Canard Forum: Show,Discuss, Learn - Page 376 - RC Groups
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Nov 25, 2011, 09:51 AM
Registered User
Hi nick

That's the philosophy of a fixed canard, stalling before main wing means u can't use max lift on main wing. I am referring to authority to pitch down to recover from stall after main wing has stalled. Like euro fighter the entire canard can point almost straight down and thus get the nose down to recover from stall

Best regards, nice video

Armour
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Nov 25, 2011, 10:02 AM
What could possibly go wrong?
nickchud's Avatar
hmmmm
Quote:
get the nose down to recover from stall
I think there's a fairly short window after the main wing stalls before it starts to slip backwards. At that point, whatever you do with the canard has the opposite effect until you use the motor to get going forward again. As I proved (IMHO) with my Short EZ.

Of course, it might be worth trying, and certainly interesting, to try pulling the elevator all the way back to see if that pushes the nose down, causing another change of direction back to the front again.

Cheers

Nick
Nov 25, 2011, 10:09 AM
Registered User
Sometimes with an aileron/elevon defeflected down the AOA on that side increases and instead of that wing moving up in response to control input it stalls the wing and rolls into that wing opposite to applied control.

In a power off stall I agree not much benefit but for slower/shorter take off and landing especially with power on greater benefit will be realized because you can now fly the main wing to max lift coefficient
Nov 25, 2011, 10:11 AM
Registered User
Yes opposite on canard means u slipping backwards, I didn't read carefully
Nov 25, 2011, 11:26 AM
Registered User
Don Stackhouse's Avatar
If the wing stalls first, the sudden forward shift of the center of lift tends to result in a very violent pitch-up that typically is much faster than a human pilot's reaction time. Therefore, having an all-flying canard "like euro fighter" will not solve the problem. This is especially true of a low-inertia aircraft like our models. Without something fast enough to outpace the aircraft's behavior (such as fly-by-wire with active artificial stabilization, the real reason why the Eurofighter can perform controllably at very high alpha), the plane will still diverge, possibly flipping over completely backwards.

The reason for the all-flying surfaces on planes like the Eurofighter is not stall recovery, it's because multi-element flying surfaces (such as stab+elevator) tend to get in trouble with unstable local shock waves forming and disappearing at transonic and supersonic conditions.

In subsonic applications, you can usually get more control authority from a multi-element flying surface than you can from an all-flying surface, because it changes both angle of attack and camber, not just angle of attack like an all-flying surface does. An all-flying tail usually needs to be bigger than a multi-element surface to achieve the same control authority. An exception is in very-low-Reynolds-number applications, where too much camber can cause problems.
Last edited by Don Stackhouse; Nov 25, 2011 at 11:36 AM.
Nov 25, 2011, 01:08 PM
Registered User
U right I didn't think of reaction time
Nov 25, 2011, 07:01 PM
Registered User
flyboy529's Avatar
Can anyone recomend a 4+ch fasst reicever for a starmax edf 64mm f/a 18?
Nov 26, 2011, 02:36 AM
What could possibly go wrong?
nickchud's Avatar
Fasst receivers: These people have some deals on at the moment

Nov 27, 2011, 07:20 PM
Registered User
[QUOTE=Don Stackhouse;19967551]If the wing stalls first, the sudden forward shift of the center of lift tends to result in a very violent pitch-up that typically is much faster than a human pilot's reaction time. Therefore, having an all-flying canard "like euro fighter" will not solve the problem.

just a thought

with an all 'control' canard (not on a servo) that is hardly loaded except when deflected to induce a moment, would not the canard 'weathervane' into the airflow, unless manipulated otherwise, and therefore not produce that drastic pitch up you mentioned?
Nov 27, 2011, 07:59 PM
ShopCzar
Endlesslag's Avatar
Maybe you could have a full flying canard, on the elevator channel with a gyro just on the canard, not on the actual elevator. then on a violent pitch-up, it would compensate for that and allow you to maintain control...
think that would work?
Nov 28, 2011, 06:24 AM
Registered User
on a real aircraft with cable or rods the canard would probably follow the airflow but a servo tends to hold it in place untill it is moved. im not sure if its possible to set up a servo tp work like a real aircraft.
Nov 28, 2011, 07:27 AM
Registered User
Trevorh's Avatar
I imagine that if, rather than operating the control surface directly, you made the servo operate a trim tab on the control surface, then the control surface would try to maintain its set angle to the airflow. I'm not sure what the effect would be - flutter, probably!

Trevor
Nov 28, 2011, 07:55 AM
Registered User
that would probably work if the canard is balanced with Cg at about 25% of its chord and hung(pivots) forward of about 20% of Cg.(?)
Nov 28, 2011, 08:53 PM
Registered User
Don Stackhouse's Avatar
It's still going to go where the trim tab tells it to go, and it's still going to continue making the same lift force even as the plane begins diverging in pitch. The problem started at the wing, so monkeying with the canard after the fact is not likely to help. The horse is already out, it's too late to close the barn door at that point.

However, the system using the rate gyro to snuff out sudden uncommanded pitch excursions stands a pretty good chance. In effect, that's how an artificial stabilization system works; a gyro or other similar sensor detects when the plane tries to do something on its own that it was not told to do, and then the system applies a control correction to stop it. You might have to experiment with the system "gain" (sensitivity; how big of a response the system makes for a given sized disturbance), but once that is set, it could help.

The down side of that system is that you have now made the gyro and its associated system a "safety of flight" item, and added a whole raft of new failure modes, most of them pretty critical. Whenever you add something in one area of a plane's design, you almost inevitably have to take away something somewhere else.
Nov 29, 2011, 09:51 PM
Flutter-Bys are fun
Conehead's Avatar
I know this is a tad off topic right now, but I found this roaming around here on RC Groups and i thought some of you would like this airplane.

I can't build it, but I am sure that someone here could.

https://www.rcgroups.com/forums/show...9#post19986471

post #7173

Conehead
Orrin Eldred