Understanding Marcel Jurca

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Ozarea5140

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Sep 23, 2020
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G'day All
I've started this thread in hope that some of you can help me understand Jurca's English and his theory behind some of his designs...
I have 3/4 scale Spitfire and P-40 Plans and comparing..
The 4 pictures below show
1. MJ10 Spitfire trim tab following the profile of the Elevator and made from a solid 3/4"ply block...OK
2. MJ12 P-40 "Overall plan" shows Rudder trim tab same profile as the Rudder but made from 2024 with a timber trailing edge..
3 & 4. MJ12 P-40 "Elevator and Rudder plan" showing his "curious trim tab cross section" as square not tapering with a heavy solid nylon block in it..

Can anyone explain why this cross section ? and what is he trying to say about it....Pic 4 . "Dear Amateur" and American and Canadian Schuberts?
Anyone speak French

Thanks
Joe
 

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llemon

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A squared off edge can lower control forces on ailerons/elevators/rudders. I think the rule of thumb is to make the edge ~1% of chord. TOWS would give the pertinent details.

I think the bit in pic 4 is a way of saying "stick to the plans" that was translated poorly.
 

Shayde

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Not really sure what he's getting at with his "curious trim tab cross section" there, but the badly translated text looks like it should be: "Don't follow suit of certain poor Canadian and American plagiarists? who have modified Jurca's plans and then claimed they created them."
 

TFF

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The eccentric designer doesn’t want you to change stuff. Strength or aerodynamics, the plans show how to do it. I think the warning is if you change these areas because you think you are smarter, don’t call it a Jurca design. If you do, you are a lier for thinking you designed the plane.
 

cluttonfred

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I don't get the "Schubert" reference as there may a slang meaning that I am missing, but otherwise, as others have said, he's just emphasizing that you should follow the plans and that making small changes doesn't mean you can then steal his design and sell it as your own. As for the rest it seems clear enough to me.
 

ScaleBirdsScott

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So is the trim tab here supposed to be built as a block, sure to upset some but ultimately being correct for aero reasons?

Or is it a paper city, as such? Maybe in part to catch out those naively copying the base plans? It mentions the RFA, are you supposed to buy something of a "key" that corrects these parts? Or is it a bit of a CYA thing in that someone doing it the "sensible" way can be disclaimed as having not followed the plans when their estate comes wearing a Law-Suit?
 

rv7charlie

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What's the trailing edge of the elevator look like? (matches the elevator trailing edge design?)
How much will the stiffness of the tab be compromised if it's shaped as a wedge instead of a rectangle? (flutter)
 

WonderousMountain

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The angle is a stop limiter, so you have 45° degree like down flap when deployed. Inside the thin aluminum is a nylon block, uncommon, but not unknown. 15mm is small and does not contribute a lot to all up weight.
 

Ozarea5140

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What's the trailing edge of the elevator look like? (matches the elevator trailing edge design?)
How much will the stiffness of the tab be compromised if it's shaped as a wedge instead of a rectangle? (flutter)
The P-40 elevators are of aerofoil tapered design following through from the horizontal stab ,same as any but with a 1/2 inch square trailing edge .(not rounded and he stipulates it must be square).The trim tab is of rectangle shape about an inch thick like a block .
The MJ10 Spitfire plans show a normal aerofoil trim tab wedge design and there are quite a few of these flying.
The MJ12 P-40 shares a similar elliptical style horizontal stab and elevator but for what reason does he use his "curious design" block for a trim tab ????? high drag???...instead of the same as the Spitfire
 

Dana LaBounty

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The block shape requires less deflection to gain the desired trim. 15mm and .020" skin is just over 5/8" just in case any Schuberts out there want to steal the design. Obviously a slight directed toward Falconar Avia and others that proposed working with Marcel then sticking it to him and essentialy stealing his designs.
 
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