Greg Covey

Our maiden flight test on the VERTIGO Rebel ended abruptly similar to Bob’s. The clear video and photos (although blurry) show a perfect take-off and we didn’t even get as high as Bob’s flight before the Rebel had an epileptic fit and crashed. The camera didn't want to focus on the silver/grey airframe from a distance. Team Futaba’s Devin McGrath said that he made no change to the elevator input after liftoff when it suddenly veered to the ground.

Our guesses are that the flying stabs are possibly causing an issue or the CG is off enough to give the Delta wing design stability problems. Since the Rebel has been severely damaged, I cannot look into repairing it, if even possible, until the winter building season. It’s been a lot of work, mostly fun, but there is something strange going on with the design.

[link=http://www.gregcovey.com/reviews/RebelPavementFlight.wmv]VERTIGO Rebel Maiden Flight[/link] (4meg)

[link=http://www.gregcovey.com/reviews/RebelCrashHD.wmv]Rebel Crash in Hi Res[/link] (1meg)

Airframe Inventer

I want to thank you Greg for all your hard work.

This is what I saw when anylyzing the video in Windows Movie Maker.
I have been studying the video for a while.

0:53.23Rollout and acceleration was good.
1:01.90Up elevator
1:02.24 Down elevator
1:02.73 Neutral elevator, good attitude
Level attitude top of Cissoid C1 curve
1:03.05 Nose beginning to drop
1:03.22 Up elevator
1:03.58 Full up elevator
1:03.78 Impact, left wheel colapse
1:03.80 Right wheel colapse
1:03.87 Airbourne from bounce
1:04.07 Up elevator
1:04.37 Down elevator
1:04.43 Full down elevator
1:04.86 Neutral elevator
Level attitude top of Cissoid C2 curve
1:05.12 Up elevator
Beginning of nose down attitude
1:05.33 Up elevator
1:05.36 Full up elevator
1:05.95 Near neutral elevator
1:05.99 Second impact
1:06.17 Catestrophic rudder, elevator and right wing twist
Comes to rest
End video



Overall it was definately flying.
Power was sufficiant to fly.
Down elevator was oversensitive from being too nose heavy.
If anyone can add to or dispute my findings please post here.
Thanks, Richard

Devin McGrath

Richard,

I am glad you took a close look at the video because knowing that I did not give any down elevator input and that you saw down elevator confirms to me that the stab design is not only wrong in theory but that is what caused the Rebel to crash. Because of the large amount of elevator area that is in front of the hinge or pivot point the elevator will always have a tenancy to hunt away from neutral. When designing an airplane with a full flying surface the pivot point should be at 33% or less of the mean aerodynamic cord which the rebel is not. Also using a full flying surface doesn't give you a real benefit at this size and speed of air frame other than looking different. for anyone who is going to try and fly one of these again I would suggest changing the elevator to a conventional stab where you will have a portion of it fixed to the air frame. I would also do the same to the rudder and cover over the leading edge slats with some clear covering just to make sure they are not causing anything weird to happen.


Devin McGrath

Airframe Inventer

I appreciate the criticism and assessment of your view, but I must defend my design.
As you may or may not know, the elevators are balanced on the pivit point.
Because of the short tail moment, the throws only need to be about 15mm.
What I saw in the video was a lot more than that.
If anyone were to also take the time, they will see that too.
The movement to the elevators was made by the servo not the passing air.
It was only flying about 40 mph.
I'm sure the servos were stronger than that.
If they were weak servos, then you would have an arguement.
I have heard the suggestion before of a more conventional stab and rudder.
Take another look at the design of the elevator, it goes up and down, powered by a servo.
This is very basic, I don't see anything wrong with that.
Well, the work goes on. It's been well over a hundred degrees here everyday and will be till September.
We'll have another one ready in the near future with a much better CG and lighter power system.
Thanks, Richard

papajeff

With a positiveintent to add another perspective to the resolution of the issues thatall of ushave encountered with
the Rebel, here is what I observed. Eventhough my camera (Canon 60D / 18mp / 18-270mm IS Auto focus lens)
could not lock in a decent focus on the thin, sliver,rear profile of the Rebel against the green background of the
far tree line, I did get an interesting look at the takeoff through the camera viewfinder. What I observed was as follows;

-nice smooth acceleration down the runway
- all flying surfaces looked to be in perfect trim position
- adequate speed attained for rotation and nose wheel lifted
- as speed increased, a slight up elevator was noticed and the Rebel lifted the main gear off the runway
- the rotation attitude of the Rebel looked to be perfect with no more than the slight up elevator
- suddenly the nose dropped as the Rebel gained speed
- the up elevator correction for the dropping nose was not full up, in my view, and the Rebel just continued
to nose in to the runway
- once the nose suddenly dropped.....it was all over in just a few seconds and no amount of correction
could have saved the Rebel
- why the nose suddenly dropped (with all flying surfaces in the proper position for a perfect ROG) is the
real problem to determine as it seems, to me, NOT to be a problem with the pilots.

