flap? I've never seen a plane do that before.
Lowell, thanks for the open line of communication.

Countryboy

My Feedback: (25)

I have to agree with Jack from what I can see in the video. Looks more like and up/down flapping... perhaps the stab pivot broke or the support structure through the fuse let loose. After such a devastating crash, it may be really hard to find the cause as to why.

Sorry to see this Dantley, hope your able to bounce back.

Jack Diaz

My Feedback: (1)

Good point Lowell.
Unfortunately it will be impossible to come up with a final answer after a total destruction like that. So, all we are left with is theories.

I am not familiar with the internals of that plane, but looks like whatever structure that gives integrity to the rear section, may have failed. It is obvious that the stab was able to move up and down like a bird flapping wings.
If the structure did not fail, then the rear section is too weak. Its elasticity induced the flapping when in resonance.
Could be illusion, but also looks like the fins were moving left and right.
Again, just theories.

Jack

EDFJim

My Feedback: (2)

Man so sorry this happened.. I had some flutter and found the bolts coming loose from the bulkhead for the whole stab assembly I got it on the ground and went to larger washers epoxy onto the bulkhead. so sorry man was looking forward to flying together one day.

ianober

My Feedback: (12)

Here is a photo of the elevator mechanism. The servo arm was swapped for a SWB metal one so the plastic WAS NOT used. This is by far the most ridiculously overbuilt mech I have ever done. the arms are .25 solid aluminum tapped at both ends for 4-40 threaded rod which secures aluminum rod ends. I HIGHLY doubt this part failed pre-flutter. I calculated the MAC and pivot point and it would appear that it is right at about 23% which should be acceptable. Maybe some of the more experienced in aerodynamics can offer up some theories.


*** hmm pictures not working again, shocker. Dantely, how did you get your pics on? ***

Ok, you can see the elevator pic I was trying to post in this gallery: [link]http://www.enthuzeus.com/event/gallery/NpWUH[/link]

jet time

My Feedback: (75)

Jack, I know...this is tough to try to figure out. If we try though, maybe someone will hopefully have a good "theory". At least it gives us areas to look at. Again, this is the only one at this point, so, it is a common problem. The good news is we are open to find out what it was and Dantley and I have had some good communication on it.

Lowell

EDFJim

My Feedback: (2)

was the bulkhead the carbon fiber or plywood?

jet time

My Feedback: (75)

Ian, no doubt that you (and always do) a very thorough and great build. That is not a question here at all. PERIOD! You did mention the pivot point and that is another thing that Dantley and I were looking at. Ken has been flying his for over a year and has had no issues. So why or what was different here? That is what we need to figure.

ianober

My Feedback: (12)

Bulkhead was plywood/honeycomb. Personally I think this could be the issue. The bulkhead was not ply all the way around. It was honeycomb all around and then re-enforced with ply in certain areas. Sure wish I could post pics. I do have a pic of the rear bulkhead on my phone and it is clearly not all plywood.

marcpamjoce

I know what the deal is, it is a Chinese ARF, "Made in Taiwan" junk.  Look at other thread about this and you will see.  Hate to say it, but you guys buy this garbage and then act surprised that something like this happens.  Anyway, Lowell is an excuse maker and tends to prop up this garbage.  He is no different than a Whole Life Salesman or Timeshare Spokesman.  In other words, can't admit that he is peddling garbage that is a bad investment.

EDFJim

My Feedback: (2)

only "weak points" (hard to say that) i can think of are the internal servo and where the bolts go through the bulkhead.

jet time

My Feedback: (75)

WOW! I don't know you Mark, have never had corospondence with you (to my knowledge) and this isn't what this forum is about. Before you defame someone you should know more about them. I do care...otherwise I would have told him to pound sand!! I think you need to get a life...No rebuttle needed. And be the way...I fly what I sell and stand behind and am always willing to help someone.

Jack Diaz

My Feedback: (1)

Dantley, flutter is a very complex phenomena.
To make it simple:
The flutter originates when the anchoring point of the control surface starts vibrating or oscillating.
For example, if the anchoring point moves down, it is equivalent to the control surface moving up. Then aerodynamics will force the control surface to move down, pulling the anchoring point down; but it was already going down !! And so on.
If those forces get in resonance, you get flutter.

In other words, the cause is in the anchoring structure. If the disturbance on the control surfaces do not distort its anchoring structure, there is no flutter. They may be moving up and down some, but that is not flutter.

Does it make sense?

jack

EDFJim

My Feedback: (2)

The four bolts put a big rotation load (looking forward) on the bulkhead.

dubd

The plastic arm was only used for mock-up purposes. SWB arms were using in the plane.

Jack Diaz

My Feedback: (1)

Lowell, please "don't step on those banana peels" as we say in my country.
Ignore non technical comments so that we can keep this thread constructive. Don't feel the need to defend anything.

Jack