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Old Jun 14, 2007, 01:19 PM
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Firebat/Foxbat

Quote:
Originally Posted by canard addict
...
My Idea must have had it's start with the Firebat which has been flown as a delta or with canard.
...
That's how it was named in the United States.
Anyone knows, where I could still buy one?

Ordering them from Europe is a little bit expensive.

http://www.acteurope.de/html/foxbat.html

regards
Uli
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Old Jun 14, 2007, 01:51 PM
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They were sold out years ago,Uli, but you may find one on Ebay or in someone's home stock. Charles
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Old Jun 14, 2007, 02:04 PM
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Tom Smith's Avatar
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Darn shame too, becaue with a little work they flew great, at least mine did. I would like to buy a "new in box" one, if anyone out there is hording one. Hint hint! Tom
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Old Jun 14, 2007, 04:16 PM
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Here the firebat my friend built and modified. I will use elevons and canard both when she flies again. The elevons are wood and further back,and the vert. stabs are canted like the f-22 and set-up with dual rudders with perfect geometry. Covered completely in ultracote and shock absorbing lg were added along with a steerable nosewheel.
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Old Jun 15, 2007, 04:22 AM
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My only canard experience so far was an original design with an all moving canard. That was about 12 years ago. It was a total disaster, whenever I used up elevator, it dived into the ground. I tried many different CG locations and always got the same result, so I eventually chucked it in the rubbish. I think it probably would have flown if I had used a hinged elevator on the canard, but I didn't think of that at the time. Elevons probably would have helped solve the problem as well.

The conclusion that I draw from these experiences is that all-moving canard foreplanes are prone to stalling abruptly. Maybe if the movement is small enough and coupled with elevon control it would not be a problem. This is not really new information since I have read that hinged control surfaces are less prone to stalling than all-moving ones. The explaination given was that hinged control surfaces result in an effective camber change of the airfoil, rather than a step-change in angle of attack.

What I don't know is how much the deflection of a conventional control surface (or flap) can affect the stall behavior. There are two cases: 1) The use of up elevator on a canard foreplane (flap down) may cause it to stall earlier. 2) The use of down elevator on a canard foreplane (flap up) may cause it to stall later. Hopefully its not a big deal, as I suspect these effects are much less dramatic as the all-moving control surface. If using coupled elevator and elevons, I think the advantage is the control deflections needed will be smaller - possible less likely to invoke a stall. Secondly there should be more control authority if either the foreplane or main wing approaches a stall. Personally, for my first flyable design I will first try to keep it simple using a hinged canard elevator, but I watch Charles' design progress with interest.
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Old Jun 15, 2007, 10:34 AM
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Firebat

Mustang,Did your friend extend the nose and /or move the canard forward? On my second Firebat, the one which went hard to the ground, I used some exponential on the canard and reduced the movement or end points to around 1/4 inch. What I did not realize was that the TRIM did not reduce itself in proportion to the main elevator control. I found that the TRIM lever gave almost as much movement as the main stick did. When I trimmed for level flight at moderate power, the canard may have had some DOWN trim and the main elevator control was not quite enough to correct the TUCK down before the crash. I have seen more than one Wingo TUCK into a dive where the elevator had no effect trying to fight the pitching moment of the highly undercambered wing. I learned to cut the throttle and save my Wingo. I then filed the bottom of the wing nearly flat which elimenated the tuck. It would then fly fast and level and won several pylon races. Charles
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Old Jun 15, 2007, 11:02 AM
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John235, Thanks for your thoughts which I agree with. I also do not care for full canard movement. I am a bit nervous over the short coupling of my new design with the low aspect ratio canard. I have hope that the rear elevon and front elevator control will work. Don Stackhouse can you help!! I feel that aileron rear and elevator front may be risky because the COG is not far enough back. I feel good about the thrust line location. I will try to go ahead with the build with a positive attitude. Charles
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Old Jun 15, 2007, 02:25 PM
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Quote:
Originally Posted by John235
...It was a total disaster, whenever I used up elevator, it dived into the ground...The conclusion that I draw from these experiences is that all-moving canard foreplanes are prone to stalling abruptly. ... I have read that hinged control surfaces are less prone to stalling than all-moving ones. The explaination given was that hinged control surfaces result in an effective camber change of the airfoil, rather than a step-change in angle of attack.
More or less correct, to a point. An all-flying control surface oonly changes angle of attack, while a multi-element control setup (such as a stab+elevator) changes both the angle of attack and the camber, and the camber increase MAY increase its max lift coefficient.

