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Anyone using other than the standard flat bottomed foils? Any one tried thinning any of the “tried true” sections fromback when climb speeds were a bit slower? If so, what is the minimum thickness, that with geodetic, or warren truss configuration, and taut covering such as Polyspan or Ultracote, can come up with the needed torsional rigidity. Can it be achieved with multi spar L.E., or is a sheeted “D” box the only way? Planning a new ship to replace 10 year old D. Mathis “Trigger” based ,310 sq in 1/2A. TIA & Merry Christmas;-d d
Hi Deezel-don
While we do not fly AMA classes in Oz we do use models that fit the Classic requirements and while those with little building time stick to the flat bottom sections, the undercambered wing around 7-9% thick, full Dee box leading edge and covered with Polyspan is far stronger. You could build an undercambered wing using geodetic construction and reinforced with carbon like the “Classical Gas” design but for 1/2A, a bit excessive.
The real benefit of a thinner undercambered wing is climb speed under power (less drag) and lighter if you pick your timber carefully. A tip, use an “I” beam main spar and diagonal ribs between the spar and leading edge then sheet top and bottom with 1/32 quarter grain timber. A good wing jig is essential.
I have built 1/2A’s, F1J’s and B gas size models all using undercambered airfoils to the above specs, no more flat bottoms for me. 😀
Ployd
Undercamber will probably give a superior glide. Flat bottom will be easier to build. I have’ Marval’s in 3 sizes with flat bottom foils. I really cannot complain about the glide. Even the C ship, which is really too heavy -43oz,725 sq ” – has a decent searching glide.
Flat bottoms certainly allow easier wing shifting/shimming during the trimming process. Whatever you decide to build, it needs to be rigid. At the speeds modern engines generate, any wing flexing will result in inconsistant patterns. Really loose wings will of course result in needing to clean dirt out of engines while glue on the new plane you’ll need to build dries.Think of it this way. On a 2 minute flight, the engine only runs for 6.5 seconds and the rest is glide…..I have definately changed my mind on things- Get a great glide and a good (decent) power pattern and you will outperform a fantastic climb and mediocre glide…..Not as spectacular, but the stopwatch is the only thing you are trying to impress….
Our F1J model gets up very good, but it does outglide most things around, which ultimately has saved us a few times….
Be careful comparing the F1J to regular gas planes. They are similar as are Kharman Ghias and Shelby Cobras — Both roll.
AMA Cat 111 planes need tp perform at 4 secs. That is truly the line of demarcation. I imagine the F1j will 3 1/2 minutes on 4 secs in dead air. A true FF plane on 4 secs is a bout 2 3/4 min in dead air- maybe. By the time you get to the 6th flight at a meet, you’re no longer flying in ‘dead’ air – lift or not , it’s turbulent and dicey. I have some recent experience with this. Very recent. Oh well, that’s another story.You would do better if you had aurto surfaces and bunt!! HAHAHAH!!
We know that our J model will do 3 1/2 on 3.75 sec. And, as you state, when it gets turbulent it can get dicey. I guess I have to look at some of our experince with the most recent model and its glide has saved us more than a truly great power pattern.
Also, I have found that because of the large glide circle we fly (typically 1 minute or so in duration is what we try for) the models tend to glide out of down air but when it hits lift it does tighten up. I used to run very tight circles on my AMA stuff but now would take and open everything up.
Undercamber and aspect ratio is all good. No doubt about it. Also, thinner foils seem to help when U/C is used. Would I do it on a conventional AMA model? It is more work but I think it would be worth it.
Heck, my new Dakota has it!!! And, just so you will be happy Dan, No auto stuff!!
Is the Dakota the destination for worn-out 1cc engines?
I think the Dakota actually has auto-surfaces. Doesn’t the fin automatically disengage from the airframe upon landing?Hey Dan & John
My Dakota has both automatic disengaging rudder and landing gear fairings.
I think it’s designed this way to keep the vertical surface areas in balanceRudy in Cleveland
Hey Rudy, I forgot about the ‘landing’ gear. It should probably be called something else on a Dakota.
The late Bob Meuser in his American Aircraft Modeler ‘Duration” column “Spotters Manual” noted the apparent similarities, and pointed out the differences in two great designs; the “Starduster 900” & the late Jim Clem’s “WitchDoctor 800” He noted “the Doctors had four-break dihedral, with flat center,— the Doctor has higher aspect ratios especially in the stabilizer, and an undercambered wing profile” “Jim and his friends tried bothflat-bottomedand undercambered wing sections before settling on the somewhat more difficult wing.” Also noted was the Doctors tapered wing tips, as compared to the constant chord planform of the Starduster.
Anyone know what that Witch Doctor 800 undercambered section looks like? How thick? Are A.R. and undercamber related? Whats the reasoning for the higher aspect ratio stab ?How much of the flat or undercambered choice resulted the the kit makers decisions? TIA Cheers d-dWow, that is alotta questions…
Now, the Dakota…So far I have the possible removable rudder on it, but still haven’t put on the fairings.I can build with only balsa!!
Funny that the questions on the WItch Doctor came up. We have one here that is about 400 years old and John wanted a C gas model, so we glued the broken fuselage back together and his orders are: Fly it. If you crash it, crash it good..We want nothing rebuildable! It has an older ST 40 RR on it so it should either go or crash well….
I can’t answer all the questions about the WD tho. It does have a big stab, but everything back then did, with short tail moments and underslung fins. It is a thick under cambered foil with about 1/4″ undercamber (guessing as the wing is buried). It is nothing spectacular. Remember that the ‘Duster has a big, blunt LE, a coupla turbulators, and a pretty thick section. Alotta this is because of structural requirements. As far as the high aspect ration stab…My feeling is that you need “X” amount of area. Typically you have a stab airfoil that is 2% or so thinner than the wing. Because of this, if you have a low aspect ratio, the stab will be very thick even with a 2% thinner foil. Some times it is structural (again) as higher aspect ratios will be thinner, thus weaker. It gets to be how do you package a big stab….
Thin, undercambered, D boxed, good spars, long wings- That is all ya need to win!!
have fun…
Square tips, rear fin, big pylon…..Ahhh, the good old days….In fact, I do have a 1/2A Peral in my stock here. Only difference is that it has an added center section on it for a bit more wing area. Maybe I should pull it out and then kick luddite butt….
For pure performance ya can’t beat high aspect ratios, thin foils, etc…But, you need consistancy more than anything. Good friend of ours always said he wanted an engine that started every time, ran the complete run at full power, and had all the systems work every time. Then you look for performance…
Now that I am thinking about it, I have a 5 oz, TD .049 power 1/2A that is truly killer. It climbs like a bat and has a good glide- And it is flat bottomed with no auto stuff. I do have allotta stuff here…
Youse flies what youse wants to fly….
The closest I get to anything called “Dakota” is a german bar/restaurant in Detroit called the “Dakota Inn”.
I’ve come out with a crooked rudder, but still intact!
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