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Old Feb 18, 2013, 04:28 AM
GH crash - I practice a lot
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New topic - Need flight trimming advice

OK, you experts. How about a change of subject.

Iím hunting for good advice on flight trimming an indoor micro canard; 18" wingspan, 32 grams flying weight. The plane is somewhat unique in that it has a fixed canard with no control surfaces; the only control surfaces are the elevrons on the main wing.

I scratch built this canard and it flies but needs a good amount of up elevator trim to get it to fly level. What do I need to change to get straight and level flight without needing up elevator trim? Re-designing the plane may be the answer, but I don't want to add control surfaces to the canard.
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Old Feb 18, 2013, 04:59 AM
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ghcrash, can you give us the dimensions of the canard stabiliser? It looks really small, and I have doubts whether it is generating useful lift. I make that comment due to physics of very small wings (really low Reynolds numbers). My first idea is to increase the area of the canard before doing other experiments. You will need to adjust the CG forward to compensate for any added area.

I would consider adding trim tabs (flaps) on the trailing edge of the canard. They can be fixed in place and maybe just add around 5 degrees of downards flap deflection for the canard. After that is done it should be easy to fine tune the pitch stability of the model using elevons.
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Old Feb 18, 2013, 08:52 AM
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Charles, I owe you an apology. I re-read your earlier post, and noticed that I had missed the point that your initial statement referred to the lift components in level flight. I had turns on my mind, and misinterpreted your initial statements.

That said, that still does not support your claims that the solution is a larger canard, nor do you give any explanation why you don't believe it's a tip stall, even though that scenario is a perfect fit with the symptoms John described.

Making the canard larger will require moving the C/G forward. This has a number of effects, some positive and some negative.

First of all, it will decrease the "fin effects" of the nose and the canard tips and support structure, while increasing the effectiveness of the vertical tail. OTOH, it will increase the mass at the extremities, putting more demands on the available yaw damping. The two effects should largely cancel each other as far as dutch roll, but will worsen any tendency towards spiral instability. The size of that "sweet spot" between the two conditions is likely to get smaller.

Having more canard area, and the need to add more load to it (i.e.: move the C/G forwards) to maintain static pitch stability and the canard-must-stall-first requirement means that the canard will now carry a greater portion of the aircraft's total weight. Since the smaller canard is less efficient at making lift than the larger wing, this means the aircraft's L/D will decrease. In addition, the canard will be making more disturbance and downwash in the wing's inflow, further hurting the wing's lift distribution and efficiency, with further losses in the aircraft's L/D. (Note, these concerns are why it's a good idea to keep canards as small as possible)

Assuming the C/G can be moved forward without massive amounts of ballast (could be a problem on a sailplane), the increased total canard+wing area will lower the aircraft's overall wing loading. This does mean that for a given lift coefficient, the plane will not penetrate as well, but will be able to reduce its turning radius. This will help thermalling ability. However, the whole reason for all of this was an attempt to cure a tip stall problem. The tighter turning radius will increase the discrepancy in airspeeds between the wingtips during the turn, which will make any tip stalling tendencies worse.
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Old Feb 19, 2013, 07:25 AM
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Don, Your apology is warmly accepted but after reading your two great replies on the subject I see it was hardly needed. When I reviewed my first sentence it was misunderstood by me and came close to an edit.

I associate tail wagging and wing wobble to short coupled models with small tail areas, dihedral and non tapered wings. These observations apply to models and not necessarily to full scale types. The most stable models that I recall are the Sig Kadets and DeBolt designs. Two wobblers that come to mind are my original Georgia Goose and my Icon A5. I really love to fly the A5 because of it's beauty and good manners in every other way except the slight wag when coming out of a turn.I feel sure that the full scale version is fully stable. My original Goose had a shorter fuselage, dihedral in both wings and not enough canard area. My full scale experiences are limited but I feel that the non tapered wings of the J3 Cub and the Stearman are capable of more violent tip stalls than the Cessna 180 types.

Regarding the sail plane of John 235, I still believe that the small canard is carrying too much load and is stalling in a nose down turn.

Charles
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Old Feb 19, 2013, 09:26 AM
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...I associate tail wagging and wing wobble to short coupled models with small tail areas,
It's the net combination that matters. Too little area, and/or too little moment arm, can result in inadequate damping, unless the other parameter is big enough to compensate. Of the two, the moment arm is more important. Dynamic stability is linear with tail area, but goes with the square of the tail moment arm. Planes that are famous for outstanding handling properties in these regards, such as the Spitfire, Ryan Navion, Bucker Jungmeister, DJ Aerotech Monarch and Chrysalis (sorry, just had to sneak those in there) typically have small tails on long moment arms.

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dihedral
Sufficient dihedral to overwhelm inadequate yaw damping results in dutch roll.

[quote] and non tapered wings.[quote]

This is not generally a factor, other than the wider tips will generally (but not inherently) have more mass, which makes the damping's job more difficult. More mass does mean more inertia in roll and yaw, and therefore energy that has to be dissipated to stop the oscillations. The designer does need to make an effort to keep the tips light.

