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Plane faster when flying on the Bottom Half of the tank?
http://t18.net/thorpforum/viewtopic.php?f=8&t=7935		 Page 1 of 2
Author:  Jim Mantyla [ Wed Apr 27, 2016 12:47 pm ]
Post subject:  Plane faster when flying on the Bottom Half of the tank?
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
Author:  fulcrumflyer [ Wed Apr 27, 2016 4:01 pm ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
Aircraft lighter = less induced drag
CG further aft = less trim drag
Net result = faster airplane

Spanky
Author:  James Grahn [ Wed Apr 27, 2016 10:01 pm ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
I wish I was good enough to notice a 2mph difference. You rock Jim!
Cubes
Author:  Bill Williams [ Thu Apr 28, 2016 4:39 am ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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.
Author:  Ryan Allen [ Thu Apr 28, 2016 7:22 am ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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.
Author:  Doug S [ Thu Apr 28, 2016 8:28 am ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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).
Author:  James Grahn [ Thu Apr 28, 2016 9:28 am ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
Haha. That's awesome Doug!
Cubes
Author:  cluttonfred [ Thu May 05, 2016 7:27 am ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
Except when you get down to the reserve, when the plane seems to go slower, like a watched pot. ;-)
Author:  KWK [ Thu May 05, 2016 8:41 am ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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.
Author:  fytrplt [ Thu May 05, 2016 9:33 pm ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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?
Author:  fytrplt [ Thu May 05, 2016 9:45 pm ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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?
Author:  KWK [ Thu May 05, 2016 10:30 pm ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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
Author:  fytrplt [ Fri May 06, 2016 6:58 am ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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.
Author:  James Grahn [ Fri May 06, 2016 1:55 pm ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
That's why the new wing is set to zero.
Cubes
Author:  KWK [ Sun May 08, 2016 9:06 pm ]
Post subject:  Re: Plane faster when flying on the Bottom Half of the tank?
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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