rc bugman

My Feedback: (30)

When we balance the conventional airplane for its initial flight, we often bias the balance toward nose heavy for the initial flight. This way, the elevator is not too sensitive while we work out the kinks.

But, what about the planes where the elevator in on the front wing?

Should I bias the plane slightly tail heavy for the first flight? This would take some authority from the elevator which is in front.

Confused as usual

Elson

iron eagel

No it would make the elevator much more sensitive giving it way to much authority.

Lnewqban

Same procedure.

CG should be forward the center of lift created by the two wings or the stab and wing.

If there is fuel tank and it is located aft the CG, it should be full as normal for proper balance (worst tail heavy condition in real flight).

My personal preference is level fuse rather than nose slightly down.

Rotaryphile

For forgiving handling, a canard should be balanced slightly nose-heavy by about the same amount as would be indicated for a conventional tractor layout. A good way to check this is to make up a profile fuselage small cardboard hand-launched glider with the same relative wing and stab areas and tail moment arm, and test for stable glide with various CG locations, courtesy of paper clip nose weights. A six inch wingspan is adequate for the little glider, and flat-plate non-cambered airfoils are fine, as far as CG is concerned. This little trick should get you within a couple of percent of wing chord of the best CG location, although test flying of the full-size version may indicate that some fine tuning of CG is needed. Always start a bit on the nose-heavy side.

Villa

I fly a Canard all of the time. Search for the Canard CG calculator and go by that. DO NOT MAKE IT TAIL HEAVY.

da Rock

You mean this one?

http://adamone.rchomepage.com/cg_canard.htm

the canard CG locator application......................... Look familiar?

Villa

Hi da Rock
That is the one. Thanks for posting it.

rc bugman

My Feedback: (30)

Thanks for the replies. Somehow the responses are not logical. Here is why.

With conventional aircraft (wing in front, elevator in rear), a tail-heavy airplane puts too much authority on the elevator which makes the elevator too sensitive and the plane becomes difficult to impossible to control. Move the weight forward, the elevator loses sensitivity and the plane becomes better "behaved". Move the weight too far forward, and the elevator becomes insensitive and the plane becomes difficult to fly.

With a canard, the plane is backward with the wing in the rear and the elevator in front. Logic would suggest that too much nose weight would make a canard difficult to control like a tail heavy conventional plane. Elevator would have too much authority. Move the weight back and the fore mounted elevator would lose authority.

A tandem-wing biplane is yet another beast. Two equal sized wings located on either end of a fuse seems to be naturally stable where ever the CG is located between the wings.

Elson

Lnewqban

According to that logic, a tailless aircraft couldn’t be either nose heavy or tail heavy.

However, pitch stability is not determined by the location of the stab respect to the main wing.

The more or less authority of the stab, wherever it is, comes from how much the airstream helps it or not to stabilize the main wing.

In conventional, layouts the stab works against the lift produced by the wing.
In canard layouts, the stab helps the main wing to lift the aircraft, and then a smaller wing is possible.

This website has animated pictures with very clear explanations.

http://www.geistware.com/rcmodeling/...rorgravity.htm

Just click on the tabs under each picture to activate the animation.

Craig-RCU

You have to suspect that it is your logic that is flawed when your logic leads you to reason that, on a canard, "nose heavy=tail heavy." You are making an incorrect inference that the effect of moving the weight toward the elevator of a conventional design is going to apply to all configurations. The correct relation between C.G. and elevator sensitivity has to do with the C.G.s position relative to the aerodynamic center of an aircraft not the elevator.

I think part of what is confusing your logic is the names that we give to various control surfaces. Control surface names have no bearing on aerodynamic stability. We just call these different control surfaces different names out of convention more than anything else. For example, flaps on a conventional design can have the same function as the elevator on a canard: flaps go down-nose pitches up: elevator goes down on a canard-nose pitches up. You could even fly a conventional design with just flaps for pitch control without any movable elevator on the horizontal stab at all. It's the same with a canard configuration, no movable elevator on the horizontal stab. is needed. Pitch control can be had with elevons on the larger, aft wing only. We just tend to put pitch control surfaces on the wings furthest away from the C.G. for leverage reasons.

Desertlakesflying

My Feedback: (28)

I'd call Scaled Composites and get the word from the horses mouth myself.

