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Old Feb 02, 2012, 12:53 AM
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Charles, I Looked at your diagram from post 5666. The trouble is the diagram doesn't show the leading edge detail for most of the ribs. Without showing the leading edge it isn't possible to determine the true chord line for each rib. If the tip airfoil has increased camber, the chord line won't be in the same position as the line drawn for the root airfoil. So I think there is probably one of two explainations: 1) I am wrong to say the tip rib as increased camber, or 2) The chord line shown in the diagram is only correct for the root airfoil.

If its really true that your tip airfoil is more like a flat bottom profile with increased camber then my attached diagram can illustrate how geometric washout will be built in when you assemble the wing with the traling edge flat on the building board. Both sections shown have the same thickness, but the tip airfoil has 3% camber and the root section has 2% camber.
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Old Feb 02, 2012, 01:06 AM
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Originally Posted by Don Stackhouse View Post
However, we would have to do an aerodynamic analysis of both airfoils to determine of there is any aerodynamic wash-out or wash-in. Until then, the geometric wash-out or wash-in is just interesting trivia.
Agree with Don, but I want to add two other considerations.

1) If we really want to analyse the lift distribution and stall behaviour of the main wing, we need to consider the upwash and downwash distribution from the canard wing as well.

2) Tip stall in canard models can be less of a problem than in convetional models. If the model is correctly designed the canard wing will always stall first, so we will hopefully never get to find out what happens when the main wing stalls. In the case of this model with tractor propellor on the front, the stall of the canard wing can be delayed by the use of engine thust. That has the potential to make it more interesting for the pilot.
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Old Feb 02, 2012, 09:59 AM
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Geometric Twist

John 235
Quote:
Charles, I Looked at your diagram from post 5666. The trouble is the diagram doesn't show the leading edge detail for most of the ribs. Without showing the leading edge it isn't possible to determine the true chord line for each rib. If the tip airfoil has increased camber, the chord line won't be in the same position as the line drawn for the root airfoil. So I think there is probably one of two explainations: 1) I am wrong to say the tip rib as increased camber, or 2) The chord line shown in the diagram is only correct for the root airfoil.

If its really true that your tip airfoil is more like a flat bottom profile with increased camber then my attached diagram can illustrate how geometric washout will be built in when you assemble the wing with the traling edge flat on the building board. Both sections shown have the same thickness, but the tip airfoil has 3% camber and the root section has 2% camber.
John, Your diagrams are a pure delight!! Thank you! My old brain saw that the leading edge was lower at the tip which said "wash out" BUT the sudden change in curvature at the top said stall which acted like wash in. The low Reynolds number at the 5" tip chord should give a reduction in lift relative to the center section? Hopefully, the performance will be OK in docile maneuvers.
I took a few pictures to back up your diagrams.

Charles
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Old Feb 02, 2012, 10:10 AM
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Originally Posted by canard addict View Post
...The low Reynolds number at the 5" tip chord should give a reduction in lift relative to the center section?...
Yes, but that is NOT a good thing.

An airfoil stalls when it exceeds its max lift coefficient, not a particular angle of attack (note, normally angle of attack and lift coefficient go hand-in-hand, but things like sweep and planform effects can alter that). Because of the low Re at the tip, the max lift coefficient at the tip is reduced, which makes it stall SOONER.

On one hand, there is the lift at each local segment of the wing that the lift distribution demands from that location. On the other hand, there is the max lift that each segment is actually capable of producing (and lower Re directly hurts that capability). When the demand on a particular segment exceeds its capabilities, that segment stalls. Like the weakest link in a chain, whatever segment is the first one that's demanded to exceed its max capabilities is the segment where the stall begins. If that happens to be near one of the tips (and normally it's just one, not both, since the conditions at both tips are are rarely, if ever, exactly identical), things tend to get "interesting" very quickly.
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Old Feb 02, 2012, 10:27 AM
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Looking at your photos, it's tough to say. We really need to see the entire root airfoil.

However, the tip airfoil has the high point way too far forward, which hurts its max lift, and the flat upper surface aft of there will all let go at the same time, for an extremely large and abrupt loss of lift. Not good.
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Old Feb 02, 2012, 11:26 AM
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Looking at your photos, it's tough to say. We really need to see the entire root airfoil.

However, the tip airfoil has the high point way too far forward, which hurts its max lift, and the flat upper surface aft of there will all let go at the same time, for an extremely large and abrupt loss of lift. Not good.
I agree,Don, and that was my original concern. Check post 5666, page 378 for a shot of the center rib which has a chord of 9 inches and peaks at 2-3/4 back where faint marks are shown between ribs 6 and 7 as I recall.
All of my wings whether flat bottom or symmetrical have similar tip shapes but this one is the most extreme. I have reported apparent tip stalls twice on two models during high speeds with sudden changes in direction. My conventional low winger Pursuit with symmetrical airfoil will drop a tip on take off with too much rate of climb. Thanks for your great thoughts. The learning will be reflected in the future.

Charles
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Old Feb 02, 2012, 09:18 PM
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Sorry, with my workload these days I barely have time to skim the more pertinent threads here, so when I do chime in, sometimes it takes me a little to get fully up to speed with the latest discussion.

Looking at your ribs drawing, it's apparent that all the ribs have a common chord line and a common flat portion of the underside, so the geometric washout should be zero.

However, there are some possible trouble areas when it comes down to actually building it. There is quite a bit of unsupported underside at the root, so it would not take much to introduce some error. In addition, the wing's internal alignment is not truly fixed until you "close the box" with the last of the D-tube sheeting. That's on the bottom, and you flipped the wing over to apply it, so there is some possibility for error sneaking in during that operation.

