HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Feb 08, 2013, 01:56 PM
glider pilot in training
webdragon's Avatar
United States, AR, Searcy
Joined Aug 2006
647 Posts
Is it possible to have a 2 channel, rudder elevator canard?

I ask because i was given a box full of sailplane bits an pieces, including a H9 aspire 2 meter wing and a unknown sailplane balsa fuse with some random rudders and elevators.
webdragon is offline Find More Posts by webdragon
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 08, 2013, 03:34 PM
Registered User
numanair's Avatar
United States, WA, Kirkland
Joined Sep 2010
288 Posts
Quote:
Originally Posted by webdragon View Post
Is it possible to have a 2 channel, rudder elevator canard?

I ask because i was given a box full of sailplane bits an pieces, including a H9 aspire 2 meter wing and a unknown sailplane balsa fuse with some random rudders and elevators.
Yes, just like a conventional configuration it will need dihedral for stability.
numanair is offline Find More Posts by numanair
Reply With Quote
Old Feb 08, 2013, 04:41 PM
glider pilot in training
webdragon's Avatar
United States, AR, Searcy
Joined Aug 2006
647 Posts
I hope the Aspire wing has more than enough dihedral.

I might dig though the box a bit more for a bigger rudder, this one looks alright but i worry it's too small.
webdragon is offline Find More Posts by webdragon
Reply With Quote
Old Feb 08, 2013, 07:18 PM
Registered User
Joined Jun 2005
2,465 Posts
webdragon, The rudder seems to have enough leverage about the CG which will be forward of the LE near the screws but lots of rudder area is always good. The large polyhedral wing will not respond to aileron movement because the tips will resist downward aileron control since they are there to hold lateral stability. It would be good to control the turns with rudder and elevator. I hope that you have some positive incidence on the canard to assure that it will stall before the main wing. Best of luck with it.

Charles
canard addict is offline Find More Posts by canard addict
Last edited by canard addict; Feb 08, 2013 at 07:40 PM.
Reply With Quote
Old Feb 08, 2013, 07:37 PM
Registered User
Joined Jun 2005
2,465 Posts
Mid motor Canard

The build activity on my model has resumed after a thirty day delay. The attempt to save weight seems to be working. Here is the present view of the canard which I estimate to weigh about three ounces finished.

Charles
canard addict is offline Find More Posts by canard addict
Reply With Quote
Old Feb 08, 2013, 07:44 PM
glider pilot in training
webdragon's Avatar
United States, AR, Searcy
Joined Aug 2006
647 Posts
Quote:
Originally Posted by canard addict View Post
webdragon, The rudder seems to have enough leverage about the CG which will be forward of the LE near the screws but lots of rudder area is always good. The large polyhedral wing will not respond to aileron movement because the tips will resist downward aileron control since they are there to hold lateral stability. It would be better to control the turns with rudder and elevator. I hope that you have some positive incidence on the canard to assure that it will stall before the main wing. Best of luck with it.

Charles
i found another rudder in the box that is about 4 inches taller an 3 inches wider. I plan on using it. as for the positive incidence i'm planning to trim the area for the canard mount to have 4 degrees positive.

your very close on the CG, you have a pair of very accurate eyes there, the Cg calc says 18 inches from the leading edge of the canard which drops it in 1/2 behind those screws.

since you seem to know quite a bit about canard aircraft. when designing one, do you calculate a rudder like you would when building a conventional plane.

roughly 50% of the tail/canard surfaces?
webdragon is offline Find More Posts by webdragon
Reply With Quote
Old Feb 09, 2013, 10:02 AM
Registered User
Joined Jun 2005
2,465 Posts
webdragon
[QUOTE]since you seem to know quite a bit about canard aircraft. when designing one, do you calculate a rudder like you would when building a conventional plane.

roughly 50% of the tail/canard surfaces? [/QUOTE

Thank you WD for the kind words. My knowledge about model planes has come from many years of experience. Rudder size became apparent to me from flying the Sig Cadet models with their large fins and rudders and from testing 1/3 scale gliders of my designs. In more than one case the rudder size and distance from the CG has greatly improved stability. Also experience has shown that a larger than scale horizontal stabilizer area is beneficial in models with lower Reynolds numbers to hold the rear of of a conventional type high at slow speeds especially on long coupled models. Our friend Don Stackhouse is highly qualified to discuss proportions for light weight models to full scale types.

Charles
canard addict is offline Find More Posts by canard addict
Reply With Quote
Old Feb 09, 2013, 05:06 PM
Registered User
numanair's Avatar
United States, WA, Kirkland
Joined Sep 2010
288 Posts
From this thread: http://www.rcgroups.com/forums/showp...&postcount=151
Looks similar to what you are doing, webdragon.
numanair is offline Find More Posts by numanair
Reply With Quote
Old Feb 09, 2013, 05:15 PM
glider pilot in training
webdragon's Avatar
United States, AR, Searcy
Joined Aug 2006
647 Posts
Quote:
Originally Posted by numanair View Post
From this thread: http://www.rcgroups.com/forums/showp...&postcount=151
Looks similar to what you are doing, webdragon.
Yes it does.
did a little research today while at my rc club meeting, I talked to the gentleman i got the glider parts from. turns out the fuse an tail i'm planning to use was a never completed skimmer 600. the plans were lost years ago an that was as far as the builder got. His loss my gain a suppose.

i pulled some of the sheeting off to get an idea how much room i'll have to mount everything i can as far forwards as i can so i can balance it with as little lead as i can get away with.