Craig-RCU

What word?

iron eagel

If you try to fly a canard tail heavy it will fly once and be pretty uncontrollable, reducing the weight the canard carries is akin to enlarging the flight surfaces, or much larger throws, the exact reverse of what happens to a rear mounted stabilizer.

Here is exactly what will happen if you fly a tail heavy canard...
The nose will come up before the main wing is flying the wing will stall and snap roll into the ground.

here are some things to look at:

http://www.desktopaero.com/appliedae...stability.html
http://yarchive.net/air/canard.html
http://www.mh-aerotools.de/company/paper_3/yaka.html

"Logic is a wreath of pretty flowers, that smell bad!"

— Mr. Spock

A good book to read:

iron eagel

Burt Rutan designed many canards, he should be able to explain the aerodynamics...

rc bugman

My Feedback: (30)

Thanks for all of the input. It appears that it is time to build a canard and play with the CG. That is the problem being a scientist by profession. You have to hit your own fingers with a hammer to believe it hurts.

I don't dis-believe your explanations but somehow I am missing the logic.

Thanks again

Elson

iron eagel

Here is a little experiment, build a flat panel canard glider in the proportions you intend to use out of balsa or even cardboard weight it for the proper cg location.
Toss it and see how it flies, then set it up tail heavy and try it again, quick and easy, you will see what happens.

Lnewqban

Let's try the logic of the arrow:

Feathers in an arrow create an artificial drag that is necessary in order to achieve a stable flight.
The CG (located by the middle point of the arrow) is forward of the center of drag, or vice-versa.

Then, the airstream will keep that center of drag directly behind the CG (around which the arrow rotates for any disturbance).

If the arrow starts flight with the feathers pointing forward (case similar to a tail heavy airplane, for which the CG is aft the center of drag or aerodynamic center or neutral point), the airstream will force the arrow to rotate around the CG by dragging that point of resistance or center of drag, and placing it directly behind the CG, as it should be.

In other words, a tail heavy airplane (regardless the type or configuration), will be forced by the air stream to fly tail first, with disastrous consequences.

combatpigg

My Feedback: (3)

I tried scratch building a 30 inch span canard for .049 power some years ago. The internet was full of hearsay and mumbo-jumbo, but no definitive CG calculators that I could find. The usual advice was to "build a chuck glider".
I found that in order for the canard to function smoothly and effectively, it needed to have a "stoopid lookin' amount" of foreplane area, no matter where I put the CG. With a "stylishly proportioned" foreplane [as in smaller] the model behaved like it had a toggle switch for pitch control.
It flew level just fine.
I ended up cutting the foreplanes off and flying it with elevons.

Craig-RCU

Yes, I figured that is what Cowboy meant. I was asking for specifics about what he thinks Burt Rutan, or anyone for that matter, would disagree with in my post #10. A comment without a specific criticism like Cowboy's comes off as a disrespectful, trollish hit and run. I'd like to have a more productive conversation than that. Do you have any specific criticisms of my post #10?

iron eagel

I think your post #10 was fine.
I just wasn't sure if you knew who was involved with SC, when you asked what word, that is all...
They, as I, would agree with what you had said.

Craig-RCU

I'll try again. Moving the C.G. toward the elevator produces tail heavy behavior only when the elevator is toward the tail of the airplane. It's just that simple. Moving the C.G. toward the elevator on a canard produces nose heavy behavior because the elevator is toward the nose. In all planes, moving the C.G. toward the nose produces more nose heavy behavior and moving the C.G toward the tail produces more tail heavy behavior. The location of the elevator is irrelevant to this.

Your logic lead to the contradictory result that moving The C.G. toward the nose of a canard would cause it to behave as if it were tail heavy. The error in your logic is of thinking that the specific case of conventional designs applies to all cases of various airplane designs. This is an error of inductive reasoning. An example of the error is this argument. Bill is tall. Bill is a person, therefore all people are tall. Your reasoning follows that same form. In the same way it does not follow that because Bill is a tall person that all people are tall, it does not follow that moving the C.G. toward the elevator on a conventional design has the same effect on all designs. Any clearer for you?

Craig-RCU

Sorry, I guess I took your tutelage of such basic information about Burt Rutan, SC and their connection to canard designs to be a sign that you don't think that I know the most basic things about canards. Maybe I took Cowboy's post the wrong way too as a criticism?

Bozarth

My Feedback: (15)

Craig-RCU,

I think we think alike, logic-wise.

Kurt