"Plank" wings and pendulum stability - Page 2 - RC Groups
Thread Tools
Jan 31, 2013, 10:30 AM
You know nothing....
Stuart A's Avatar
If you read through this thread https://www.rcgroups.com/forums/show....php?t=1654485 it should give you some ideas.
Good place to do a build
Stuart(Nurflugal recruiting officer)
Sign up now
to remove ads between posts
Feb 01, 2013, 06:55 AM
An itch?. Scratch build.
eflightray's Avatar
Nurflügel, still means 'flying wing' no matter how posh it sounds.
Feb 01, 2013, 10:26 AM
Registered User
Feb 01, 2013, 11:53 AM
Registered User
ShoeDLG's Avatar
das ist richtig.
Feb 01, 2013, 07:42 PM
derp express's Avatar
Lots of good information here that I'll have to do a lot of further research into. Thanks guys!
Feb 04, 2013, 10:25 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar

Thanks for the equation and explanation.

I have built plank sheet gliders - 1/16th flatplate, with 12" span 2" chord - dihedral and a rudder. They glide very well unless you throw them hard or add camber to the chord, which fits your discussion.

So, if I had added a small amount of camber and a tall rudder the models might have still had a flat glide. That leads to a design similar to the one you posted a while back - a tall fin with a split rudder. Two servos could provide control of both drag and yaw.

Assuming such a scheme would work - what are the economics? A drag rudder would have to provide the same force as a reflex section to balance Cm. So, where's the benefit? Perhaps the rudder has an advantage because of it's longer moment?

Feb 04, 2013, 12:59 PM
Ascended Master
Sparky Paul's Avatar
I was going through some old stuff and ran upon this one...
Rudders and drag rudder... using the sliding tray method.
Prolly programmable today.
Feb 04, 2013, 02:05 PM
B for Bruce
BMatthews's Avatar
Tom, while such a thing could be made to work it would be a draggy sort of solution. Which is why either reflexed airfoils or tailplanes are used instead. These other solutions generate the required pitching moment with less drag involved.

There was an interesting little test concept glider that used to be printed in a lot of the old Solarbo adverts in Aeromodeler. I've attached a sketch of how I remember it below. It wouldn't take much to make the wing from some 1/32 sheet with a modest amount of camber courtesy of a few simple arc shaped ribs to give the wood about 3/32 of camber and see how it works out. 3/32 camber over 2 inches would be roughly 4.5% camber.

Then make the same model that has a curled wing of 1/32 that has 3/32 camber at around 1 inch back and some reflex over the last 1 inch. Use the same swept forward fin with no drag plate. See which has the better glide ratio.
Feb 04, 2013, 02:42 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Good idea, but I have too many projects already heaped up around this computer and workbench.

That is why I posed the question. It seems that a reflex airfoil would have less drag - unless there is a moment advantage to the rudder drag. In your example that would mean moving the drag plate to the rudder TE.

I may have one of those models laying around someplace. Could be a quick test.

Feb 04, 2013, 07:48 PM
Cognitive dissonance
kcaldwel's Avatar
The usual plank lay-out with a reflexed airfoil will be far better than any design with drag up high to provide stability.

The only advantage a high split drag rudder might have is that it might allow you to add some camber with the elevons during landing, rather than trying to slow down by reflexing the airfoil more.

But that is a different thread anyway.

Feb 10, 2013, 09:58 PM
Electric Coolhunter
Thomas B's Avatar
Originally Posted by slebetman
I'd just like to comment a bit on this:


There have been many successful planks in the history of aviation. It's just a lesser known planform which is why it is so surprising to see one flying.

One of the earliest is the Stabiloplan.

The marines used to operate a plank style UAV. It was called the Dragon Eye:

And the French aircraft designer Charles Fauvel built a series of planks, mostly gliders:

There are many others. These were just the few I remembered and managed to google in the past few minutes.
Technically, planks are constant cord unswept flying wings, so the Fauvel wings with tapered planforms are not exactly examples.

Two well know full scale flying plank design series are Al Backstrom's series of planks and Jim Marske's XM-1 plank design that lead to the Pioneer series of flying wings.

There have been many R/C plank designs over the years.

While pure planks are easy to build, it makes more sense to taper the wing to reduce tip losses with a smaller wing tip cord for any glider/sailplane type plank.
Last edited by Thomas B; Feb 10, 2013 at 10:14 PM.
Feb 11, 2013, 07:49 PM
608 km/h!
josh18's Avatar
Hey, this seems like an interesting exercise. But the notion that you have to run a plank with lots of reflex for stability is just really old school. You can fly pretty much any airfoil as a plank with the right CG and reflex but unless its a propper flying wing airfoil it will have some usually have some pretty bad tendencies, especially as you go faster as the reflex causes ballooning (pitching up). Luckily, you dont have to worry about this any more, as there are some realy slippery plank airfoils out there now which require no reflex of the surfaces and have great pitch stability and no need to adjust trim at any speed. Look up the PW51 airfoil, it is a well known airfoil in glider planks and wings and has been proven to over 250mph in dynamic soaring. If you start at a point you know will work then you will have a good base to start playing with the pendulum concept.
Also, looking at the nasa plank at the start, I couldnt help but think that all the pylons would be needed to hold the very flexy wing off the ground when not in flight.
Feb 12, 2013, 12:45 AM
B for Bruce
BMatthews's Avatar
I hate to bust your bubble Josh but the PW airfoil does have some reflex. It's simply that the reflex only shows on the camber line instead of the outer shape as it's such a slight amount.

The minimal reflex is combined with a slightly more forward max camber point to produce a low camber airfoil which is marginally but still positively pitch stable. And with models of the sort which use this airfoil "marginally" is enough.
Feb 12, 2013, 03:08 AM
608 km/h!
josh18's Avatar
Dont worry your not bursting my bubble, I know it has reflex. But its definitely not like getting your good old clarke-Y, whacking the cg to 20% and dialling in 5mm of reflex is it? Thats more what I was getting at, and wasn't that one of the problems the OP was trying to solve?I was simply saying that the problem has really been solved already, and if you want to play with a really interesting concept you may as well go with a proven method to get a plank to fly then try the pendulum system on that. Then at least you will be able to really see if it is making a difference.
IMO, YMMV, etc etc.
Although, looking at the pic of the nasa plane at the start, it does look to have a very flat bottomed foil- maybe it does have pendulums inside after all.
Feb 12, 2013, 06:47 AM
Registered User
slebetman's Avatar
Originally Posted by josh18
Dont worry your not bursting my bubble, I know it has reflex. But its definitely not like getting your good old clarke-Y, whacking the cg to 20% and dialling in 5mm of reflex is it?
Technically both are reflex. Reflex simply refers to an upturned camber at the end of the airfoil. To avoid the confusion of referring to "built-in" vs "trim" reflex I never use the term "reflex" to refer to elevator trim. I would personally describe the Clark-Y thing as: using an airfoil with no reflex with lots of up elevator trim.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion A concept for a weight trimmed plank wing with secondary pendulum stability BMatthews Nurflügel 67 Jan 19, 2014 06:45 PM
Question Wing tips on a plank Stuart A Slope 3 Feb 16, 2013 12:10 PM
Discussion Straightening a warped balsa plank/wing? Piece The Builders Workshop 8 Jan 07, 2013 12:26 AM
Discussion Using pendulum to stabilize models zultone Free Flight 11 Jan 11, 2009 05:40 PM
Discussion The Pendulum Rocket Fallacy - Helicopter stability chench Micro Helis 90 Dec 11, 2008 12:31 PM