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Author Topic: The Airplane Performance Balance  (Read 2700 times)

Offline phil c

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The Airplane Performance Balance
« on: January 21, 2017, 04:33:10 PM »
Even though many fliers are buying ready-made F2D planes these days performance still varies.  Through a lot of experimentation I've found a few key things about plane design, in addition to getting the best motor(lots of issues there too) you can.  I've found three key design parameters that are easy to measure and compare,  span loading- ounce/inch are useful units, wing loading square inches/ounce, and speed- good old mph.(or km/sec).  About a 5% difference in any of the three can be significant.  Using these units makes it easy to see how much of a difference there is.  400sqin/18oz=22.2  500sq.in./20oz.=25.  Which is better?

Speed is probably the least important.  In AMA Fast it is very easy to fly too fast and get into trouble.  Most pilots over the years seem to have settled in at somewhere between 110 and 120 mph.  F2D, with the planes so similar it's all engine.  Some are a bit better but it is very hard to get a group of engines that is consistently good so flyers end up trying to match engines against specific opponents.  Of course, for speed limit events you don't have much choice.  Pick a combination of prop/fuel/engine/and plane that makes sense to you.

The plane is where it gets interesting.  Span loading, the amount of weight each inch( or in the case of full size each foot) of span carries to a big extent determines the rate of climb.  Our planes are so light and overpowered that ROC turns into how fast it will do a loop.  For example, almost all the F2D planes produced have a span of 45.5in., due to international postage regulations dictating a box must be no more than 46in. long. Most commercial F2D's weigh about 250 grams or a bit more.  A really good builder can shave that down to 230 grams. Add about 185 grams for the motor+bits and we get 415-435 grams(a little under 15.5 oz.)  The lighter plane is just about 5% lower in span loading.  That is enough to make a noticeable difference.  You could get the same improvement by keeping the same wing area and extending the span 47.5in.  Two inches of span is neglibable weight out at the tips.  Making the heavier plane slightly longer span would help keep it up to speed with the lighter plane in turns.

Wing loading- take the same two planes at a fairly typical wing area of 425 sq.in.  The lighter one has a wing loading of 1.02sqin/gr.  The other one has .977sqin/gr. The lighter one has an advantage of about 4.4%.  But if you can't build that light you can make the ribs ~1/2in. longer adding 18 square inches.  The wing loading now will be the same.  The heavier plane will turn with the lighter one but at a small cost in speed.

If you do both, add 2in. of span and about .4in of chord would bring the heavier build very close in turning radius and speed loss in maneuvering.  Another way to gain some small advantage is to think about other ways of weight reduction.  The F2D shutoff is a good place to look.  They weigh about half an ounce.
phil Cartier

Offline Paul Smith

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Re: The Airplane Performance Balance
« Reply #1 on: January 22, 2017, 02:09:39 PM »
You have started a HUGE topic.

Let me weigh in on a key issue.  Fore-and-aft CG.

Flyers with top notch reflexes who can handle a tail-heavy airplane are winners.  
If you're getting rusty and need to fly nose-heavy, you will get beat by somebody who can fly tail-heavy.

Tail-heavy goes fast and turns quick.  
Nose-heavy can fly low inverted, but that's about it.

« Last Edit: January 23, 2017, 08:03:59 AM by Paul Smith »
Paul Smith

Offline phil c

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Re: The Airplane Performance Balance
« Reply #2 on: January 22, 2017, 08:31:30 PM »
Balance point doesn't have a big effect on absolute performance of the plane.  It does reduce drag a smidge.  If you've got the reflexes and practice to be able to fly exactly where you want with a very sensitive plane you'll do well.  Unfortunately reflexes can't be taught.  Drills and drills and lots of practice actually flying matches helps more than anything.
phil Cartier

Offline Paul Smith

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Re: The Airplane Performance Balance
« Reply #3 on: January 23, 2017, 08:08:44 AM »
I have to differ on that.

A nose heavy-airplane needs UP elevator to fly level and thus creates a lot more drag.  A plane that is set up neutral flys clean & fast.

To go a step farther, a plane that needs bit of down fly level is faster and cleaner yet, but very unstable.  WWI planes flew that way and got into spins as a result.

In real airplanes we moved passengers and cargo aft when we had too much UP trim.  Speed increased and fuel consumption went down.
Paul Smith

Offline phil c

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Re: The Airplane Performance Balance
« Reply #4 on: January 24, 2017, 11:39:32 AM »
Paul, I'm not trying to argue about trimming the plane.  What I'm talking about is the physical layout.  Agreed, the further aft the balance point the more responsive the controls.  That has to be trimmed to suit the pilot.  The balance that suits the pilot and their skills will get the best results for that pilot.
phil Cartier


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