Fowler flaps, airbrakes, servos and my mental health...

[SeditiousCanary]

Hey all.

I was turned on to this site from RCU and saw a good bit of know-how being shelled out by the members here, so I figured I'd ask some of the questions that I have been struggling with the past few weeks.

Quick update on me. I used to fly, but haven't in about fifteen years or more. When I did, it was all control line stuff. I am mechanic by trade (bicycle, car, computer, non-living devices, etc...).

I am building a Sterling Diamant as my first return trip to modeling. I am bashing it pretty hard so that it will have a different tail configuration, elevators, ailerons and maybe flaps. I am also designing a glider to scratch build as my next project in AutoCAD. It's the scratch build that I have questions about.

Currently, I plan on it having about a six foot wingspan, minimal dihedral (2 or 3 degrees), rudder, elevator, ailerons, fowler flaps, retract wheel and about a 10:1 aspect ratio. With this in mind, here is a list of what I am wondering about:

I'm trying to design the fowler flaps so there are no protruding parts when the flaps are up. I'm planning on using a grooved track as a guide for the flap to move back and pushrods to actuate the rotation. The leverage and math involved in this design is beyond me. Anyone have any suggestions on how to do this?
Is 20 degrees of throw enough for flaps?
Is 45 degrees of throw enough for airbrakes?
See pic for rough design:
http://members.cox.net/seditiouscanary/S7012%20Flap.jpg
How much torque should I need for servos (All control surfaces are going to be about ten inches long and 1.5 inches wide)?
Who do you guys like for servo makers?

Thanks all. I'm sure I'll post and ask more as I realize what I have left out.

[Ollie]

If any of the following links don't work, you can still access them through a google search of "fowler flap mechanism" where they are cached.

See:
http://www.aerotechmodels.com/P47.htm#flapmec
http://homepage.mac.com/mikejames/rcu_flaps_site/rc_setups.html
http://www.betsybyars.com/guy/soaring_symposia/71-desig.html

"Questions And Answers
Question: (Bob Ball) You mentioned the BJ-4 which has Fowler flaps. Did you imply that the Fowler flaps degrade the climb performance of the BJ-4?

Answer: Fowler flaps have remarkably higher lift coefficient, of course, and also much higher profile drag, and in normal thermals the high drag coefficient is worse than the better lift coefficient. Therefore, I feel that it fails to gain performance with the use of Fowler flaps in low speed circling. And you also have much lower aspect ratio since you increase your wing chord and your wing area. The lower wing loading does increase the climb performance but I feel that in normal thermals you lose more than you win. In very narrow thermals or extremely strong thermals, as in South Africa, you may be ahead in this case; but in normal conditions-no."

In my opinion the use of Fowler flaps on model sailplanes is counter productive because the disadvantages far out weigh the benefits. If you want to do it because the challenge is your idea of fun, then go ahead. If you want to do it because you think there will be a net benefit to the model's performance, then you are barking up the wrong tree.

[Sparky Paul]

Boid, true Fowlers, moving back and down, are a PITA using normal model techniques.
Sliding type motions are easily jammed.
A screw-drive would be more reliable, but add a ton of weight.
I believe most flapped sailplanes use the upper skin as the hinge, with a cover on the bottom that moves with the flap..
But also, flaps have been used, linked with the ailerons, as full-span flaperons, so up motion has to allowed for when coupled.
And then there's "crow", with the flaps down and ailerons up..
The options are numerous, and can be confusing... Starting a tad simpler to get a feel for what works well for the size plane you want.. simple flaps would be a better start than a full complex monkey-motion setup.

[SeditiousCanary]

Originally posted by Sparky Paul
Starting a tad simpler to get a feel for what works well for the size plane you want.. simple flaps would be a better start than a full complex monkey-motion setup.

True enough. I am starting simpler for my first build in over 15 years, but this is for a design I'm working on that I suspect it will take a while to finsh, let alone build. I like to research things well in advance so I don't wind up painting myself into a corner and then relying on brute force, liquid cash or the trying to find the one person who knows the answer. I am mechanically inclined, so I'm not really worried about the building part, just the better understanding of how the dang thing works.