Canard Balance Location

[CloudyIFR]

Does anyone have experience with Martin Simons neutral point calculations from his wonderful book "Model Aircraft Aerodynamics"?

The reason ask is that I have made/used spreadsheets for about 17 years using his formulas for normal tailed models and the calculations work flawlessly.

He says the formulas are the same for a Canard and all I would have to do is change one symbol from plus to minus.

If you've built a canard and it flew successfully with his calculations perhaps I'll make up the spreadsheet for others to use.

I have no reason to believe they are incorrect, I'm guess I'd like some verification.

Thanks
Curtis
Montana

[eflightray]

Here's a CG Calculator for Canards (http://adamone.rchomepage.com/cg_canard.htm)

Hope it helps.

[HerkS]

In your spreadsheet use the tail for the wing and the wing for the tail.

That is use the canard wing area and layout in the tail portion and the canard tail portion in the wing part of the spreadsheet. It should work out to be correct.

You can use minus numbers for the tail length, but that can get very confusing.

[CloudyIFR]

Herk,

That's easier said than done! :)

I may be close though.

I do have a question.
In Martin's book he has the formula to find the Np for a normal tailed model as:

h n = h 0 + ns * V s ( a1 / a) * (1 - (de / da ))

He then goes on to say that for a Canard the foreplane is destabilising i.e. it brings the Np forward and the appropriate sign change must be made in the formula:

Canard:

h n = h 0 - ns * V s ( a1 / a) * ((de / da ))

But then when I look at the formla he also removed the 1- from the de/da.

Is that a mistake or should that also be removed?

They symbols are:
where
h n neutral-point
h 0 aerodynamic center of the wing, typically 0.25
h s stabilizer efficiency typcially 0.6 (0.9 for a T-tail)
V s stabiliser volume coefficient
as lift curve slope of stabiliser
aw lift curve slope of wing
de / da change in stabiliser downwash angle versus change in wing angle-of-attack, typically 0.5 to 0.33

In my normal tailed models I'm sing a de/da of .35 which has worked well. Should this be changed for a Canard?

I need to find some online plans of known models that are flying well to verify my calculations are correct.

Thanks for the help.
Curtis
Montana

[HerkS]

Hi Curtis,

I think that's a pretty serious mistake because if the downwash value was set at zero the formula would not work. Calculating NP without including downwash should normally get you pretty close to the correct NP. The first equation works quite well with it set at zero.

Efficiency for the horizontal tail of a canard would be 1 since it is at the front of the aircraft. For a canard the efficiency factor would apply to the wing not the forward surface. And for a basic canard it would apply to only part of the wing.

I think downwash should be neglected for a basic canard calculation. The forward surface normally operates at a higher lift coefficient than the wing, so the part of the wing behind the canard surface is experiencing strong downwash. However the tip vortex formed by the canard surface will produce a strong upwash over the outer part of the wing. Depending on lots of variables this could completely cancel the negative contribution of downwash to the stability calculation, or it could have a strong effect. Picture a tandem wing aircraft - equal span and equal chord. In that case the downwash affects the entire aft wing.

You might consider a tandem wing case to test your calculation. If you leave downwash completely out of the calculation, the NP should be halfway between the quarter chord lines of the two wings. Then as you reduce the size of the forward wing the NP should move aft proportionately.

Or, in other words -- DUH - I dunnow????????

[CloudyIFR]

Herk,

I have Model Aircraft Aerodynamics 2nd edition. I asked a gentleman who had the 4th edition and yep, the formula was changed to:
Curtis,

Hn(canard)=Ho{Ns x Vf x Af/A x {1 - DEf/DAw}]

So now back to the spreadsheet to see what that gives me.

Curtis