However, those are generalities. The details of a particular circumstance can vary from that. For example, at very low Reynolds numbers, airfoils can become very uncomfortable with anything but the smallest of changes in camber. Any more than that results in flow separation and loss of lift. In that case, an all-flying control surface might work better. Also, a typical 2-element flying surface typically moves just the aft segment. If the incidence angle of the fixed forward portion is not correct for the airflow coming into it, then that airfoil is not going to perform its best.

Furthermore, all airfoils, be they single or multi-element, have at least two places where they stall, one for positive lift and one for negative. The stall characteristics can vary as well, such as the extremely abrupt case of a stall that starts at the leading edge, or a gentle, progressive stal that begins at the trailing edge and gradualy moves forward, or something in between such as a stall that begins as an abrupt separation of the entire surface of an elevator or flap, followed by gradual rogression forward from there onto the fixed portion of the flying surface.

The net conclusion we can draw from this is that you should avoid blanket statements such as "all-moving canard foreplanes are prone to stalling abruptly". There are so many parameters involved that absolute statements and generalizations are likely to let you down sooner or later, most likely sooner. I could design an all-flying canard with extremely gentle, gradual stall characteristics, and I could design a 2 or 3-element flying surface with abrupt stall characteristics.

In the case of the Firebat, the canard airfoil looks like it could have some pretty iffy stall characteristics, among other problems.

Quote:
...Secondly there should be more control authority if either the foreplane or main wing approaches a stall...
More control authority in pitch just puts you that much closer to stalling the wing before the canard, which is not a good thing. It's a juggling act; more elevator authority lets you use more of the main wing's lifting ability (allowing it to be smaller for a given mission) at the risk of the plane suddenly wanting to swap ends on you in flight, while less elevator authority guarantees better low-speed behavior, but means you need a bigger, heavier, draggier main wing to accomplish the mission. Another case of "...there ain't no free lunch."
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Old Jun 15, 2007, 02:48 PM
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Looking at the issue of high-speed "tucking" on the firebat, I took a look at the instructions, which are available as a .PDF file on the Tower website. There are a couple things that stand out.

As far as aerodynamic pitching moment being the culprit, I'd have to vote a definite "no" on that one. Yes, as you fly faster, the moment forces due to the airfoil camber increase. However, so do all of the other aerodynamic forces that are counterbalancing it. Assuming the C/G is located where it provides positive static pitch stability, an increase in airspeed should result in a tendency to pitch up and slow down.

Aeroelasticity (where the air loads on the structure cause the structure to distort into a new shape, which then changes the air loads) looks like a more likely cause. In this case, the Firebat appears to have a very weak attachment between the canards and their pivot shaft (basically just the soft, spongy Styrofoam epoxied to the relatively tiny diameter of the shaft), so that the canard can probably twist on the shaft a fair amount for a fairly small amount of applied force. In addition, the canard airfoil has a fair amount of "aft loading" (i.e.: there is a lot of positive camber near the trailing edge). This is good for squeezing more max lift out of the airfoil, but it dramatically increases the aerodynamic pitching moment coefficient. As the plane flies faster, this will increase the aerodynamic pitching moment, which then twists the canard on its mounting shaft, causing a nose-down trim change.

If the shaft was a bit longer so that the outboard 1/2" or so could be bent 90 degrees, parallel to the chord, giving a far more rigid attachment between the canard and the shaft, the tucking problem might go away. Sanding the airfoil to eliminate the undercamber at the trailing edge might also help. Of course the loss of lift from that could result in insufficient "up" elevator authority for things like flairing for touchdown on landing, resuting in higher landing speeds.

