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Jim Mantyla
PostPosted: Wed Apr 27, 2016 12:47 pm 
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Location: Barrie, Ontario,Canada
Guys,

I was out last night for a stress relief flight and I noticed that the plane was indicating about 2 mph faster than it did before. I rechecked my power settings and the only difference I could find from previous flights was that I only had about 3/8 of a tank of fuel. This got me to thinking so I landed, filled the tank and went up again. The plane was 2 mph slower. I am guessing that the CG is further back and this is resulting in less drag. Has anyone else noticed this?

regards,

Jim


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fulcrumflyer
PostPosted: Wed Apr 27, 2016 4:01 pm 
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Aircraft lighter = less induced drag
CG further aft = less trim drag
Net result = faster airplane

Spanky


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James Grahn
PostPosted: Wed Apr 27, 2016 10:01 pm 
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I wish I was good enough to notice a 2mph difference. You rock Jim!
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Bill Williams
PostPosted: Thu Apr 28, 2016 4:39 am 
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I can trim N30WW with a slight nose up trim and hold a little forward pressure on stick and gain 4 or 5 MPH. Seems the Thorp likes to be up on a step.


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Ryan Allen
PostPosted: Thu Apr 28, 2016 7:22 am 
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I fly 99.9% of the time by myself. Last weekend, my father flew with me. He weighs 215#, and I noticed I was cruising faster than normal while at my normal cruise RPM setting. I would say somewhere in the 3-5 knot range.


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Doug S
PostPosted: Thu Apr 28, 2016 8:28 am 
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When I was a younger brat, my dad would throw me in the rear jumpseat for the CAFE speed runs at OSH - I felt real important (as ballast).


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James Grahn
PostPosted: Thu Apr 28, 2016 9:28 am 
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Haha. That's awesome Doug!
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cluttonfred
PostPosted: Thu May 05, 2016 7:27 am 
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Except when you get down to the reserve, when the plane seems to go slower, like a watched pot. ;-)

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A site for builders, owners and fans of Eric Clutton's FRED

« Voici ce que j'ai fait...vous pouvez en faire autant! »
"This is what I have done...you can do the same!"
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KWK
PostPosted: Thu May 05, 2016 8:41 am 
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Bill Williams wrote:
... Seems the Thorp likes to be up on a step.

Which brings up a question about wing incidence. Is there any benefit to altering the incidence for the expected cruise speed of, say, a "mouse motor" as compared to an IO-360? It's not obvious which incidence the fuselage best sits in cruise; perhaps it makes so little difference on drag as to be moot.


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fytrplt
PostPosted: Thu May 05, 2016 9:33 pm 
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Here's one for you to ponder:
The first set of ordinates on the wing profile of the T-18 are 0,0. On the S-18, they are -.5, 0.

The design chord line is the same on both sets of drawings.

What does this do to the apparent angle of incidence?

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fytrplt
PostPosted: Thu May 05, 2016 9:45 pm 
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BTW, I was out fooling around with power settings the other day at 5, 000 ft in smooth air. I put the plane on autopilot, both heading and altitude hold. I ran in some nose up trim and let it stabilize. I gained the 2-3 knots on the GPS that Bill Williams has experienced.

Who knew?

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KWK
PostPosted: Thu May 05, 2016 10:30 pm 
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fytrplt wrote:
The first set of ordinates on the wing profile of the T-18 are 0,0. On the S-18, they are -.5, 0.

I don't have S-18 plans, only T-18. There are coordinates for an LDS-2 airfoil on the University of Illinois airfoil database, and these are said to be the S-18 airfoil. They go through 0,0 at the nose. Either it's not the S-18 airfoil or the coordinates were rotated to match the convention of 0,0 at the nose.

If you don't use Xfoil or its ilk, I'd be happy to compare the two for you if you send me the coordinates. Send me a photo of the coordinate table, and I can work from that.

As I recall, I estimated the T-18 would cruise at about 155 mph with the waterline of the fuselage at zero degrees to the free airstream. Higher speeds would cruise with the tail higher. I forget, though, at what altitude I made this estimation. (It was only a quick estimate.)

Karl


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fytrplt
PostPosted: Fri May 06, 2016 6:58 am 
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I don't have the plans at hand, but just looking at the situation, lowering the leading edge would seem to decrease the angle of incidence. In this case, it would more align the wing with the fuselage at the higher speeds the high HP ships attain.

Lu's original plan was to raise the trailing edge, but this did not prove to be practical. I had the privilege to talk to him at length on this subject. The shorter aileron of the S 18 was also discussed.

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James Grahn
PostPosted: Fri May 06, 2016 1:55 pm 
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That's why the new wing is set to zero.
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KWK
PostPosted: Sun May 08, 2016 9:06 pm 
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fytrplt wrote:
... autopilot... altitude hold. I ran in some nose up trim and let it stabilize. I gained the 2-3 knots on the GPS that Bill Williams has experienced.

That still puzzles me. Changing the anti-servo setting and fighting it with the stick will, it seems, slightly change the camber and angle of attack of the tailplane; however the (down) lift can't change much given nearly the same cruise speed. I'll take a pencil and paper and ponder it all (and the meaning of life, etc.) further while on vacation this coming week. (Airliner rides are boring.)


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