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Old Nov 08, 2008, 09:39 PM
INDORUS EXPARAMINTO
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Quote:
Originally Posted by Don Stackhouse
I presume you're referring to turbulation to prevent boundary layer separation. That really only applies to lower Reynolds numbers, such as models or on relatively slow full-scale aircraft. At higher Re's, the efficiency-improvement problem is usually more about trying to keep the flow from becoming turbulent.

There are cases of turbulation (such as vortex generators) used on higher Reynolds number applications, but there it is generally to prevent or delay stall of that portion of the flying surface. It does not generally improve efficiency (in fact it normally reduces it), but it can increase the max lift coefficient possible from the surface. It's often used as an add-on to fix handling problems, such as loss of aileron effectiveness near stall, or to increase control authority.

For example, used on the fin of a twin-engined aircraft they can allow more lift to be squeezed out of a given size fin+rudder, allowing the tail to handle more asymmetric thrust in an engine-out situation. This can fix an existing problem, or it can allow a new design to get away with a smaller vertical tail than it would need without vortex generators. However, the benefit here is not from a direct increase in fin efficiency in cruise (as before, it reduces it), but rather by allowing the fin (which in a twin is sized by an emergency situation) to be smaller. Note, vertical tails on twins are normally several times larger than what yaw stability alone would call for, just to get enough authority for the engine-out situation.



A canard with a control surface on it IS a variable geometry canard. The majority of canards already out there fall into this category. A canard with variable sweep (such as on the Beech Starship) is a more radical example of variable geometry. In that case they have a wing with flaps, and need a multi-mode canard to keep the canard's lifting ability matched to that of the wing in its different modes. Beech paid a pretty high price in the form of a fairly complex and extremely flight-critical jackscrew linkage coupling the canard sweep to the wing flaps in order to pull that off. A failure of that mechanical system could immediately make the airplane incontrollable. Failure modes end up making a lot of the decisions in aircraft designs.

In all cases, for decent stall characteristics you need to make sure the canard stalls before the wing, unless you can limit the canard's capability by some other means, such as through a very "smart" fly-by-wire system. If you don't, you run the risk of a violent pitch excursion at stall. As Burt Rutan pointed out to me once, you almost never want to add more lifting ability to a canard (the typical reason for more elaborate variable geometry techniques) unless you first increase the lifting ability of the wing.

In any case, it's important to remember that it's the wing's job to support the plane, and the tail's (or canard's, in this case) job to take care of stabilizing and controlling the wing. When you start to blur those two tasks, overall aircraft efficiency generally goes down.
Well dang Don.
You took the word right out of my mouth once again.
Thanks for such great points to ponder

Larry
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Old Nov 09, 2008, 07:29 AM
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How about this canard----------->

http://commons.wikimedia.org/wiki/Im..._Jaeger_02.jpg

http://yukikazeb03.deviantart.com/ar...eger-103089273
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Old Nov 09, 2008, 11:06 AM
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Ruffled Feathers? No Way

There are lots of intelligent viewers here. I am here to have fun with something different. The present discussion goes from super sonic full scale high Re no. to models with low Re. The range is great and nothing, in my opinion, is cast in stone including my views. All angles presented are valuable to me. Yukikase, The delta is interesting to me with the lifting wing up front. It could be helpful to raise the nose and create drag to land like a bird? There seems to be a tendency of our full scale fast aircraft small and large to move toward the delta shape. It also seems that the front elevator is a new trend. My Dixie Delta will mush in on landing but drops it's nose at the last second. Maybe a canard elevator would help? Charles
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Old Nov 09, 2008, 01:17 PM
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Deltas have some advantages, including more lifting surface for less wingspan and can be stronger for less weight penalty. The disadvantages include some handling problems at low speed and higher drag.

1) Forward canards enable more pitch stability & more gentle rolling?

The farther forward they are, the more effect they have on pitch rate - think of it as a longer moment arm. This is good because you can make them smaller which reduces weight and drag.

Forward positioning of a canard has very little effect on rolling, although there is some linkage between the various degrees of freedom. Generally the canard control surfaces move at the same amount and are limited to pitch.

2) Canards just in-front of wings provide less stability and more unstable maneuverability?

Stability is generally a function of the separation between the center of gravity and center of aerodynamic lift. If the weight of the canard remains constant as it is moved forward on the design, the cg and cl will also move forward. Generally, the canard gets smaller as it gets moved forward which can tend to mitigate the movement of the cg and cl. So, it really depends on what you are moving and how much.
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Old Nov 09, 2008, 08:30 PM
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Martimer, Those are beautiful explanations and I agree. Reminds me of John 235. Are you still with us John? Charles
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Old Nov 10, 2008, 07:51 AM
What could possibly go wrong?
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Quote:
Originally Posted by martimer
The farther forward they are, the more effect they have on pitch rate - think of it as a longer moment arm.
I've been working away on my Beechcraft Starship - concept tester. here's a link I haven't had much chance to fly it yet due to bad weather. Maiden flight was last week. I was delighted with how well-behaved it is. Pulls out of a dive nicely. Yank and bank works fine and there was a nice steady glide to land. I didn't try adding a little power to flair on landing as I didn't want to take any chances on stalling near the ground. Can't wait to get it out again.