Meanwhile, the canard airfol also has a rather sharp leading edge, which could make that airfoil tend to stall abruptly from the leading edge. This could explain the stall characteristics John observed. Of course eliminating this problem might broaden the stall enough for the wing to stall before the canard runs out of elevator. It's all a juggling act.

Canards CAN have excellent stall behavior, but (contrary to popular belief) they do not automatically have excellent stall behavior. That only comes as a result of doing your design homework very thoroughly, or else being very lucky. Counting on "happy accidents" is not a consistently effective strategy for airplane design.
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Old Jun 15, 2007, 06:29 PM
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Thanks for the excellent discussions, Don. There is lots of food for thought there. I especially enjoyed the thoughts on the Firebat canard's mechanics and aerodynamic behavior. The control arm of the canard had a tendency to distort the shaft and slip. I believe some modelers drilled a hole to try and prevent the slippage. The Firebat was a real popular plane due, I believe, to it's good and different looks but lost popularity fast because of it's flight problems. The success stories on the threads used the stock speed 400 power and recognized as Tom did, the problem with speedy dives. I can do without those problems. Charles
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Old Jun 15, 2007, 07:46 PM
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Don, Thanks for your response. When I build my canard I had never seen a firebat. Your comments reminded me that my design also had sharp edge on the bottom leading edge of the canard airfoil. This would have contributed as you wrote to the "extremely abrupt case of a stall that starts at the leading edge". I appreaciate your comments since I don't want to design my next model "by accident" - happy or otherwise. All the same.. I think I'll skip the all moving foreplane.
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Old Jun 16, 2007, 12:10 AM
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Charles, the canard on the firebat is in the stock location. I wasn't on here at all until earlier tonight since I flew the Long EZ four times today
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Old Jun 16, 2007, 12:29 AM
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possible giant scale conversion in the works soon

http://www.rcgroups.com/giant-electric-planes-477/
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Old Jun 16, 2007, 03:24 AM
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Very interesting discussion.
I am trying to learn as much as I can about canards.

Quote:
Originally Posted by John235
Don, Thanks for your response. When I build my canard I had never seen a firebat. Your comments reminded me that my design also had sharp edge on the bottom leading edge of the canard airfoil. This would have contributed as you wrote to the "extremely abrupt case of a stall that starts at the leading edge".
....
There are pros and cons for sharp edges (also thin airfoils).
If there is a stall, it will be abruptly.
On the other hand does a sharp edge help to get a more stable airflow up to the stall, because the airflow is turbulent right from the leading edge.This helps at low Reynolds-numbers too. But turbulent airflow causes alot more drag then laminar airflow.

Quote:
Originally Posted by John235
....
I think I'll skip the all moving foreplane.
A friend of mine, Dieter Schall from Germany, told me, that he has never seen an (R/C)-Canard with all-moving foreplanes without some bad behaviour because of that. So his advice is not to use all-moving foreplanes (for R/C canards). It is just an ADVICE (due to experience), no general rule!
And I don't like to have a bad experience, that has been made xx times before.
Dieter Schall is one of the experts in Europe for (RC)-Canards and designed alot of them. The ENTEX-software is his work (Design-program for canards).

http://www.rc-network.de/forum/archi...p/t-13576.html
http://www.rc-network.de/forum/showthread.php?t=53937
http://www.modell-aviator.de/content...oad/entex.html
(in German only, sorry)


I bought a Foxbat / Firebat in Germany. Unfortunately it has to wait for me there, until I go back in 2 years.
But I will change the canard from all-moving to a "standard" one. This should eliminate the stall problem and the bending problem mentioned in other threads. I did not have the chance to fly it.


Up to now I had bad luck with RC-canards.
But it was not because they were canards. Most of the crashes were initiated due to equipment failure (power loss in fresh charged batteries, BEC or receiver malfunction) and a wrong CoG from the manufaturer.
But as Arnold said: "I'll be back !"

regards
Uli
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