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Old Mar 30, 2011, 10:04 AM
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The length of your revised fuselage looks better from an aesthetic and functional point of view. I wasn't expecting to see so much canard area. I was expecting something closer to the Georgia goose with the area at 30-35% of the main wing. As Don posted above about the airfoil selection, many considerations are related to each other. So there are sure to be difficulties in considering any one parameter in isolation.
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Old Mar 30, 2011, 10:30 AM
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John 235
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For symmetrical canard airfoils on a model of this size, I think you would get better performance from your own designed duck airfoil. I don't think think the E168 was ever intended for low reynolds numbers less than 150,000. I'd avoid symmetrical airfoils with any more than 10% thickness for this application.
Yes John, Thanks for that. It now stands at 13.3% but can easily be dropped to 10% since it has a 9 inch chord. I am adding lots of area to the bottom of the 197 to make it full semi symmetrical.
The canard area is 58% of the main wing area to move the CG forward to center just as the Delta Duck's canard area was reduced down to get the CG back to the center. I hope that a large front wing will not be a negative.
Charles
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Old Mar 30, 2011, 11:38 AM
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Seems odd that John beat me to the punch about the airfoil thickness issue, but he's absolutely correct.

Back in the early days of RCHLG (similar Re's to where your plane will operate), a number of the popular designs used those airfoils or similar ones. Our "Monarch" RCHLG was the one that caused the paradigm shift to much thinner sections, a trend that continues today.

We found that at these Re's an airfoil section thicker than about 8.5% actually causes a loss of max lift, as well as efficiency. The flow uses up most of its tolerance for adverse pressure gradients just getting around the airfoil, with little left to use for making lift. Too much camber was also bad, and what camber there was needed to be concentrated well forward on the airfoil.

As John indicated, you should not count on those airfoils having the same stall characteristics at your Re's, or even stalling at the same alphas, as they do at the Re's for that published data.
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Old Mar 30, 2011, 11:46 AM
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Your wing aspect ratio appears to be higher than the canard's, which is one factor (albeit out of many) arguing in favor of the wing stalling first.

Yes, you have plenty of canard area, but that by itself doesn't say much, one way or the other. C/G location, and how it determines what percentage of the load is carried by the canard vs. the amount carried by the wing, will be a critical factor.

Note, C/G influences whether the wing or the canard stalls first, but it also determines static stability. It's very possible that those two requirements could conflict with each other.
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Old Mar 30, 2011, 12:55 PM
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Quote:
Originally Posted by canard addict View Post
John 235
Yes John, Thanks for that. It now stands at 13.3%
WAAYYY too thick!

Quote:
but can easily be dropped to 10% since it has a 9 inch chord.
Still too thick.

Quote:
I am adding lots of area to the bottom of the 197 to make it full semi symmetrical.
Why?? That's going to increase the thickness even more. It's also going to make it into something that is no longer even remotely a 197, including its aerodynamic behavior.

There is far more to airfoils than "flat-bottomed, semi-symmetrical and symmetrical". Airfoil designers don't even use those terms anymore, other than in a very general sense.

A flat-bottomed airfoil is nothing more than a section where the camber is exactly half the thickness.

Camber shifts the Cl vs alpha plot up or down. You use whatever amount of camber necessary to make the airfoil's most efficient operating Cl correspond to the Cl the plane needs from that flying surface at the plane's most important operating point. Odds are that is not going to be a camber of zero, unless you are making an aerobatic aircraft that needs to fly the same inverted as it flies upright.

Quote:
The canard area is 58% of the main wing area to move the CG forward to center just as the Delta Duck's canard area was reduced down to get the CG back to the center. I hope that a large front wing will not be a negative.
Charles
So the canard is roughly 1/3 of the total area, but carries about half the weight? Good for stability, bad for aerodynamic efficiency.
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Old Mar 30, 2011, 09:01 PM
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Since, I feel that the thick airfoils will keep my models slower but also since I value what has been proven best, a compromise to 9% will be made.

If my calculations are correct, the main wing will carry 70% of the load and the canard will carry 30%. The CG is about half way but the large rudder is included. In order to start the build, the airfoils will need to be finalized and printed on card stock. The support from each of you has been great and after so much drill over a period of time, then maybe some of it will remain in my memory.
Charles
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Old Mar 30, 2011, 09:13 PM
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Drag does not help a plane fly slower. It will help it decelerate.