Your root airfoil has its high point at 30%, which at these Re's is too far aft. Your tip has its high point at about 17% as best as I can measure, which is about 5% too far forward. Normally, having the high point at the tip more forward than at the root would be a good strategy for avoiding tip stalls. However, going from significantly too far aft to significantly too far forward, plus having the root marginally too thick and the tip way too thick, plus having the tip so flat on top that 83% of the upper surface is likely to let go all at once (instead of progressively), and since the tip is at the aft portion of the wing and will tend to cause the plane to pitch up and worsen the stall, there is plenty of potential for trouble.

Keep your speed up, and "fly" the plane on for landing until you've verified the plane's stall characteristics, both power off and power on, as well as in turns, at a safe altitude.

Not meaning to be a pessimist. However, a good pilot hopes for the best, but plans for the worst, so either way they're covered.
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Old Feb 02, 2012, 09:44 PM
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Reggie's Canard Crash - Again! 5th Time is a Charm!

RC Crash Canard with 10 foot wingspan CRASH ! Again! Number 5 (7 min 1 sec)
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Old Feb 02, 2012, 10:45 PM
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I'd have to agree with the folks on the video, "It's the wing flex!"

When the wing flexes lke that, the hinge line of the ailerons becomes curved, which forces the ailerons back to zero angular deflection, pretty much regardless of what servos you are using. In fact, it can back-drive even some very high-torque servos, frying them in very short order (guess how I know).

The solution is either reinforce the wing so it doesn't flex (pretty impractical in many cases), or else cut the ailerons into short segments, with couplers in between, so they can "breathe" in the spanwise direction as the wing flexes.

Cut your present ailerons with chordwise cuts, so you have three or four short ailerons instead of one long one. Make the cuts about 1/16" wide. Glue a piece of 1/16" brass tubing about 1" long along the trailing edge between each cut (reinforce it well with glass tape, so it can't come loose!), so that it straddles the gap between adjacent aileron segments.

Cut the brass tube in two at the gap between segments, with the gap in the tube the same width as the gap between the aileron segments.

Cut a piece of 1/32" music wire about an inch long, then bend the last 3/32" or so 90 degrees. Slip it though both halves of the brass tube, so it couples the aileron segments back together. Tape or glue the bent tip of the wire to the underside of the aileron, so the wire can't slip out.

This allows the aileron segments to move apart or together as the wing flexes, but still keeping the entire length of the aileron coupled to the servo. This will allow the ailerons to follow your commands despite the wing flex.
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Old Feb 03, 2012, 12:39 AM
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Reggie, Polyhedral wings work well on a conventional plane with rudder control. The yaw steers the wing tip into the wind which will cause a banking turn which can be corrected with opposite rudder. The polyhedral wing lays on the air like a boat in water and opposes roll. I have found that ailerons have little or no effect on a polyhedral wing and in your case the flexing just adds more polyhedral. My experience came from trying to turn the Wingo into a canard before the days of this thread. I gave up after many failures. You may be able to either remove the curved tips of both wings or just drop them down to a flat position. Due to the flex, it probably would be best to remove them since you seem to have plenty of area. Shorter wings enhance the roll while longer wings oppose it. Also, aileron action is best near the wing tip. Using Don's fix for the ailerons with the tips removed may be your best bet. Your model does seem to perform well in all other respects.

Charles
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Old Feb 03, 2012, 07:52 AM
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Reggie. I agree with all of the above. Those curved wingtips must be more trouble than they're worth, specially since you don't have a rudder. Also, can I chip in with the idea of removing the inboard half of the ailerons? I'm sure you don't need all that much.

Nearly there!

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Old Feb 03, 2012, 10:21 AM
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Originally Posted by canard addict View Post
.... I have found that ailerons have little or no effect on a polyhedral wing ...
Well, um... NO.

Poorly designed ailerons have lots of adverse yaw (the drag of the down aileron causes the plane to yaw away from the direction the aileron is trying to roll the plane, see Al Bowers' video I posted earlier). If you have lots of polyhedral or dihedral (or even sweep), the adverse yaw can couple with the dihedral and cancel out the rolling effect of the ailerons.

However, if the ailerons are efficient, the adverse yaw is minimal, and the ailerons work just fine.

We proved this on the 'CX' 4-channel version of our "Monarch" 1.5 meter RCHLG. It had the same polyhedral setup as the Monarch 'C', its 2-channel sibling, but it also had full-span flaperons. Roll response from the flaperons alone (no rudder input at all) was excellent, and with a little rudder added, it was almost scary, approaching the "Pitts Special" category.

The key here was that the flaperons were very wide chord, almost half the chord of the wing at the root, and roughly 2/3 of the chord at the tip. At low Re's, the 20-25% chord ailerons common on full-scale aircraft tend to just cause separated flow and drag. By going to very wide-chord ailerons, the amount of angular deflection required to get the job done is far less, the adverse pressure gradients are lower, the flow stays attached and the ailerons are much more effective, with much less adverse yaw.
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Old Feb 03, 2012, 03:06 PM
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I will attempt to post an old picture of my first try of converting a Wingo into a canard without flattening the wing. The ailerons were ineffective. Mickey Duck canard from a Soar Star flew well with flat wings
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Old Feb 03, 2012, 03:15 PM
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Those ailerons were ineffective mainly because they were too far inboard, not because of the polyhedral. They also had aerodynamic issues that got in the way.
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Old Feb 03, 2012, 06:36 PM
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Starship

Here's a link to a fabulous 25% Starship

He's got me interested in making a giant Starship. Except I think I can build one with 92" wingspan and AUW of aproximately 80oz. Using this aero depron. I'm optimistically doubling the dimensions of the depron Starship I made last year.

All suggestions will be gratefully received. I'm hoping I can get away with motors as light as 130gms or so. And 11.1 volts. Will that work?


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