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Author Topic: Spitfire Mk VII extended wing horizontal stabilizer desing  (Read 742 times)
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Chris A.
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« on: February 12, 2020, 04:37:48 PM »

I'm modifying a Comet Spitfire Mk IX into a long wing Mk VII with the large rudder. I"m sure that the horizontal stabilizer needs to be enlarged for stable flight, but any ideas on the percentage of increase will be appreciated. Wing tips will be brought to lower edge of canopy height. Anyone out there ever built the long wing FF version?
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charlieman
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« Reply #1 on: February 13, 2020, 03:54:41 AM »

I built Comet's "P-51A", their P-47 and the Dauntless, from the old Blue and white boxes. 1966-67. They were stable on their scale areas. In the Model builder plans book, the Spit Mk 1 has enlarged fin/rudder AND stab. Not sure it needs it, but the models do fly well.

I used to fly indoor with a group from Sacramento, Ca. 1978-80. One of the better builder/flyers was Nick Bressi. He would make an oversize horizontal out of light 1/16" sheet slide it in then test fly. Once it was flying to his satisfaction, he would begin to reduce the stab area with scissors until the model was unstable. If the plan stab was larger than the final unstable temp area, he;d' build plan stab to scale outline, use his preset incidence and fly it with confidence. Today, one could easily experiment with foam plate material, instead of balsa.

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Chris A.
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« Reply #2 on: February 13, 2020, 04:38:51 PM »

Charlieman; thanks, I'll be trying your suggestion. The Comet Mk IX fuselage backbone has a slot for the stabilizer that can be lengthened so a "try it" larger stabilizer can be pushed through. I'll also make the slot deeper for incidence trim adjustments.
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charlieman
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« Reply #3 on: February 14, 2020, 03:34:05 PM »

It's been awhile since I built a kit. IIRC, it's usually possible to enlarge the forward end of "slot" in keel, to form a long tri-angle cutout.  When framing stringers (both sides) follow the same cutout, top and bottom. Now,  it's possible to cover to those stringers, which also form a stable lateral stab mount. Shim front of stab (again, both sides) for trim adjustment. Back of stab can be held with dab of rubber cement, or other flex material, to act as a hinge.  When all trim has been established, GLUE the shims in place and cover the cutout.
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lincoln
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« Reply #4 on: February 15, 2020, 01:09:56 AM »

If you have a similar model that flies well, you can compute the tail volume and match that on the new model. You might read How Big the Tail? by Stan Hall. It's about full scale airplanes, but the same principles apply. Easy to find on the net.

William McCombs' book Making Scale Model Airplanes Fly is a good resource for questions like this. According to rumor, it's still being advertised in the NFFS newsletter classifieds. While you're at it, buy a big magnifying glass to read it. It's just a bunch of stapled pages with poor print quality, but it's very useful.
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Terry Fitzpatrick
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« Reply #5 on: March 01, 2020, 12:25:39 AM »

Re Spitfire tail area. I was confident that the long span version would have the same tailplane, but I thought I better check first. This is what I found:
Mk 1 Tailplane area 31.40 sq.ft
Mk VII Tailplane area 31.46 sq.ft.
Mk IX area not stated but the ref. book has a drawing of the original tailplane and the Mk IX drawn together & the Mk IX is larger in span and roughly 30 to 35% greater in area.
So if your Mk IX kit is accurate, you already have the large tailplane. I would expect that the increase in tailplane area has come about as a desire to improve handling or steadiness as a gun platform & doesn't relate directly to the high altitude long span wing. I think 31.40 sq.ft & 31.46 sq.ft  indicates a change in tape measure operator or perhaps a small revision to the elevators.

My ref. was "Spitfire the history" Eric Morgan & Edward Shacklady

best regards Terry Fitzpatrick , Sydney Australia
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charlieman
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« Reply #6 on: March 06, 2020, 11:58:33 AM »

 "So if your Mk IX kit is accurate,..."   
Yeah, that 'IF' can be a real corker! Grin

Years ago, I was checking Spit tail plane dimensions and found a discrepancy I always meant to run down. Prototype tail span,  according to Harry Robinson scale drawing, is 10'-0", while Mk I production assembly drawings shows  10'-6'(same as Mk 9?) and Spiteful/late Spitfire span was increased visibly with large constant chord inboard extensions. This something that I'd still be interested in understanding better.
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tom arnold
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« Reply #7 on: March 06, 2020, 03:51:01 PM »

I can highly recommend this article by Don DeLoach regarding the eternal question of "how much do I need to enlarge the stab" that we all go through. I have put his approach into practice and it is dead on accurate and it has improved my flights tremendously. (this article is found on the Volare website along with a TVO calculator)

http://flyingacesclub.com/PFFT/RubberScaleFlyingbyDeLoach.pdf
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Chris A.
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« Reply #8 on: March 15, 2020, 06:31:49 PM »

Tom A., thanks. Measuring the Comet Mk IX it is very close to scale. With the long wing Mk VII, there is more wing area for a negligible weight gain. Now to read Professor D Loach's writings on model aeronautics to learn about span effect on stability.
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Chris A.
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« Reply #9 on: April 08, 2020, 03:10:23 PM »

The Comet horizontal stabilizer scaled out very close to actual, the only real difference i could see is in the actual shape of the elevator itself. That doesn't effect the outline so I'll stick with the Comet version. However the late series Mark VII had an enlarged fin and rudder, both taller and larger in area. I'll build both as either can be used, and keep the one which seems to work best. The only problem with the Comet Spitfire structure is keeping weight aft  of the CG to  minimum. Is it common practice to build in extra motor mounts and shorten the rubber on Spitfires rather than add nose weight?
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MKelly
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« Reply #10 on: April 08, 2020, 03:24:34 PM »

Chris,

If the stab is correct scale size it's probably too small to give you a good-flying model, especially if you plan to fly outdoors.  I have a 28" span Earl Stahl Spitfire that flies very well - the stab is considerably enlarged, giving a tail volume of .86.  The model also has a lot of washout (aft edge of the tip rib is elevated 1/4" compared to the root rib).  The fin/rudder is scale size.  Recommend you look through the article Tom Arnold linked to and calculate the tail volume for your plan.  If it comes out less than .65, you'll likely want to enlarge the stab.

BTW, you can easily measure the wing and stab areas if you have a PDF file of the plan.  Open the plan file, Select "Tools", then select "Measure", then click on "Measuring Tool", then select the filled in triangle to measure area.  Click along the edges of the wing/stab plan, when you get all the way around the perimeter of the surface it will calculate the area.  If you click on the line shown in the measuring tool box you can use that to measure the tail arm.

Hope this helps...

Mike
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flydean1
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« Reply #11 on: April 08, 2020, 06:39:04 PM »

Chris,

By all means MOVE THE REAR MOTOR MOUNT FORWARD!!.

It should be not far behind the wing trailing edge, and will allow you to run a much longer motor without needing to load the nose with "Demon Lead".
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