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Old Jan 28, 2013, 02:18 PM
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Denmark, Central Denmark Region, Silkeborg
Joined Nov 2009
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Hi Tick Point!

To me the stall itself doesn't seem that abrupt... Maybe it's is the spinning that surprized you?
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Old Jan 28, 2013, 05:16 PM
Lighting Up the Darkness
United States, GA, Loganville
Joined Jan 2013
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Tick Point: Man, what a rush of a flight once it was in the air! FAST. I really enjoyed the flights, and impressive construction (strength). How nice to have the constant wind - no worries regarding power systems, props and such. Mother nature knows what she's doing when it come to wind I suppose. Great camera work too. I'm planning to use a small flight camera on the little canard project I have underway once it's ready for the air. We'll see if it can handle the extra weight - I think it can.
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Old Jan 28, 2013, 05:27 PM
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United States, OH, Bradford
Joined Jun 2005
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Dave,

Stepping through the video frame by frame, it appears that the plane departs (and rather violently!) first in yaw, not in pitch, The pitch departure ("stall") comes later.

This plane has two problems:

1. Not nearly enough vertical surface aft of the C/G to compensate for all the side area of that long nose. It wanted to swap ends sideways almost as soon as it was launched, and as soon as it started to yaw even a tiny bit, it went violently divergent in yaw immediately. A case of badly negative static yaw stability. This is a typical problem with Shindens.

Moving the C/G forward increases pitch stability, but it also increases the effective tail moment arm for the vertical surfaces, improving yaw stability, which explains the vast improvements we see in the end of your video. Note, the C/G that results in acceptable static pitch stability is not necessarily the same C/G that results in acceptable yaw stability. Those are two entirely separate parameters.

2. Yes, it does appear that the wing is stalling before the canard, and that it's a tip stall (resulting in a snap roll). Part of this is due to the extreme yaw angle, plus interaction with the fuselage, but a problem with the stall characteristics of the canard vs the wing is probably involved as well.

You mention that:
"The stall is so sudden and not at all like the soft stall I thought I'd be dealing with on a canard."

This is a popular myth about canards. They DO NOT INHERENTLY have benign stall characteristics. It is NOT a natural characteristic of them, rather it is a design requirement that must be designed in. Besides having the loadings and other characterisitics between the canard and the wing adjusted to provide positive static pitch stability, you must also design so that the forward lifting surface stalls first, a totally separate requirement.

Note, it is also a requirement that the forward lifting surface of an aft-tailed aircraft must stall first. However, in that case it tends to happen naturally most of the time (although not always, the early Cessna Cardinals being one exception).

However, in canards it is something that does not happen naturally, it has to be designed in, and getting it to do so tends to be quite a bit more difficult than with aft-tailed arrangements. It involves determining/predicting with a fair degree of accuracy what the actual stall angle and stall characteristics of the two surfaces are, something that is difficult to do reliably, even at higher Reynolds numbers ("Re") and with the best wind tunnel and computer analysis tools. A little study of the deep stall histories of various canards such as the Curtiss Ascender, VariEze, LongEze and Velocity will corroborate this. At our Re's, it's even more difficult. Add that you must also have enough lifting capability in the canard to lift the nose (but not too much!) with a C/G that provides positive static stability, and you the designer will find yourself maintaining an extremely difficult balance between multiple conflicting requirements, with serious consequences if you do not simultaneously satisfy all of them.

Note also that some simple tricks with incidences, such as setting the canard at some "X" degrees of incidence higher than the main wing, is also in NO WAY a sure fire way to solve this. The true solution almost always requires some combination of serious design efforts, extensive testing and development, and/or sheer luck.

Sorry to be a wet blanket, but you need to know what you're really up against.
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Last edited by Don Stackhouse; Jan 28, 2013 at 05:45 PM.
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Old Jan 29, 2013, 12:55 AM
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Joined Mar 2008
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Don, thanks for the analysis. I agree I need to increase vert stabs because of the side forces on the fuse that caused the yaw stall. The wind was strong and running more RIGHT to LEFT then straight in. A launch more into the wind may have helped. I think I see that long fuse getting blown away, literally.

The soft stall I'd imagined was in pitch not yaw (the yaw was a surprise). The video of the last launch shows a buffeting of the nose in pitch right after launch. What's that?

I have no training in aeronautics and appreciate your language.
I am a member of SOPWAMTOS
and proudly apply concepts of TLAR.
It's gotten me to become a decent builder and I have a flying canard, yahoo!
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Old Jan 29, 2013, 06:48 AM
What could possibly go wrong?
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Market Harborough
Joined Apr 2006
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Don!

It's good to see your input again. As you know, I've valued the advice you've given for a few years now. I must have been very lucky because I've had a lot of fun and quite a lot of success with a wide range of different canards.

the point I want to make is that it's not as difficult as you sometimes make it sound. We do have a reasonable margin for error. In particular, I've looked back over my models and the movement of the canard elevator panel allows me to adjust the effective incidence of the canard by at least 15 deg from one extreme to the other. The panels are 1/4 to 1/3 of the canard chord and I've restricted myself to 30 deg travel each way.

