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Old Feb 10, 2013, 10:14 AM
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Don Stackhouse's Avatar
United States, OH, Bradford
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Quote:
Originally Posted by gyrocptr View Post
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.
That is likely to be a quirk of a specific design, not a general principle, and not for the reasons you probably expect.

As I said before, adding dihedral to the canard makes it act a little like a V-tail mounted on the nose, or the same effect as adding a small vertical fin to the nose. This reduces yaw stability.

There is a balance between fin effect (yaw stability) and wing dihedral. If you have too much dihedral and/or not enough fin, you get a plane that's prone to "dutch roll", a sort of rolling+yawing oscillation that looks something like a falling leaf.

If you have too much fin and/or not enough dihedral, you get spiral instability, the plane wants to "overbank" itself into a "graveyard spiral".

It sounds to me like your example had that second problem, too much fin. By adding some dihedral to the canard, yes, you increased the total dihedral effect a little bit, but probably only a minuscule amount. The biggest effect was that the dihedral in the canard acted like you added a fin to the nose, counteracting some of the fin effect on the tail, and thereby reducing the total yaw stability to what the total dihedral could support.
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Old Feb 10, 2013, 10:42 AM
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Originally Posted by nickchud View Post
...But, the fact that those fins are a long way from the centre line ... hmmmm I don't think that makes any difference.

Yes and no.

The tail moment arm is considered to be the distance PARALLEL TO THE AIRCRAFT CENTERLINE from the aerodynamic center of the wing to the aerodynamic center of the tail surface in question. So, in that sense, no, the fact that the fins are out at the wingtips should not make any difference. What matters is how far aft they are.

However, if those fins are NASA winglets on the tips of swept wings, such as on typical Rutan canards, there is another factor that comes into play.

Those winglets are designed to lift inwards, so that their lift opposes the wingtip vortex (see pic). Properly done, they actually harvest the energy from the wing vortex flow, reducing the induced drag of the wing.

But, in the case of a swept and/or dihedralled wing that has those fins significantly aft of the C/G, there is another effect that comes into play in turns. In a turn, the wingtip on the outside of the turn has a higher airspeed than the one on the inside of the turn. If those tips carry winglets on them, then the inward lift of the outside winglet will be greater than the inward lift of the inside winglet. If those winglets are significantly aft of the C/G, then this difference in winglet lift will tend to yaw the plane towards the outside of the turn.

If the wing has sufficient dihedral effect (either due to dihedral, and/or due to sweep plus lift coefficient), this yawing will then make the plane try to roll back towards level flight, even in a perfectly coordinated turn. The plane will have naturally positive spiral stability. A single, centrally mounted fin of the same total area and moment arm would not have this effect, so yes, in this case the fact that those winglets (not just fins) are out at the ends of the wings, far form the aircraft centerline, does have a significant effect.

I should also clarify the matter of dihedral effect due to sweep. Sweep can act like dihedral, although not always. There is an old rule of thumb that says three degrees of sweep is about equivalent to one degree of dihedral. Of course by now most of you probably know how I feel about rules of thumb.

The problem is that the amount of dihedral effect caused by sweep varies with the lift coefficient. That three degrees rule of thumb is roughly in the ballpark for typical arrangements, IF the plane is at high lift, such as near stall. Takeoff, climb and on final approach to landing would be examples. However, if the plane is at low lift, such as cruise, the dihedral effect of sweep is reduced, and at zero lift it goes away entirely.

Now, recall my earlier comment that there has to be a balance between fin and dihedral, or you can have problems with dutch roll and/or spiral instability. A plane with sweep can act like it has too much dihedral at high lift conditions (and problems with dutch roll during things like landing approach), AND act like it has too little dihedral (and problems with spiral instability) when at high speed/low lift. And yes, this is not just theory, I have seen this in practice.

Note on planes like the VariEze and LongEze how the wing is set with a little bit of anhedral (downward dihedral). This is to compensate for the dihedral effects of their swept wings. In typical flight modes, that sweep would act like too much dihedral, causing dutch roll problems. The anhedral cancels out enough of the sweep's dihedral effect to avoid this.
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Old Feb 10, 2013, 05:14 PM
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United States, WA, Bellingham
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Quickie 380 wanted

New to the site and not sure where to post. I am looking for a quickie 380 to buy. Believe they were made by rich models but no longer made. Anyone selling one?
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Old Feb 10, 2013, 11:58 PM
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Quote:
Originally Posted by Don Stackhouse View Post
...
If you have too much fin and/or not enough dihedral, you get spiral instability, the plane wants to "overbank" itself into a "graveyard spiral".

It sounds to me like your example had that second problem, too much fin. ....
So...... this suggests that if you only have R&E control on a canard aircraft, and if you add lots (excess) of rudder-fin area to increase yaw-roll control, it may be helpful to add dihedral to the canard?

