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Author Topic: Identical twins  (Read 353 times)
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Viperkite
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« on: February 26, 2021, 09:43:55 AM »

Has anyone else built two identical models where one flies like an eagle and the other like a penguin. I built two 'Doonies' , one barely stays in the small flying field (fantastic flyer) and the other barely stays in the air.
Looking at them they are identical, so I have had to mark the duff one so I don't pick it up by mistake.
No logic to it.
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Starduster
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« Reply #1 on: February 26, 2021, 10:40:56 AM »

All the time-

I have done the CAD work for several airplanes for Bob Holman Plans. Once the design is done, I will usually build two prototypes from the LASER short kits. If the wing has under-camber, I will (almost) always build an assembly fixture, so the two sets of wings are as nearly identical as possible. Invariably, no matter how careful I am, one airplane will fly better than the other. Granted, these are hand-crafted, so there are bound to be differences between the airplanes.
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stupid
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« Reply #2 on: February 26, 2021, 11:29:21 AM »

Herb Kothe  who is an accomplished free flight designer and contest flyer, came out to our flying site in the spring with three brand-new BA Cabin models & started to trim them for flight.

While I was watching he had 2 of them flying really well, the third one he was having troubles with and put up three more flights adjusting the model each time to see if he could get it to fly better. I was amazed and astonished when Herb looked over the model, took the propeller off and set it on the ground and with his right foot stomped it into pieces. When Herb saw the shocked look on my face, he said to me "some of them you just can't get to fly".
I still remember this like it was yesterday.
Here's a video of Herb flying his Stahl Hurricane.

https://www.facebook.com/FreeFlightDigest/videos/2780293132209262/
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Viperkite
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« Reply #3 on: February 26, 2021, 11:54:54 AM »

Strangely enough it's usually the first one that flies the best. I think I have only ever have one model that flew as well as the first one.
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lincoln
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« Reply #4 on: February 27, 2021, 02:37:51 AM »

If the models fly differently, something measurable will be different, though it may not be easy to measure. I think, in some cases, a few swipes with sandpaper can make a significant difference.
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vintagemike
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« Reply #5 on: February 27, 2021, 04:31:19 AM »

From experience its usually the leading edge of the wing being sanded too sharp, I have done this, built two "identical" models and one doesnt fly. There was some discussion in SAM 35 speaks about it several years ago. In my case some sanding of the wing l/e solved the problem
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Kevin M
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« Reply #6 on: February 27, 2021, 04:54:58 AM »

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From experience its usually the leading edge of the wing

I agree that the LE profile can be a critical factor, just the first few mm. Had an Auster that flew horribly until I increased the LE radius.
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billdennis747
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« Reply #7 on: February 27, 2021, 05:47:08 AM »

But wasn't there a time when sharp LEs were in vogue? Lucky Lindy has a LE you could sharpen a pencil on.
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Kevin M
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« Reply #8 on: February 27, 2021, 06:51:16 AM »

True. Pros and cons of everything where aircraft are concerned.
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Viperkite
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« Reply #9 on: February 27, 2021, 10:32:04 AM »

Just checked the duff flyer and the L/H wing LE was sharper than RH. So sanded it to match, but will have to wait until I get back to the field. Flies ok as a garden flyer (40 feet before it hits something).
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Knightflyer
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« Reply #10 on: March 17, 2021, 07:44:12 PM »

Yup. Built two Andreasson BA4B peanuts from the Peck kits back in the 80's. The pretty one flew so-so, the not so pretty would routinely turn in 45 second flights. Not bad for a self-taught high schooler at the time.
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lincoln
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« Reply #11 on: March 18, 2021, 05:35:50 AM »

But wasn't there a time when sharp LEs were in vogue? Lucky Lindy has a LE you could sharpen a pencil on.

That depends on the application. I think for some models that fly at one speed and have low Reynolds numbers and thin could, a sharp l.e. can be pretty good. Also, for some supersonic airplanes, such as the F-104. On other types, there might be problems with sharp stalls or low maximum lift. I've been told that flat plate airfoils have consistent stall behavior and are useful on small aerobatic models. I'm guessing that this also holds for sharp leading edges. Sharp leading edges on delta wings may enhance vortex lift. I'd guess that holds for strakes as well.
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