My Starship has a canard which is only 14% of the main wing area and, as an attempt to compare the moment of elevator control, the distance from the canard LE to the AC is 66% of the canard LE to the wingtip TE. I usually give myself 10% static margin for the first flight and adjust it later.

My Long EZ has 27% and 60% respectively. Other planes vary between those two.

Here's a video showing canards I had built up till Dec 2010. I've done lots more since then, but you can see that it is possible to get away with some variety. When disasters have been fatal, I can point to a lack of rigidity in the structure as the main cause of trouble.

I've posted this video before, sorry for the repetition, I'll just have to update it.

Canards Taking off.wmv (3 min 54 sec)


In case you're interersted, here's a link to what I've been up to.
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Old Jan 29, 2013, 09:10 AM
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Nick that was a really nice vid! Did the canard improve the stuntcat?
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Old Jan 29, 2013, 09:22 AM
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Just checked the link. Fantastic! u r an inspiration Nick. I get the feeling ur best flyers were prob the Polaris stuntcat and gripen. Am I close?
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Old Jan 29, 2013, 11:09 AM
What could possibly go wrong?
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Market Harborough
Joined Apr 2006
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Thanks Capt.
Quote:
Did the canard improve the stuntcat?
No it didn't. But turning the Stuntcat into a Polaris was very successful.

My best flyer is always the next one I'm about to build.

There's lots of good advice to be found on this thread, specially from Don. The best I ever had was from Charles the Canard Addict himself. I was getting more and more timid about launching my pride and joy into the wild blue yonder to face its destiny. I even considered building a wind tunnel. He said "just get out there and do it."

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Old Jan 29, 2013, 12:19 PM
Lighting Up the Darkness
United States, GA, Loganville
Joined Jan 2013
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Two Equations to ponder for maiden flights

Disappointment = Expectation - Reality
Joy = Love - Fear

I always try to keep a couple of the key variables in check as I solve for x, especially on maiden flights! (not sayin I'm a pessimist, I'm just sayin)
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Old Jan 29, 2013, 03:18 PM
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Sounds great Nick. How does the Gripen compare with the Polaris? I get the feeling that the polarstuntcat is probably a nicer flier than the Polaris.
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Old Jan 29, 2013, 04:32 PM
What could possibly go wrong?
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Market Harborough
Joined Apr 2006
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Quote:
How does the Gripen compare with the Polaris?
It depends what you're trying to do. The mission is everything..... The Polaris is one of the best .. it takes off and lands on any surface, it flies accurately and slowly, it's easy to manage. The Polar Stuntcat was as good as that and had the extra benefit that it was my own design - you can't beat that.

My effort at the Gripen was a holiday project, very few tools and materials, it didn't look a lot like the real thing. But it went well. I think the KF airfoil was a factor in the success, and that's another cat among the pigeons. Very good in terms of fun to fuss.

On the other hand scale planes have a special beauty about them. Even if they're not accurate up close, they can still be a real thrill in flight. They're worth the extra work. If you can make a model that looks in flight like a Lancaster or a Catalina or a DH88 Comet, or a Vampire, or a Starship, well, what can I say.... I've had a lot of fun. And I don't have a lot of special skills. It can be done.

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Old Jan 30, 2013, 06:34 PM
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Joined Mar 2009
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canard elevator question

Ok I have what I am sure is a dumb question, but after perusing the forums for awhile now I haven't found what I am looking for. I recently purchased at auction a Sig tristar with a .10 glow motor. Only gave $5 for it! The motor and fuel tank was the only gear left in it. All the electronics, servos, and everything had been stripped out so I decided to convert it to electric. I'm all done with my setup, found a manual online that show cg placement so I'm good there. But then I got to thinking, gee the elevator is on the front of this thing. So having absolutely no experience with canards I find my self wondering what direction is the elevator supposed to travel. Is it the same setup for a regular plane with elevator on tail or should the elevator move opposite directions? As I said, probably a dumb question, but I just can't wrap my mind around it. If anyone could clue me in I'd appreciate it!
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Old Jan 30, 2013, 07:27 PM
OY-5542
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Denmark, Central Denmark Region, Silkeborg
Joined Nov 2009
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Hi falcn63!

The canard should deflect the air downwards when you want the nose to go up...
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Old Jan 30, 2013, 07:47 PM
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So elevator movement would be backwards to a regular airplane with elevator on the tail? If the elevator goes down the nose goes up?
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Old Jan 30, 2013, 08:13 PM
OY-5542
Chrizz's Avatar
Denmark, Central Denmark Region, Silkeborg
Joined Nov 2009
730 Posts
Quote:
Originally Posted by falcn63 View Post
So elevator movement would be backwards to a regular airplane with elevator on the tail? If the elevator goes down the nose goes up?
Yes, when looking at the trailing-edge.

PS I've found this short video showing the controls movement on a canard:
Ligeti Stratos Canard Demo (0 min 50 sec)

Hope it helps, don't hesitate if not (I think everybody had to think twice when figuring-out canard-controls the first time :-) ).

Happy landings :-)
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Last edited by Chrizz; Jan 30, 2013 at 08:28 PM.
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