Query: since the wing is typically behind the CG in a canard aircraft, would large amounts of wing dihedral also have the effect of extra fin, thus contributing to a "too-much-fin" condition where dihedral in the canard would helpful?
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Old Feb 11, 2013, 04:17 AM
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Sydney, Australia
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Quote:
Originally Posted by Don Stackhouse View Post
It sounds to me like your example had that second problem, too much fin. By adding some dihedral to the canard, yes, you increased the total dihedral effect a little bit, but probably only a minuscule amount. The biggest effect was that the dihedral in the canard acted like you added a fin to the nose, counteracting some of the fin effect on the tail, and thereby reducing the total yaw stability to what the total dihedral could support.
Hi Don, I'd like to ask your opinion about an alternative explanation for this problem of canard turning stability. My suggestion is the problem actually starts as a dutch roll condition that is exacerbated by low dynamic yaw stability in a canard model. In other words, a lack of damping allows the dutch roll oscillation to initiate a tip stall of the main wing. Even if a canard model has just about enough static yaw stability, the damping may still be too low due to the short tail moment. Do you think the addition of a forward fin can actually increase the dynamic yaw stability in a helpful way in this scenario?
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Old Feb 11, 2013, 05:44 AM
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Market Harborough
Joined Apr 2006
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1/4 scale Long EZ

I'm getting towards the end of my build now and waiting for more enlightenment from this discussion before settling on my winglets. Mine isn't truly a Long EZ because I used the outer wing panels from a much larger Starship. Studying the pictures of EZs on line there appears to be quite a bit of variation amongst them.

So, if I understand the discussion so far - fins normally work to counteract dihedral or sweep. Large fins give positive spiral stability where the plane in some conditions needs outward rudder in a turn to stop it getting tighter and tighter. I noticed this with a Catalina – no sweep or dihedral, just a large parasol wing and a very large fin.

Dutch roll results from too much dihedral or sweep and not enough fin. I’ve seen this on a Polaris, close to the stall when it starts to wobble. Perhaps that’s not a Dutch roll but it is the only unwanted roll instability I’ve experienced.

I did have a plane once without enough fin or moment arm and that used to go a bit sideways when gliding in for a landing. Nothing unstable about it.

A lot of military jets have anhedral and also pretty large fins.

With regard to Starships and Long EZs I have studied the various 3-views and I’ve never seen either dihedral or anhedral, though I know that 3-views are not always reliable. The Starship has its winglets tilted inwards 2deg. I think that’s in order to direct the “inward lift” in a straight line which passes through the CoG. The Long EZ doesn’t have that, maybe just because it was simpler for the home builder. They both have rudders which move outwards only. This means that, in a turn, the rudder increases the inward lift of the inside winglet and it does nothing to the outside winglet.

I guess that long thin canard on the EZ will help to dampen any roll instability. It has no dihedral or sweep.

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Old Feb 11, 2013, 04:28 PM
Lighting Up the Darkness
United States, GA, Loganville
Joined Jan 2013
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Nightlight Canard Update

Greetings canard friends. Here are a few shots of the latest and greatest little GWS Slow Stick Canard project I've been occupying myself with on this very rainy day. I tell you, Charles makes stuff look so easy, he'll even tell you it's easy.......but it's not! I'm almost done with it, while it is a very well reinforced little bird, over powered and all.....it feels heavy as a BRICK. The 4 inch ultra light wheels had to go as they could not support the weight. I'm going with a more sturdy wheel, a bit more weight though. I took a chance on a little all foam canard, with lots of dihedral. I think it will fly, but I'm convinced that there is not enough area or throw to the elevon (I think that's what it's called). I plan to sheet over the foam with thin balsa on both sides of the control surface only, to add a little more throw, and I'm planning to install hinges since it's a very sketchy foam seam hinge and I just don't trust it's integrity. Next nice day I am planning on making a maiden flight video, so we'll see. I will post it for your viewing pleasure (or cringe whatever the case may be!) Take care,
Russ
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Old Feb 11, 2013, 05:01 PM
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Canard/tandem with lots of dihedral on both flying surfaces has been tried (see pic)...

Seriously though, one thing you can do that will improve the plane significantly is tape over any gaps in the middle of the wing and canard. Can't see for sure in your pics on the Slow Stik wings you're using, but the old GWS "Pico Stik" or "Lite Stik" had a pretty substantial gap in the middle of its wings. The middle of the flying surface is the part that works the hardest at making lift. Having a hole/gap there, even a small one, is like trying to fill up a bathtub with the drain open. The gap can nearly double the induced drag.
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Old Feb 11, 2013, 05:25 PM
Lighting Up the Darkness
United States, GA, Loganville
Joined Jan 2013
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Thank you Don. Great advice, much appreciated and you're exactly right.....huge gap on that main wing, then many holes, etc. on the repurposed canard. I'm going to lay in the floor and envision a wind tunnel situation. One great thing about this type of project......a wide piece of strategically applied packaging tape can fix a problem in short order.
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Old Feb 11, 2013, 05:36 PM
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There used to be a website with all sorts of mods for the GWS Pico Stik, including a canard version (I checked for it just now, but it doesn't seem to be available anymore). There were a few notes of mine on it, including taping over the gap, as well as how to cut the outboard ends of the wing free about 6" in from the tip, then sanding a bevel and taping them back on with polyhedral. Dramatically improved the roll response and stability, without all the strings hanging out in the breeze that other folks were using.