Note: if the video could be viewed in slow motion, from the time that my hat comes intoframe till it goes
out of frame is the time period that the problem ocurred.

These are my observations and I leave the assessments up to all of you with the hope that some of this is helpfull.

"Papa Power"

Airframe Inventer

Thanks Papa Jeff.
Slow motion as described in post #126 can be duplicated by anyone using Windows Movie Maker.
Thanks, Richard

Greg Covey

Here are some frame grabs from the raw .avi film at the top of the flight curve. I think that videos can be deceiving, which is why we initially thought differently about Bob Ruff's flight. After our flight, we can see that the Rebel's initial veers were not due to stick input. At the top of the curve, where the Rebel is already veering, you can see there is no control surface offset. What I see is the entire airframe wrapping around the curve. Use the position of the nose wheel in relation to the other two wheels to determine the plane angle.

On Bob's flight, we blamed it on the pilot and never thought that the wing is actually blanking out the elevator in neutral flight, making it lose control until the airplane is at an angle of attack either positive or negative. An ascending or descending angle is when you get the elevator control back again.

Airframe Inventer

So far the opinions have been unanimous from people who have
<o></o>seen this video is that the model too was nose heavy.
<o></o>This is why it pitched down so easy.<o></o>

I also grabbed some frames.<o></o>

<o></o>

1:01.90 Up elevator<o></o>

1:02.24 Neutral elevator<o></o>

1:02.73 Down elevator<o></o>

1:03.74 Full up elevator<o></o>

<o></o>

You can not only see the angles of the elevator, but you can see
<o></o>the amount of light reflecting off the top surface of the elevator.<o></o>

<o></o>

Regardless of what happened, our next attempt will be with a better CG.<o></o>

Devin McGrath

In regards to the elevator how are they balanced at the pivot point? Also where and how did you come up with the figure of 15mm of deflection is necessary? and from where do you measure this deflection. All servos will have some degree of play with the surface as it is now at higher speeds I will guarantee you will see flutter with this stab design.

After reviewing both videos a few more times each I am convinced that the issue with the rebel is the stab. Not just size shape and where the hinge point is but its location on the aircraft itself. the elevator so close to the wing and directly in line with it. This places it right in the turbulent air coming off of the wing which causes the wing to blank out the elevator in level flight making it not nearly as effective if at all. also the elevator is acting as a lifting surface and as it comes in and out of the wings turbulent air you are changing the center of lift which changes your correct cg. creating a whole world of pitch sensitive issues and instability.

The Rebel may have been nose heavy but its main issue in my opinion is the stab location and design.


Edit: and I really do not see how you see any down elevator in the picture you posted. Its pretty much a shot in the dark if you can see any and I know I didn't add any....

Airframe Inventer

Up until today, all mentioned here is theory and opinion.
When the day comes, the first part of the next flight will be to take it up to a safe altitude.
From there it can be evaluated.

Greg Covey

Richard,

I am starting to think that you are forgetting your own design. The attached image from my build clearly shows how the light reflects at neutral position due to the flying elevator stab.

Therefor, I think you may be misinterpreting the control offsets in the video.

Airframe Inventer

This will be my last post on this thread.
This thread has deteriorated to the point of petty insults.
I will create a continued thread on RC Groups.
http://www.rcgroups.com/forums/showt...9#post18886127
It appears that I have hurt some feelings here.
All I did was share my observations and findings.
I'm on the Outside looking in so to speak.
Because we don't agree, there should be no animosity.
All I ask is that you do not use this thread to continue to make me look bad.

I know that if I continue here, all it will do is continue todeteriorateand
keep people on the edge of their seats waiting to read the next exciting post.
I am not here for that entertainment value, I am all about results.

I really do appreciate the work that you put into it, Thank you
I don't want to burn bridges.

Anyone who is interested can see our first test flight that shows a nose heavy airplane
http://www.youtube.com/watch?v=-o1d3dJsAkU


It is not about pilot error or design yet. We don't really know do we?
Your team did not fly it. My job is to be sure it flys and flys well.
We will continue on our end here...

Greg Covey

Richard,

I am sorry that you interpreted my text as derogatory. It was my not intention but adds further proof that things like video and text can have different meanings to different readers. My team was there and you have rejected valuable input from the national caliber pilot, photographer, and other pilots in attendance at a full-scale airport. At the very least, simply consider our feedback.

I see that you re-posted the [link=http://www.rcgroups.com/forums/showthread.php?t=1054518]original thread[/link] on RCG from 2009. In it you state that you assembled a glider unit to "confirm the CG". From your original design theory, and glider confirmation testing, what has changed to invalidate this data?

Regards.

amwolf3195

My Feedback: (4)

I watched both video. I didn't see any down on the control surface in greg's video. It seem like the plane was fine until level flight. If i may ask did you do a powered test with the proto type. and if you did would you be willing to show the video of that test flight