Other than that, no, wider tips should not cause dutch roll or similar problems.

Quote:
These observations apply to models and not necessarily to full scale types.
The equations and laws of physics are the same. The principle difference would be that on full scale the inertia tends to be a bigger player. It's the old law of squares and cubes, areas scale with the square of the size change, but volumes (and therefore weight) scales with the cube of the size change. This tends to make models a little more forgiving in these matters.

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... My full scale experiences are limited but I feel that the non tapered wings of the J3 Cub and the Stearman are capable of more violent tip stalls than the Cessna 180 types.
Not sure where you're coming up with that, it's definitely at odds with both analysis and with my own experience.

I have a lot of time in Cubs (including a lot of experience spinning them, as well as other similar types such as the Aeronca 7AC "Champ" and the BC12D Taylorcraft), not so much in Stearmans, although I am familiar with them on a secondhand basis. I have no firsthand experience with the 180, but a lot of time in other high-wing Cessnas like the 172 and 150. In my experience the Cub's stall behavior is far superior to the Cessnas. Tip stalls are not an issue with Cubs, in fact their general low-speed handling is very benign. They will spin if you force them to, but are not prone to do it on their own. Wing drop at stall on the Cub is far less than the Cessnas in my experience, both power on and power off. The Cub's airfoil, the USA-35B, has some of the most gentle stall characteristics of any airfoil in the books, with an extremely rounded-off Cl-vs-alpha curve before, during and after the stall. The curve's shape is almost a semicircle! The DeHavilland Chipmunk has the best stall characteristics of any full-scale I've flown, and it uses a NACA 2412 at the root along with stall strips, transitioning to a USA-35B at the tip, with a moderate taper ratio, about 0.7. Truly delightful airplane, it's been literally a let-down flying anything else ever since.

Unfortunately, that's full-scale. The USA-35B is too thick to work well at model Re's, although it might be retailored to compensate. It would need to be thinned and recambered, and the high point moved a little further forward. Of course at that point it's no longer a USA-35B. The same sort of concerns apply to the famous Clark Y, excellent full-scale airfoil but mediocre at our Reynolds numbers. Too thick, too much camber, high point a little too far aft.

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Regarding the sail plane of John 235, I still believe that the small canard is carrying too much load and is stalling in a nose down turn.
"Stalling in a nose-down turn" ? WHY?? That does not fit John's description. He notes a sudden steepening of the bank angle into a steep spiral, which indicates a stall of the inside wingtip. A canard stall would result in a nearly constant bank angle, with the nose pitching outward and downward, perpendicular to the wings, widening the turn in the process. Doesn't fit the evidence.

As far as wider tips being more prone to tip stalling, actually the opposite is true. A wing with wider tips has more tendency to stall first at the root, with a gradual spreading of the stalled region outboard if the angle of attack is increased further beyond the initial stall at the root. The last thing to go is the tips. Note, that's in a level-flight stall. In a turn, the difference in airspeeds along the wing can shift the stall more towards the inside wing tip, but even there, the wing with wider tips will be more resistant to tip stalls than a wing with narrower tips.

The down side of wider tips, besides the increase in yaw inertia from the extra structural weight out there, is that they hurt performance. They have too much area in the tips, resulting in lower lift coefficients (which is precisely what makes the tips more stall resistant!), so the tips are not working as hard as the rest of the wing. This means they are not making as much lift per square inch of whetted area, resulting in a skin friction drag penalty, as well as a less elliptical lift distribution that causes an induced drag penalty (although at high lift the lift distribution is still surprisingly close to elliptical).

For a "rule of thumb" for typical sport models (and you know how I feel about those), keeping the taper ratio (i.e.: tip chord divided by root chord) at or above about 0.6 to 0.7 will usually keep you out of trouble with regard to tip stalls. Thermal sailplanes that have to make extremely tight thermal turns get somewhat more complicated, requiring some additional considerations. Narrower tips can also have good stall characteristics and tip stall resistance, but it requires more cleverness and effort on the part of the designer.
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Last edited by Don Stackhouse; Feb 19, 2013 at 09:44 AM.
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Old Feb 19, 2013, 10:16 AM
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Great discussion, Don, I enjoyed every word especially on the J3 Cub which I only assumed would have a tendency to have a violent tip stall in high nose attitudes. I have had lots of models of the J3 and remember at least one which would snap to one side under pressure. Many hours were spent in the Stearman with my Brother who loved to force it into a rather violent whip stall in a climbing left turn. The Stearman would do a great tail spin.My models of Cessnas have all been very gentle and stable which led me to give credit to the tapered wing design.

Charles
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Old Feb 19, 2013, 11:23 AM
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Great discussion, Don, I enjoyed every word especially on the J3 Cub which I only assumed would have a tendency to have a violent tip stall in high nose attitudes.
Nope, it will stall and will spin, but compared to my experience with the high-wing Cessnas (especially the 150, which I consider absolutely vicious by comparison), it's a pussycat.

The Cub is extremely easy to fly, but difficult to fly well, which is what makes it such a good trainer. It will not let you hurt yourself, but if you are less than perfect, it will make you look foolish in front of everyone else at the airport.

