"Squirrel" - Construction - Speed 400
by Martin Irvine
Speed 400 sportsman class racer and sport plane
Click image to enlarge.
Click image to enlarge.
Last year Rob Pike pushed us(!) into Sp400 racing. We had a very successful series of racing in two classes. The plane presented here is my attempt at designing a Sportsman class racer. These models are meant to be an introduction to racing and have to fit a set of specifications. Wing area must be 180sq. in or more, minimum wing thickness is 1/2", minimum weight is 16 oz. and power is restricted to a Sp400 and 7 X 600mah cells (or smaller). Races are 4 minutes long. (3:50 scores 0!)
The Squirrel presented here did not win any races, but was easy enough to fly that it placed 3rd in its first race, which was also its test flight! I consistently launched it by myself and often found myself grabbing it first when the weather was good. A little plane like this is great to have in the car for those lunchtime opportunities.
<- Martin's "Squirrel"
Build the wing first. It is a constant chord wing, so one master rib of plywood will suffice. Centre section ribs will need to be trimmed to allow for sheeting. The webbing between the spars is very important! Don't leave it out. It gives a lot of strength.
The leading edge is two parts. The first is used to give you something to glue the edge of the leading edge sheeting, and the second
forms the true leading edge.
In the centre section, include some block to anchor the wing dowel and back up the wing mounting bolt hole.
The trailing edge and ailerons are cut from light sheet. Torque tubes are bent from spring steel and brass tubing. To mount the aileron servo, cut the lower sheeting and attach the servo into the wing as far as you can. I have suggested a 1/32" ply "tail" to tie the trailing edge together and cover an open rear fuselage. This gives a lot more room for the servos.
The fuselage is pretty straight forward. The fuselage sides are parallel between the leading and trailing edge and are drawn togther for the nose and tail. Use the top and bottom sheeting to hold things in place. Take your time for this as you want the fuselage straight. The wing mounting plate is 1/8" ply drilled and tapped for a 6-32 nylon bolt.
The nose block is a sold piece of balsa with the grain going in the direction of flight. The secret of doing this without splitting is to cut the length accurately, but the height and width 1/2" oversize. Carefully drill and sand to a snug fit with your motor. Now CA the inside and resand to exact size. Glue this to the nose of the plane and then carve the outside to shape. Another refinement is to add a 1/16" laser cut motor mount made by Tim McDonough and sold by New Creations, and your local hobby shop if they can get Hobby Lobby things.
Sand the edges of the fuselage to a smooth shape, but be careful not to go too far. You don't want to weaken it too much by over sanding.
Cut the tail surfaces out and sand the edges smooth. 1/16" wire joins the two halves. If your weight is low, consider adding a rudder. It will let you do spins, stall turns, snap rolls and knife edge. Of course, for racing it isn't necessary.
For power, the only motor is the 6V version. The 7.2 is inefficient on 7 cells and the 4.8 volt motor is way too "hot". There may be some room for experimenting with gearing, but everyone last year went with direct drive.
The Graupner precision spinner is expensive but well worth it. It holds the prop, is small and light, and most important, is very precise. Anything else is likely to vibrate.
Props are usually 4.7 X 4.7. Graupner props are very good but rather fragile. The APC version seems close and is more durable and cheaper. Larger props work but draw enough current that finishing the 4 minute race is unlikely.
Batteries are always Sanyos - 600AA, 600AE, 500A, or 500ARs. I like the Ars because they are the most rugged and designed for fast charging. The others have to be treated with a little more care or their useful lives are greatly reduced.
Control throws ailerons + - 1/8" racing, 3/16" sport elevator + - 3/16" racing, 1/4" sport
Balance point is on the main spar. I think that's about it. Build one and race at one of the EMFSO races this summer. If you have any questions feel free to contact me.
1331 Rockwood Drive,
Kingston, Ont. K7P 2M8