Cautrell05

I understand from the books that I have and what I have read online that the X-29 was intentionally built unstable to make it more menuverable. What exactly is it that makes it unstable? Thats one thing that I havent been able to find. I know that the canards are located close to the CG and that they are mounted low on the airframe right in line with the leading edge and that they drastically affect airflow over the root of the main wing in flight. Would raising the canards up a little higher help with stability on an RC model?
Thanks
Nick

Tall Paul

This "unstable" refers to the location of the c.g. It was placed further aft than one would do normally. Moving the canards up (or down) won't have any effect on the stability of the plane.
A model would need to be automatically stable, with the c.g. forward of where it was on the full-scale.
Otherwise, it would fall into (drop into, stall into) the cliche, "..tail heavy models fly once..."

allanflowers

I always heard that most canards have the foreplane as high as possible in order to reduce the turbulence on the main wing. On the X-29, in addition to the cg location, the forward swept wing would be yaw-unstable. Generally swept BACK wings are yaw stable and act like lots of dihedral. On some planes, like the Harrier, they have anhedral on the wings in order to keep it manueverable.
At least that's what I thought.
Allan

Cautrell05

In the book that I have, The grumman X-29 by Bill Gunston, There is a lot of good info. In one section it has the following statement-

Without the canard the X-29 would be neutrally stable at subsonic speeds; with the canard fitted it is perhaps the most unstable aircraft ever flown with instability as extreme as 35 percent at low speeds. This condition has the happy result of making neutrally stable at mach numbers over about 1.2, which puts equal loads on the wing and canard and thus gives minimum trim drag.
Would the high speed stability have something to do with the CG issue that paul mentioned? It sounds like at low speeds the cg is farther back on the main wing to make it more menurverable and at high speeds things like thrust angle and wing incidence make up for the CG. I hould like to understand a little more about this plane before I get too much invested. Right now I am working on a 16" wingspan EDP50 powered model to work on CG placement. The bare airframe with no electronics will be around 2 ounces so it should survive alot of CG glide tests. Im not looking for a 3D plane when Im done just a decent flying scale plane.
Any input is appreciated.
Nick

Tall Paul

3D needs airflow at zero airspeed over the control surfaces. You can't get that with a jet.
Vertical, yes, but hovering, it will be difficult.

Cautrell05

I didnt phrase that very well. I dont want a 3D plane. I just want a decent flying scale plane.

Nick

Johng

My Feedback: (4)

The short answer is: Place the CG for stable flight, unlike NASA and users of other unstable jets - F-16, Rafale, etc. This is done on model jets all the time.

The side effect of this, is that the CG when placed for stable flight, will be farther forward than scale. So the scale location of the landing gear can put more weight on the nose gear and make it harder to pull the nose up on takeoff. If you have a model without LG, no worries.

Ben Lanterman

Also remember the Mach number effects on the static stability of the airplane. This is what Bill Gunston was talking about. As the airplane goes past Mach 1 it can gain another 20 or 30 percent or so stability (at least on the airplanes I have worked on) Equivalent to moving the CG that much subsonically. So if you have a airplane that is going to spend a lot of time supersonically and want to reduce the trim drag then you adjust the CG to make it just a little stable supersonically. Then when you look at the subsonic case you are a lot unstable. With modern autopilots you can handle the instability.

The Harrier has the anhedral in the wings (well a big driver no doubt) to get the Clbeta (rolling moment due to sideslip) term to a lower level. A shoulder wing airplane like the Harrier gets a lot of yaw into roll coupling which may or maynot desirable. It depends on what the designers wanted. For example it is great in a rudder-elevator airplane but not in a pattern ship.

A certain level of Clbeta is desireable and you generally try to work out how much before flight based on similator work and pilots comments. "I can't fly this piece of _______" gets a lot of attention.

Of course another thing that gets into the compromise is the length of the wing tip wheels and positions of air to ground stores on the wings. All airplanes seem to be compromises that result as designers work and bicker to get their way.