Squirrel model airplane!

A lot of discussion comes up about the unique wing design of the Squirrel.

I think there is a lot to think about in wing design. The wing must not only produce lift but it may be involved with yaw, roll and pitch stability. In the case of the Squirrel there are several features worth mentioning.

The winglets (refered to as tip plates below) help with roll stability.

The shim helps with pitch stability.

The winglets also may contribute to yaw stability.

Stability is usually attained through interoperability between the other parts of the aircraft such as the vertical and horizontal stabilizer. Also for pitch stability there are trade-offs between kinetic and potential energy.

Quite a bit of thought went into the wing design and it’s interesting to hear other conversations about other designers and model plane enthusiasts’ thoughts. Notice that Ding (below) has a design he’s calling “Super Squirrel) which has obviously has borrowed some of the elements of design.

Here is a discussion Hip Pocket Aeronautics:

 packardpursuit August 02, 2010
Always wanted to know if anhedral provides opposite effect of dihedral? Intuitively, I’d say yes. But that is why I’m asking. There are number of slick jet aircraft I’d like to see modeled in FF genre, but shy away from their drooping wing arrangement. Is such a fear unfounded?? How about anhedral stabs on F-16 and no dihedral wings??

Sundance12 August 02, 2010
On the F-104 Starfighter, the wing has some anhedral, and I often wondered the same thing. On this aircraft, it has a small wingspan that keeps the wings inside of the shock cone at supersonic speeds, the sweep back will be effective below mach for lateral stability but I think that the Vertical Stabilizer is so huge that this makes up for any weakness in lateral stability. At mach speeds that rudder would be doing a very good job at eliminating yaw that would introduce any roll coupling regardless wether the wings were angled up or down. my 2C…

JetPlaneFlyer August 03, 2010
packard,
You are dead right in that anhedral has the opposite effect to dihedral. Anhedral is most often used on swept wing jets because sweep back has a ‘dihedral like’ effect so the anhedral is added to offsets this with the aim to make the plane less stable and so more maneuverable. The F104 I think is a bit different, it doesn’t have much sweep back but on this one the huge (in relation to wings) vertical stab has a dihedral effect which the ahhedral is designed to counter.

You are also right that models with anhedral can be tricky for freeflight because the anhedral reduces stability, and to my eye these planes just look all wrong with dihedral added. I’ve got away with a little anhedral on some of my swept wing models but at best you have to reduce the scale dihedral angle.

DaddyO August 03, 2010
Steve, Thanks for the board Packard (and the answer Steve). There were a few (noticeably rubber) duration designs that I’ve come across which featured anhedral on the tailplanes – (makes ‘em take longer to build and is extra weight just where you don’t need it) . I had always wondered however about the aerodynamic effects of this though. Paul

packardpursuit August 04, 2010
Old US biplane trainers and seaplanes of the 1916-20′s period often had small vertical areas called “anti-pendulum” panels, usually between king posts of upper wing or between the wings, each side of the fuselage. Would these type verticals, if say added in clear plastic fences, above an anhedral jet wing (about 1/2 way out each side) possibly function to provide a workable dihedral effect?

I got the idea while looking at A-7 scale drawings which show the wings folded (about half way out each side) and stowed vertically.

Maxout August 04, 2010
The F-104 is a strange animal. There is a reason for the anhedral, and it’s easily understood via FF principles. Remember that too much dihedral combined with not enough rudder leads to dutch roll. In the extreme case, you get a plane that wants to swap ends–ie go into a spin. The F-104 does not have sufficient tail volume for the amount of lateral area ahead of its CG when there is any dihedral present. The only way to make it remotely stable in yaw is to make it unstable in roll by adding the anhedral. If you read pilots’ stories, even with that anhedral to improve the yaw stability situation, it took roughly 10,000′ at full throttle to recover from a spin.

Anyone who has flown a few Jet Catapult models knows what a spin looks like on a FF model…

JetPlaneFlyer August 05, 2010
Paul, On ‘real’ (full size) airplanes anhedral on the tail is usually there to move the tail out of the wing’s wake. Tail dihedral is sometimes used for the same reason or, when used on twins, to take the tail out of prop wash.

On models I’m guessing it’s more of a styling feature.

Anhedral on a tail will be slightly destabilising, much less so than if used on the wings though purely because of the tail’s smaller area and smaller span.

Steve

packardpursuit August 05, 2010
So would a dihedral stab make for more roll stability? Again, I’m thinking CV A-7 with dihedral stab and anhedral wings. Another aircraft which saw a flat stab and later a noticeable dihedral added was the Bristol Beaufighter. Was that to aid directional or roll stability, or as you mentioned earlier, to get stab area out of the prop wakes?

Ding October 15, 2010
I’d like to point out the inherent stability of Darcy Whyte’s (Canada) popular SQUIRREL design. It has NO DIHEDRAL, no pylon, but has vertical UPRIGHT wing tip plates. In analysis when the wing banks an impending lateral SIDESLIP is instantly countered by a roll moment from the UPRIGHT TIP PLATES. Therefore an impending spiral is halted and the wing is brought back to horizontal. This is STABILITY.

The proof might be to try a wing with an UPSIDE DOWN wing tip plates. I fear this will increase the spiral tendency and a CRASH. I’d rather not try it.

Ding

Yak52 October 15, 2010
Anhedral or dihedral in the tail has an effect on spiral stability.

The more dihedral in the tail the less spiral stability. The dihedral in the horizontal stab effectively adds to the lateral force provided by the vertical stab, to the point where the vertical stab can be removed altogether – ie a V-tail.

There’s an article on it here:

www.djaerotech.com/dj_askjd/dj_questions/vtail-angle.html

BEAR October 16, 2010
Wasn’t the Star Fighter nick named The Widow Maker??

Ding October 16, 2010
I believe the Star Fighter would not be nicknamed Widow Maker if he flew with upright WING TIP PLATES.