At the time I ran the wings through my planform analysis software. Induced drag is inversely proportional to the square of the wingspan. With a gap in the middle, the wing acts like two half-size wings, which is far worse than one wing of the same total span. My planform analysis software told me that the induced drag with the gap taped over was only 54% of what it was with the gap open. A bunch of us tried it, and all found our real-world results in line with what the analysis had indicated.

The bigger Slow Stik did improve a number of details quite significantly. It had actual dihedral built into the wing mounts, and I believe the gap between the wings is proportionally smaller. Still, even a 1/16" gap can have a surprising effect on induced drag.
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Old Feb 12, 2013, 05:24 AM
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On the subject of tail area, Don says:
Quote:
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.
If I use that formula and plug in the measurements from my model EZ, I get a number very close to 0.08. Does that mean I should use slightly smaller winglets, or just take special care with rudders and ailerons when turning? I'm bearing in mind that when landing my swept wings will have an increased dihedral effect so, for stability at that time, oversize fin area will be a good thing.

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Old Feb 12, 2013, 08:31 AM
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Are you making allowances for the side area of the nose? That has quite a bit of fin effect of its own, which would cancel out some of the yaw stability from the winglets, and call for a little more Vvt than normal.
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Old Feb 12, 2013, 09:15 AM
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Quote:
Are you making allowances for the side area of the nose?
Yes, I did think about that and I looked at the side-view drawing. Then I confess to looking at it a bit longer, sucking my teeth and thinking: "a little more Vvt than normal, OK". Well, .08 is quite a lot more than .05. After I've allowed for spats and u/c legs I don't think the lateral area in front of the CG is really very much more than I've got behind it.

When thinking about margins of error I like to err on the safe side rather than the other side, at least in the beginning. I can always cut a bit off the top of those winglets if I can land the plane in one piece.

so "Does that mean I should use slightly smaller winglets?" Probably not.

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Old Feb 12, 2013, 10:52 AM
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It's not just how much is in front vs behind the C/G, you also have to consider how far in front or behind. A lot of that nose is very far in front, while the portions behind are on comparatively shorter moment arms.

The other thing to consider is that winglets add a significant amount of dihedral effect of their own, which then requires even more yaw stability to avoid dutch roll. Canting the winglets inward (such as the Starship's) can help mitigate that additional dihedral effect.
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Old Feb 15, 2013, 04:32 PM
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Sorry about the delays getting back on your's and John's questions, it's been busy here, plus I wanted to think about these a bit.

Quote:
Originally Posted by gyrocptr View Post
So...... this suggests that if you only have R&E control on a canard aircraft, and if you add lots (excess) of rudder-fin area to increase yaw-roll control, it may be helpful to add dihedral to the canard?
Depends on what problem you're trying to solve.

Roll authority from rudder (yaw) coupling with dihedral is primarily dependent on two factors: the amount of yaw, and the amount of dihedral. Increasing either one (or both) will increase the resulting roll authority.

If you have too much fin and lots of yaw authority, and you're still not getting enough roll response, or if it's taking too much yaw to get adequate roll response, then increasing dihedral should result in faster roll response with less yaw.

Yes, adding dihedral to the canard does add some total dihedral effect. However, it's a poorly effective way to do it, unless your canard has nearly the same span as the wing. Adding more dihedral to the wing is usually a better way to go.

Basic principles:

If you want to alter roll behavior and stability, look for the widest thing(s) on the airplane. Normally that means the ends of the main wing.

If you want to alter yaw stability, look for the places furthest ahead or behind the C/G. Generally that means the tail, or the nose.

Quote:
Query: since the wing is typically behind the CG in a canard aircraft, would large amounts of wing dihedral also have the effect of extra fin, thus contributing to a "too-much-fin" condition where dihedral in the canard would helpful?
Yes, adding dihedral to the wing, if the wing is a significant distance in front of or behind the C/G, does act like adding some fin at that location.

HOWEVER, you're talking "qualitatively", not quantitatively. The changes we're discussing tend to have effects on both yaw and roll. The question is just how much, in comparison to each other. Yes, the wing adds fin effect, but the effect on roll stability is probably going to be much greater, because of the great span of the wing, compared to its relatively tiny moment arm for yaw effects. Thus, the balance of the two is probably going to favor the effect on roll.

There are ways to play these effects against each other, allowing you to control how this balance comes out, but that gets really complicated.
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