Four Star 120 Question:


Moving the thrust line "Down" 3/4 inch to center the prop hub in the center of the 4.5 x 5.5 firewall.

What effect will this have on its flying characteristics?

I have attached a pic of it being done. Need room for a gasser. Would like to use a 7' Radial Cowl (generic) from Fiberglass Specialities. I believe it is the cowl in the picture.

DR

With the Fiberglass Specialties 4 star cowl you have pictured you don't need to move the engine at all. As for moving the engine down on the firewall for a radial cowl... Doing so should cause no problem aside from losing prop clearance. Just be sure to keep the fuel tank at the right height in relation to the carb... But since you plan on going with a gasoline engine this is not so critical since the carb has a built in pump.

You are certainly right on the CAP cowl. I am thinking of going a size larger. FS has two other CAP 21's that might work. One at 5" wide x 6 3/8 high x 8" long and another at 5 3/8 wide x 6 3/8 high x 8" long. The length can be trimmed to fit, it might lose a bit of cheek, but that should be OK.

I am considering the 5 3/8" version. The additional height will be benefical has the gasser cylinder head will be even with the bottom of the cowl with the muffler pipes and spark plug exposed. On the muffler side the muffler will be close to an even match, but will have to be cleared to insure it will never touch the cowl. On the other side, the carb will protude, but I think that is beneficial.

The prop clearance issue is a killer on the radial cowl with lowing the thrust line.

I am thinking CAP now. Cowls Left to Right

SK-30 *120 Cowl 4 3/4" wide x 5 3/4" high x 6 5/8" long SK-30 WP90 Pants 9 3/8" long x 2 3/4 high x 1 7/8" wide.
GP1 5 3/8" wide 6 3/8" high x 8" long
GP-31 5" wide x 6 3/8" high x 8" long


Thanks for you response,

DR