If i mount both servo's inline with the battery in front an RX behind them it may take only 1/4 ounce in with the battery which it may end up not needing once i add a little ply for a tow hook
webdragon is offline Find More Posts by webdragon
Reply With Quote
Old Feb 09, 2013, 06:04 PM
who has rabbit ears down
Captain Canardly's Avatar
United States, MN, Buffalo
Joined Jan 2007
3,630 Posts
Web Dragon! Just skootch that rudder aft a tad and you'll be fine- consider adding Dihedral to the canard too. (helps in the turns (2 channel)
Captain Canardly is online now Find More Posts by Captain Canardly
RCG Plus Member
Latest blog entry: One printer running!
Reply With Quote
Old Feb 09, 2013, 07:26 PM
glider pilot in training
webdragon's Avatar
United States, AR, Searcy
Joined Aug 2006
647 Posts
Quote:
Originally Posted by Captain Canardly View Post
Web Dragon! Just skootch that rudder aft a tad and you'll be fine- consider adding Dihedral to the canard too. (helps in the turns (2 channel)
I was thinking about that, it would be easy to cut down the centerline an sand in a few degrees.
webdragon is offline Find More Posts by webdragon
Reply With Quote
Old Feb 09, 2013, 08:10 PM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
4,076 Posts
Quote:
Originally Posted by webdragon View Post
...do you calculate a rudder like you would when building a conventional plane.

roughly 50% of the tail/canard surfaces?
Well, that's not how you calculate "rudder" for a conventional tail-aft layout (at least not the "proper" way), so no, it would not work very well for a canard layout, either.

That's a "rule of thumb", which would give you a workable answer within a limited range of cases, which would tend to make folks believe it. However, that particular rule of thumb has an especially narrow range of usefulness, and also carries no real indicators of just how narrow that range is.

The real problem is that it only looks at less than half the picture.

Tail authority depends on a number of factors, including (and especially) the tail moment arm (how far the tail is from the wing). The rule of thumb you mention completely ignores that factor, and also assumes the factors sizing the vertical tail (including the moment arm) are close to the corresponding factors for the horizontal tail. It's still not a good rule of thumb, and grossly oversimplifies the problem to the point of invalidating itself in a large number of cases.

In the case of canards, it's especially bad because the canard is on the opposite end of the plane, usually with a different moment arm as well, and the whole issue of yaw stability in a canard tends to be more complex than in an aft-tail as well, because of the destabilizing effects of the side area of the fuselage (which tends to be more of a problem for canard layouts).

Most of the details of what you need to know on this is covered in the "Ask Joe and Don" section of our website www.djaerotech.com There is a search engine on the opening page of AJ&D. Type in "tail volume coefficients", "tail moment arm" or "tail sizing" and it will come back with a list of links to articles in AJ&D that mention those topics.

As those articles describe, imagine that you were the world's most experienced airplane designer, with thousands of designs to your credit, some that worked well and others that didn't. You could probably just LOOK at a new design and by gut feel alone you could probably make a pretty decent guess at how a design was going to behave, long before its first test flight. From what I've read, Kelly Johnson was probably an example of that. His subordinates used to comment "That man can SEE air!" And along those lines, yes, visualization skills are indeed an important part of all of this.

Most of us don't have that kind of experience. However, what if you could attach a number to the experience of any designer with any design? By studying a variety of designs, some that worked and some that didn't, we could detect patterns in those numbers, that would suggest that for a reasonably similar category of designs (say for example, high aspect ratio sailplanes with aft-tails and no flaps), if we kept the tail design within a certain general range of these "tail effectiveness numbers", we could reasonably expect that the plane we're building would probably have similar behavior. By finding a way to study the results of the entire history of aviation and attach quantitative numbers to it, we could take advantage of all of the experience of all of those other designers!

In the case of tail sizing, we have just such an approach, although we do have to be a little careful of complicating factors (more on that in a moment). It's called "tail volume coefficients".

All we do is measure and multiply together everything that helps make the tail more effective, and then divide by everything that makes its job more difficult. On planes where the effects of the fuselage on stability and control can be safely ignored (which is often NOT the case for canards), the formula for the horizontal tail is tail area divided by wing area, times tail moment arm (see "AJ&D" for the proper way to find that) divided by the wing's Mean Aerodynamic Chord ("MAC", again see "AJ&D"). It's the MAC that rotates around the pitch axis, so that's what's in the Vht (horizontal tail volume coefficient) formula. Typical numbers are around .45 to .55

For the vertical tail it's similar, except it's the span (or more often the half-span, be careful of which version of the formula was used for the numbers you're looking at) that rotates around the yaw axis. The vertical tail formula is:
Vvt = tail area/wing area x tail moment arm/wing semispan
Typical numbers are around .05 if there are no major complications.