The Cherokee, by comparison, with its "Hershey bar" constant chord, constant airfoil wing, is almost too gentle. It can actually cover up your mistakes, which leads some instructors to dislike it as a trainer.

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I have had lots of models of the J3 and remember at least one which would snap to one side under pressure.
Of the various full-scale Cubs I've flown, there were subtle variations, most likely the result of rigging. I would check that particular model for things like warps and alignment issues. Another possible factor would be inappropriate airfoils for model Re's.

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Many hours were spent in the Stearman with my Brother who loved to force it into a rather violent whip stall in a climbing left turn.
Whip stalls really don't say much about the plane's natural stall characteristics. A whip stall is basically an entry into a loop, but without enough speed and power to make it over the top. With the right technique (mainly airspeed during entry), I think you could probably get that supposedly unstallable Ercoupe to whip stall (although we never pushed the one I flew far enough to actually find out, it would always just sort of mush over, back into level flight, after running out of elevator).

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The Stearman would do a great tail spin.
There are a number of factors that can make the Stearman a real handful. The untapered wing is most definitely not one of them. It has serious mass all over the airframe, including a huge, monstrously heavy round engine and a gigantic (relatively speaking) prop with lots of P-factor and gyroscopic effects, a tall, narrow landing gear, and a bad attitude in general. Mainly it's the mass that set up most of the potential for mayhem. The military did not want it to be easy to fly, they wanted it to be demanding, like the planes the new recruits would be moving up to next.

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My models of Cessnas have all been very gentle and stable which led me to give credit to the tapered wing design.
Nope, not the tapered wings, although the taper ratios on Cessnas are moderate enough to not cause a problem. The main thing the taper does is help the efficiency a bit. Instead, credit the washout, stall strips and choice of airfoils vs. local loading along the planform for most of the stall characteristics.
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Old Feb 20, 2013, 02:28 PM
What could possibly go wrong?
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Here's my Long EZ : 1/4 scale. She flies beautifully, fast, slow, loops, rolls, very easy to control. I still have more to learn about inverted flight. Not finished yet.

Long EZ (2 min 33 sec)
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Old Feb 20, 2013, 02:36 PM
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Here's my Long EZ : 1/4 scale. She flies beautifully, fast, slow, loops, rolls, very easy to control. I still have more to learn about inverted flight. Not finished yet.

http://youtu.be/9ecGemWJ4VE
Where's the dog? Where is the dog barking at your plane?

Nice video. Looks good, seems like it flies quite nicely, no bad habits, ( that I could see) so you did good.

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Old Feb 20, 2013, 03:00 PM
What could possibly go wrong?
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Where's the dog? Where is the dog barking at your plane?
Hah! You missed him. Me too. I can't take him to the flying club and that plane needs a runway. Unless I fix a hook for the bungee launcher. Those were taken from a 5min 30 sec flight. My 2200 battery went from 12.6 to 11.1 volts.

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Old Feb 20, 2013, 03:53 PM
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Nick, Thats a really awesome flight. Really well done on the 1/4" scale sized model. Now I underestand your excitement after the maiden flight on 17th Feb. Its a gently flyer because 10oz/square ft wing loading is really low for a a model with over 2m wing span.
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Old Feb 20, 2013, 04:40 PM
What could possibly go wrong?
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Thank you John

I've scratch built and flown quite a few canards now and I would say they're generally pretty well behaved across several different mission profiles, except maybe 3-D. Has anyone managed a good knife-edge?

It's specially important to take the good advice from people like John, Charles and Don, but don't let them make it all sound too difficult. There are quite wide margins for error, fortunately.

What I would say is: make them light and rigid.
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Old Feb 20, 2013, 05:11 PM
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Originally Posted by ghcrash View Post
OK, you experts. How about a change of subject.

Iím hunting for good advice on flight trimming an indoor micro canard; 18" wingspan, 32 grams flying weight. The plane is somewhat unique in that it has a fixed canard with no control surfaces; the only control surfaces are the elevrons on the main wing.

I scratch built this canard and it flies but needs a good amount of up elevator trim to get it to fly level. What do I need to change to get straight and level flight without needing up elevator trim? Re-designing the plane may be the answer, but I don't want to add control surfaces to the canard.
nice looking plane

a lesson i never seem to learn is always try the simplest solution first...

did u try moving the cg back?
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Old Feb 20, 2013, 06:44 PM
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Originally Posted by nickchud View Post
Hah! You missed him. Me too. I can't take him to the flying club and that plane needs a runway. Unless I fix a hook for the bungee launcher. Those were taken from a 5min 30 sec flight. My 2200 battery went from 12.6 to 11.1 volts.

I remember other videos of yours and the dog, I am a dog owner and so seeing your dog in other videos, not knowing there is a difference between a local open field and your flying club's site, I just missed seeing the little rascal.

You really dropped the voltage on that battery. Hope it is ok.
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Old Feb 20, 2013, 06:45 PM
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Nick, I agree with all and your new model looks very stable. I guess that the Long EZ design is hard to beat. Mine is still flying and rock solid stable too. That was an excellent landing!

Charles
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