Feb 01, 2012, 11:53 PM
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Sydney, Australia
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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.

# Images

Feb 02, 2012, 12:06 AM
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Sydney, Australia
Joined Mar 2006
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Quote:
 Originally Posted by Don Stackhouse 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.
Feb 02, 2012, 08:59 AM
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Joined Jun 2005
2,578 Posts
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

# Images

Feb 02, 2012, 09:10 AM
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Joined Jun 2005
4,418 Posts
Quote:
 Originally Posted by canard addict ...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.
Last edited by Don Stackhouse; Feb 02, 2012 at 09:23 AM.
 Feb 02, 2012, 09:27 AM Registered User United States, OH, Bradford Joined Jun 2005 4,418 Posts 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.
Feb 02, 2012, 10:26 AM
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Joined Jun 2005
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Don Stackhouse
Quote:
 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
 Feb 02, 2012, 08:44 PM 0Gravity Orlando, Florida Joined Feb 2008 794 Posts Reggie's Canard Crash - Again! 5th Time is a Charm!
 Feb 02, 2012, 09:45 PM Registered User United States, OH, Bradford Joined Jun 2005 4,418 Posts 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. Last edited by Don Stackhouse; Feb 02, 2012 at 09:52 PM.
 Feb 02, 2012, 11:39 PM Registered User Joined Jun 2005 2,578 Posts 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
 Feb 03, 2012, 06:52 AM What could possibly go wrong? Market Harborough Joined Apr 2006 4,198 Posts 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!
Feb 03, 2012, 09:21 AM
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Joined Jun 2005
4,418 Posts
Quote:
 Originally Posted by canard addict .... 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.
Feb 03, 2012, 02:06 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
Charles

# Images

 Feb 03, 2012, 02:15 PM Registered User United States, OH, Bradford Joined Jun 2005 4,418 Posts 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.
Feb 03, 2012, 05:36 PM
What could possibly go wrong?
Market Harborough
Joined Apr 2006
4,198 Posts
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?

Nick