However, the minimum flying speed is determined by how slow you can fly and still make enough lift to support the plane's weight. The secret of making a plane fly slower is therefore maximizing its lift-making ability, NOT its drag.

Since your thick airfoils will actually reduce lift, they force your plane to fly faster, not slower. However, because they also increase drag, and because the higher airspeed also increases drag, they also increase the amount of power needed to sustain flight. That means bigger motor, bigger batteries, etc., which increases weight and increases the stall speed even more, which increases drag some more, which increases powerplant size and weight some more, which increases minimum flying speed some more, which increases required power some more...you get the picture.

No, to sustain flight at the slowest possible airspeed, you need more LIFT, not drag. And at these Reynolds numbers that means making the airfoils THINNER.
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Old Mar 31, 2011, 03:32 AM
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I concur Don! and looking at new construction methods, I'm hoping to get to 6 % for the next project.
Fellas:
Take another look at my "contraption" movie @ blog- at the end of the movie, what looks like a landing going into the wind, was actually a deep stall agravated by exactly what Don has been explaining. My thinking seemed correct- but proves very wrong. the reason the Butterfly wing flew better than the first wing (ellipse) is because of 4% difference in the foils.
been very busy with attaining employment, as the Machining world has exploded with work the last couple weeks.
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Old Mar 31, 2011, 06:56 AM
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Johnny, good to hear from you!

Of course there is that business about "too much of a good thing". 6-7.5%, about 8.5% max, seems to be a good zone for airplanes around this Re range, but going thinner can also get you in trouble.

As things get smaller, the airfoil needs to get thinner, and the high point needs to move forward some more. For a Mosquito class model (0.75 meter span RCHLG), 4-6% might be more appropriate.

However, as the thickness goes down, the exact shape becomes more critical. Simply buying that higher thickness, higher Re airfoil a membership at Weight Watchers or Jenny Craig is not going to get the job done. The airfoil needs to be optimized for the lower thickness and camber at the lower Re. Particularly for something like a sailplane, which has to have excellent efficiency over a wide range of airspeeds and lift coefficients, designing a good low-Re airfoil can be a very difficult challenge.
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Old Mar 31, 2011, 10:29 AM
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Thanks Don! I think I've been reading too much into the dlg's! I'm planning on taking the contraption to the slope- hopefully that would give some better data than learning the strengths of a new high start plus other datas! had you seen Jarel's Davinci foil shapes?
Johnny
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Old Mar 31, 2011, 10:56 AM
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Originally Posted by Captain Canardly View Post
... had you seen Jarel's Davinci foil shapes?
No, but from some of his comments, it sounds like they are "aft loaded" (i.e.: fairly large amount of camber near the trailing edge, which results in a fairly large convexity in the aft upper surface and some undercamber near the trailing edge in the lower surface). Personally I do not care for it, and I believe their track record supports that. In some cases they may do fairly well at high lift (IF you can keep the aft upper surface flow attached, which can be a problem), but at low lift and high speed the lower surface airflow tends to detach. The result is an airfoil that can be sensitive to Re, and that tends to have high drag at anything less than near max Cl.

At higher Re's, where the boundary layer flow has more of a tendency to be turbulent, and therefore has an easier time staying attached, they seem to do better.
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Old Mar 31, 2011, 11:47 AM
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time for a break!

his vids would show your assessment is accurate, I was thinking it looked "lumpy" but it seems efficient from vid views.I'm running with A.M. Beck's foil collection for the Crimson aileron wing.
Johnny
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Old Mar 31, 2011, 12:14 PM
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Those don't look as bad as the written descriptions implied. I'm not overly thrilled with them, but for less extreme Re's (2-meter or open class, with moderate aspect ratios so the chords don't get too skinny), they should be OK for a "floater".

At low Re's, you have to be very careful with what you ask from the airflow on the aft parts of the airfoil, on both the upper and lower surfaces.
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Old Mar 31, 2011, 05:03 PM
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Stealthsky Fighter

This one isn't electric because it was before the electric era (photowas 1987ish). But you might like to see it anyhow. The nifty feature was the rudders were also air brakes (servo in sliding tray mechanism). Flew good except for elevator response--should have been a full-flying stab.
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Old Mar 31, 2011, 06:10 PM
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That is one item the electrics will never deliver- the best fragrance in the world!Looks good Inari!
I wonder if our Florida pals are getting whackered with the Florida National news weather report? 12 tornadoes as I heard.
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