Unfortunately there are a number of things that can be complications. A large, slab-sided fuselage (or even more, a profile fuselage) acts like a fin+rudder, and if a bunch of that fuselage is in front of the C/G, that part acts like a fin+rudder mounted on the NOSE. This is destabilizing in yaw, possibly very destabilizing. Canards are especially prone to this problem, since the C/G tends to be close ot the wing, but there is a lot of fuselage in front of that needed to support the canard. In cases like that, you need to estimate what sort of fin effect that fuselage has, on what effective moment arm, and add enough extra fin behind (usually on a much shorter moment arm, so you need proportionately more area to make up for that).

You can (and should) try to calculate the effects, but you can often confirm your calculations by building a small free-flight model glider of your design. It's a lot cheaper and quicker than building an entire RC model and only then discovering a problem. One useful technique is to make the fin on that little test model deliberately oversize, then cut it down a little at a time until you find the point where yaw stability issues first start to raise their ugly heads.

Other factors include things like multi-engines and the possibility of asymmetric thrust if one quits (twins typically have about three times as much fin and rudder as a single-engine plane to deal with this). And yes, electric motors can and do fail occasionally, all it takes is one loose wire.

Propeller effects also come into play. A prop that has inclined inflow (such as a prop in a nose-up climbing attitude) has a moment effect, such as the well-known "P-factor" that tries to yaw a climbing plane with a right-handed prop to the left, but there is also an "in-plane" force parallel to the prop disk that pulls away from the inflow. In the case of that prop on a climbing plane, that in-plane force will try to pull the prop upwards. If the prop is in front of the C/G, it will try to pull the nose up, which is destabilizing in pitch. If the prop is behind the C/G, it will tend to pull the tail up, increasing stability. The same effects apply to yaw. On the Northrop YB-49 (the jet flying wing), when they got rid of the original XB-35's props, they had to add little fins to make up for the yaw stabilizing effects of the props, as well as the fin effects of the driveshaft housings for the props.

Bottom line is that a prop acts like a small combination vertical and horizontal fin wherever on the airframe that prop is mounted. On a large tractor prop on a long nose, you might have to add some more vertical tail to compensate for this.

Other things such as landing gear, nacelles, etc. can also act like fins, and alter the needed amount of vertical tail.

Yes, it sounds complicated, and in many cases it is. However, if it was easy, somebody else probably would have done it already. Welcome to the wonderful world of airplane design!
Don Stackhouse is offline Find More Posts by Don Stackhouse
Last edited by Don Stackhouse; Feb 10, 2013 at 09:19 AM.
Reply With Quote
Old Feb 09, 2013, 08:32 PM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
4,076 Posts
Quote:
Originally Posted by Captain Canardly View Post
Web Dragon! Just skootch that rudder aft a tad and you'll be fine- consider adding Dihedral to the canard too. (helps in the turns (2 channel)
Because the span of a canard is typically quite a bit less than the wing's, the roll-stabilizing effect of dihedral in the canard tends to be pretty insignificant. A little bit more dihedral in the wing will be more effective.

However, adding dihedral to the canard DOES make it act like a vertical fin (just like a V-tail) on the nose, which is destabilizing in yaw. There are special cases with reasons to add dihedral to the canard, but in general it will cause far more trouble in yaw than any benefit in roll stability.
Don Stackhouse is offline Find More Posts by Don Stackhouse
Reply With Quote
Old Feb 09, 2013, 10:52 PM
Registered User
Neenah, WI
Joined May 2006
387 Posts
For what it's worth: From experience with several models, adding dihedral to the canard helps hold the nose up (reduces tendency to spiral dive) when only rudder-control (and wing dihedral) are used to induce roll for turning.
gyrocptr is offline Find More Posts by gyrocptr
Reply With Quote
Old Feb 10, 2013, 03:07 AM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
3,736 Posts
Thank you Don - both comprehensive and clear, as always.

On a different subject, here's some information that might be useful. I've just covered the wings of my Long EZ with glass cloth and WBPU. It was much easier than I expected and, this morning after leaving it to go off overnight, it seems extremely effective. The foam and fibreglass wing is very stiff and the finish looks good so far. It will need some sanding and some paint to finish it off.

I needed a fairly stiff paintbrush, an old credit card to squeegee the liquid across the surface and disposable gloves. I mixed spackle and white acrylic paint into the liquid. The weight penalty, for a plane with 6' ws and 853 sq " of wing area was 2.8 oz (80gm) total. So I think it was worth it.

Now, rather than just following the scale dimensions for the fins, I'll have try Don's calculations and allow something for the fairly slab-sided nose and the spats. But, the fact that those fins are a long way from the centre line ... hmmmm I don't think that makes any difference.

nickchud is offline Find More Posts by nickchud
Reply With Quote
Reply


Thread Tools