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Old Jun 13, 2015, 06:43 PM
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The dog ate my clearance
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Guillow's Spitfire for Complete a Project 2015

This has been a work in progress for about 4 years. Maybe this will be the year for completion? This kit is from the Guillow's "400" series (first kitted in 1962), with a span of 27 3/4".

Changes I'll be making:

Modified wing airfoil
Plug-in landing gear
Paper mache engine cowl
Laminated leading and trailing edges
Electric power, with three channels of RET
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Old Jun 13, 2015, 07:59 PM
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Onward and upward.
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Looks good, Pete. I made one of these many years ago, converted it for Control line ops. It flew very well until a gust blew it into me and the lines went slack. I converted it to a Mk-IX. It was the first time I had used silkspan for covering. It will be interesting to see how you finish it.

Randy
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Old Jun 13, 2015, 08:38 PM
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Nice start. There's lots to build in a Guillows kit. For all the weight a pair of 5g servos would add, I'd have ailerons .
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Old Jun 13, 2015, 09:24 PM
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Why change airfoil section??
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Old Jun 14, 2015, 04:08 AM
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Nice project Pete, I'll post your entry and hopefully this will help with the impetus to complete it.
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Old Jun 14, 2015, 06:14 AM
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The new airfoil looks better than the old one. I'm sure the airplane will gain from it. Sharp leading edges are not recommended. Good luck.

-Paul
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Old Jun 14, 2015, 07:47 AM
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Quote:
Originally Posted by Edubarca View Post
Why change airfoil section??
The original section, totally flat right to the front with a very sharp LE looks to me as if it might have rather "sudden" stall characteristics which, coupled with the elliptical wing might well produce rather "interesting" (not interesting in a good way!) handling in a radio model. I think bs is very wise to thicken the LE up slightly and add a touch of Phillip's entry, it certainly won't make the resulting model any worse and it might well make it quite a bit better.

As an aside I have done exactly the same thing on my current build, the Lulu which also had this type of LE. Worked fine on the very light (3 ounce) F/F version but I feel my change, plus turbulator spars will produce better results in an R/C one weighing twice as much.
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Old Jun 14, 2015, 08:00 AM
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Yup a sharp L/E makes for nasty stalls.
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Old Jun 14, 2015, 09:54 AM
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Thanks, all.

Oops, I meant aileron, elevator, throttle, not RET. A pair of tiny servos will drive the ailerons.

The sharp wing leading edge is definitely something to avoid. There's a lot of discussion on it here:

http://www.rcgroups.com/forums/showthread.php?t=827861
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Old Jun 14, 2015, 11:23 AM
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Quote:
Originally Posted by Sundancer View Post
The original section, totally flat right to the front with a very sharp LE looks to me as if it might have rather "sudden" stall characteristics which, coupled with the elliptical wing might well produce rather "interesting" (not interesting in a good way!) handling in a radio model. I think bs is very wise to thicken the LE up slightly and add a touch of Phillip's entry, it certainly won't make the resulting model any worse and it might well make it quite a bit better.
Definitely a good move, IMO however the new foil now is positioned at a greater AOA.
Possibly it is worth to rise the TE or lower the LE a bit.
Nice model. Subscribed!
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Old Jun 15, 2015, 01:39 AM
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Quote:
Originally Posted by Guizzo 17 View Post
Definitely a good move, IMO however the new foil now is positioned at a greater AOA.
I have computed the polars of the original Clark Y and a flattened version for two Reynolds numbers: 70k and 50k, Reynolds numbers the Spit should encounter in flight. I don't know if the following analysis is correct but here goes.
Note: the flattened Clark Y has been normalized (length = 1) but not derotated.

The airfoils stall characteristics seem close (I'm not sure the Xfoil program captures well the stall characteristics) whereas the behavior of the flat airfoil is not very smooth for small AoAs (high speed).
Beyond that the polars are quite similar, especially the lift coefficient CL versus alpha curves. So the flight attitude (fuselage incidence) should be the same if the two airfoils are set at the same angle.
The main difference is the Neutral Point moment coefficient Cm: that of the original Clark Y (smooth leading edge) is 20% (absolute value) smaller and so requires a little less trimming (up elevator).

I would leave the wing incidence as it is and maybe change slightly the incidence of the stabilizer or leave it as it is and trim with the radio.
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Old Jun 15, 2015, 05:15 AM
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Quote:
Originally Posted by Paul_BB View Post
I have computed the polars ..........................................
....................I would leave the wing incidence as it is and maybe change slightly the incidence of the stabilizer or leave it as it is and trim with the radio.
Ok. I see that your airfoil analysis has more solid bases than my guesstimation. Go for it!
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Old Jun 15, 2015, 05:37 AM
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Quote:
Originally Posted by Paul_BB View Post
..........whereas the behavior of the flat airfoil is not very smooth for small AoAs (high speed).
.
Yep, that's what I meant by "sudden" - all the rest passed 2 feet over my head but it is always nice when the theory agrees with what actually happens!
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Old Jun 15, 2015, 08:58 PM
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hi BS-

nice project I like it !

best regards,

jeff
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Old Jun 16, 2015, 08:40 PM
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Quote:
Originally Posted by Sundancer View Post
The original section, totally flat right to the front with a very sharp LE looks to me as if it might have rather "sudden" stall characteristics which, coupled with the elliptical wing might well produce rather "interesting" (not interesting in a good way!) handling in a radio model. I think bs is very wise to thicken the LE up slightly and add a touch of Phillip's entry, it certainly won't make the resulting model any worse and it might well make it quite a bit better.

As an aside I have done exactly the same thing on my current build, the Lulu which also had this type of LE. Worked fine on the very light (3 ounce) F/F version but I feel my change, plus turbulator spars will produce better results in an R/C one weighing twice as much.
Understood, George. I also agree about the LE on any model. Thanks!!
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