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Old Nov 02, 2012, 11:59 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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Coming from someone that made his mark by "doing the math" the suggestion for a simple test glider is all the more valid.

I'd go so far as to suggest that the 1/4 to 1/6 size "test glider" use copper wire as "hinges" for the ailerons and elevator. That way you can adjust the reflex of those three surfaces from near zero up to what is needed to achieve stable flight. You'll know when you need to add some additonal reflex and then nose weight to shift the CG forward because the model will tend to dive or stall then dive when you do not have enough reflex and a far enough forward CG to match the amount of reflex. Once you get the reflex and associated CG up to where they need to be so that the model can recover from a too slow launch or recover from a stall without nosing in you'll have a good idea of how much reflex is needed for the full size model and where the CG should be located.
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Old Nov 03, 2012, 04:37 AM
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I am actually concerned about the vertical surface. the head has a lot of side area well forward of the CG, and the CG can only be moved for a pretty small range on a flying plank setup. Setting up the feet and tail as vertical surfaces helps, but might not be enough.
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Old Nov 03, 2012, 08:03 AM
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From your pictures -you are OK on relative lateral areas
even a little enlarging on the stinger will make a big difference -if you are worried.
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Old Nov 03, 2012, 02:04 PM
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Thanks guys. The maiden was successfull.
Maiden Flight of the Dimorphodon (5 min 3 sec)
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Old Nov 03, 2012, 03:25 PM
the anthropocebo effect
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Wow, flies nice Joel! Congratulations

You make some cool flying things!

Kevin
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Old Nov 03, 2012, 04:15 PM
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What CG location did you use?
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Old Nov 05, 2012, 07:12 PM
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Gary about 2 1/2 of mac.Bmatthews , you were correct that itt needs substantial reflex and elevator to maintain level flight. How were you able to predict this?
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Old Nov 05, 2012, 07:45 PM
Red Merle ALES
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Impressive Joel!

Curtis
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Old Nov 06, 2012, 09:50 AM
the anthropocebo effect
kcaldwel's Avatar
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Quote:
Originally Posted by Joel K. Scholz View Post
Gary about 2 1/2 of mac.Bmatthews , you were correct that itt needs substantial reflex and elevator to maintain level flight. How were you able to predict this?
You have a cambered airfoil that has a negative pitching moment (nose down). The CG must be ahead of the neutral point for stability, which makes a negative pitching moment.

The only way you have of balancing these negative moments is to provide positive pitching moments with the ailerons and elevator. They have very short moment arms to the CG, so they will have to make a reasonably large downward force behind the CG. The only way they can do that is to have quite a bit of upwards deflection.

A better airfoil designed for use in a plank with reflex to start with would have a zero or slightly positive pitching moment. This would provide better performance and the trim force required: less drag, more lift, less power required.

Kevin
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Old Nov 06, 2012, 11:37 AM
B for Bruce
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The 'Wack, BC, Canada
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What Kevin said.

I'm also sort of thinking that the shape of your airfoil with the rather fat folded leading edge and the steps is producing a big amount of drag when you try to slow down. Notice how on the landing approach right at the end of the video that the glide is nice and flat if a trifle fast? Then Mike eases in a little up trim to slow the model down and how the glide instantly steepens by a LOT. THis suggest to me that your KF airfoil switches from reasonably clean airflow over the shape to a very draggy big separation bubble. It short of shows this in the powered flight portion as well where when you try to climb by adding up instead of using power that the nose (head?) comes up but all that happens is that it slows down and no altitude is gained.
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Old Nov 06, 2012, 12:26 PM
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Kevin, you've nailed it. Part of the problem also was the downthrust angle was too much. I have rectifed this, but the main problem now is the plane is too underpowered. It has a good glide, but takes forever to gain altitude. I am looking for a motor now.
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