Basics of design and trimming of aconventional hand-launched freeflight glider (boost-glider orrocket-glider).

By Kevin McKiou

1. Make the dihedral 15 degrees!!!! In 1994 I had prototype whichjust would not trim out. It did death dives. It would go inverted atthe top of the hand launch (HL) and go inverted into the ground. Itdid all kinds of crap I could not figure out. The dihedral was set at12 degrees, which was plenty for spiral stability, but not enough tomake it really stable in the transition on a HL. All I did wasincrease the dihedral to 15 degrees. After that, it was easy to trim.It would roll out like a champ at the top of the HL.

2. Make the distance between the wing 1/4 chord and the tail 1/4chord about 50% of the wing span. This will be adequate for spiralstability, but not excessive.

3. Make the horizontal stab area between 15% and 20% of the wingarea. When I have gone below 15% I have had problems with excessiveelevator deflection required. Anything above 20% is unnecessarilydraggy.

4. Make the vertical fin area half the horizontal stab area.

5. If you can calculate the neutral point, set up the CG so theglider has a 15% static margin as a starting point. If this worksout, fine. It is probably close to the safe minimum. If you can'tcalculate the static margin, start with the CG at about 40% back fromthe leading edge at the wing root.

6. Trim the elevator so the glider *just* will not fly in astraight line without stalling, no matter how slow you throw it.Remember to always toss it at a point on the ground about 20 feet infront of you.

7. Now you want to induce a turn. Add about 10 degrees ofhorizontal stab tilt to the right to induce a left turn. Add about ahalf gram of clay to the left wing tip to get the turn started. Giveit another slow toss slightly down. If it glides into a left turnthat is pretty flat, you are very close to perfect. If it turns toofast, remove to tip weight. If it won't break into the turn, add atouch of left rudder.

8. Time to throw it. Throw it up at about a 60 degree angle andtilted slightly to the right. It should arch up, go briefly invertedat the top and roll out in the opposite direction from which youthrew it. Give it a real firm throw.

If it kind of slid up and did not arch back, you have the CG toofar back. Add half a gram of clay to the nose and go back to step6.

If it definitely looped back on you, try again with a throw thatis a little more horizontal. If you just can't get much heightbecause it wants to loop back (usually into the ground), the CG istoo far forward. Remove a bit of nose weight and go back to 6.

If it seemed to launch OK and pretty much stalled at the top witha really slow roll out, add just a bit more weight to the left wingtip and/or a touch more left rudder.

9. The glide after HL should be a big gentle circle to the left.If you are not getting a turn, but the launch looks good, give it abit more left rudder. That should help it into the turn. If the modeltends to glide too fast in the turn, add a bit more up elevator. Ifit seems you just can't get everything working quite right betweenthe launch and the turn (e.g., glides fine, but wants to loop on HL)add wash-in to the left wing tip. That is, bend the trailing edgedown on the outter 1/4 of the left wing. Now, as the speed builds,the lift of the outer portion of the left wing increases more thanthe rest of the wing and forces it to straighten out the glide a bit,slowing it down. As the model slows, the wing tip weight and rudderwill tend to turn the model back into the turn. Now you can back offa bit on the elevator. Use this sparingly. You can over do it andcause the model to tip stall. If the glider builds speed as itglides, with no real recovery in a second or two, the CG is verylikely too far back. Add a half gram of clay to the nose and go backto 6.

That's about it. From this point, you will just have to try it,varying each of the parameters to get a feel for what works well. Ifyou work this well, you should get very good hand launches andtransitions to glide.