In this case, practice seems to confirm that a fairly long moment arm will allow me to get away with a smallish canard. As an almost complete canard-beginner, I would say there are 2 rules for success...

1) Follow the Canard Addict suggestion for incidence, ie zero for the motor 1.5 degrees for the wing and 3.5 degrees for the canard.
2) Use the CoG calculator.

I'm not so confident about taking off from grass. It was quite windy when I tried. As soon the as the nose got up, the wind got under it, turned it sideways on and bowled it over. I'll wait for a calm day before I try that again.

I'm hooked!

Nick
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Old Nov 10, 2008, 08:31 AM
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One of Marty's rules for flying is always take-off directly into the wind unless you have enough excess power that your runway need only be two feet long. It was learned the hard way and used up several aircraft...
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Old Nov 10, 2008, 09:18 AM
What could possibly go wrong?
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I thought I was directly into the wind, I certainly wasn't far off. Maybe I didn't have those wingtip rudders set up exactly right. The ground was bumpy. The wind was about 15mph. I guess you have less margin for error if you're pushing instead of pulling! Anyway it all happened a bit too fast for me.

No real harm done.

Nick
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Old Nov 10, 2008, 12:21 PM
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Incidences and thrust line are things that apply to each individual design. There is no "magic" one-size-fits-all combination that works on all designs. If you have something that is very similar in moment arms, volume coefficients, C/G location (both longitudinal and vertical), wing and canard airfoils and aspect ratios, pitching moments, etc., then their incidence requirements may be reasonably similar. However, if you have significant differences in your design compared to the ones your "rule of thumb" is based on, then the setup will need to be different. Simply changing the desired flight condition at neutral elevator, or changing the C/G and associated static margin will alter the required incidences.

Changing the vertical location of the C/G will change the required thrust line angle.

Each design needs to be evaluated on its own merits.
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Old Nov 10, 2008, 12:49 PM
INDORUS EXPARAMINTO
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One word, Telos

Could be making a comeback
Maybe Richard from jareldesign.com has taken pity on us

Larry
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Old Nov 11, 2008, 06:28 AM
What could possibly go wrong?
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Market Harborough
Joined Apr 2006
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Quote:
Originally Posted by Don Stackhouse
Incidences and thrust line are things that apply to each individual design. There is no "magic" one-size-fits-all combination that works on all designs. If you have something that is very similar in moment arms, volume coefficients, C/G location (both longitudinal and vertical), wing and canard airfoils and aspect ratios, pitching moments, etc., then their incidence requirements may be reasonably similar. However, if you have significant differences in your design compared to the ones your "rule of thumb" is based on, then the setup will need to be different. Simply changing the desired flight condition at neutral elevator, or changing the C/G and associated static margin will alter the required incidences.

Changing the vertical location of the C/G will change the required thrust line angle.

Each design needs to be evaluated on its own merits.
Well I got lucky apparantly, so in progressing to a Beechcraft Starship, I'll stick to the incidence and thrust lines I have now!

Don, you mentioned the Starship and its variable sweep canard. I have a plan to introduce this, linked to flaps by a system of rods and cranks.

Step one in development now will be to replace the existing swept canard on my crash dummy with a narrower, straight one, closer to scale, and re-enter the details in the CoG calculator. Then see if my luck holds.

for setting up the flap / canard system, I'll run some different sizes and sweep angles through the CoG calculator till I get roughly the same CoG for both configurations.

Step two will be to build the actual model and fly the plane, flaps down and canard forward. Once I get some height, I'll try pulling in the flaps and sweeping the canard.

What could go wrong?

My experience of communicating with Don is that it may be a mistake to ask that question. But I will try to keep up

cheers

Nick
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Last edited by nickchud; Nov 11, 2008 at 06:34 AM.
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Old Nov 16, 2008, 08:29 AM
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Steve, Thank you for showing your fine work. That is not an easy project and I admire your determination. My 75% goose is still waiting to be finished. It has been on hold because of an ARF Telemaster which I assembled for a friend. Please keep us informed on your progress especially before the maiden flight. Charles
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Old Nov 16, 2008, 09:21 AM
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Quote:
Originally Posted by canard addict
My Dixie Delta will mush in on landing but drops it's nose at the last second. Maybe a canard elevator would help? Charles
Hi Charles -- The trick is to touch ground just before the "last second"

Randy
Model Airplane Engineering
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Old Nov 16, 2008, 12:30 PM
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Randy, You are right about my bad timing on landing the Dixie Delta. I envy the way you mush all the way in with yours. I did not mean to criticize your model and was thinking at the time of how a full size delta may be better controlled with a front elevator and to not have to depend on rear elevons to control the attitude at low landing speed. My 30% canards give me good nose UP control at low speeds. I feel the large size is needed on model planes just as larger rear elevators are better to deal with low Reynolds numbers. Charles
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