Cowl flaps

Last weekend a friend came over and shared his "second pair of eyes" on my project. He is a builder/restorer, CFII and owner of four fine aircraft. He suggested I check into adding cowl flaps in order to gain some speed and effeciency. It sounds like a reasonable idea, although I hadn't considered it before. Has it been done, is it effective, where can I find how-to info, anything like that would be welcome.

Bob Kelly, 90854, engine mounted

Cowl Flaps

Cowl flaps have been done, and they do work, mostly in connection with keeping engine temps down to a manageable level, and mitigating shock cooling on the faster breed of RV's. There was a big discussion about a year ago on the RV-7/7A list regarding being better able to increase cooling flow by enlarging the cutout in the lower cowl air exhaust area, using narrow internal raised strips along the inside edge of the cutout (Piper did this with the Cherokee), or installing shaped "roll strips" along the bottom of the firewall inside the cowl to speed up and ease the outflow of cooling air inside the cowling, thereby reducing cooling drag and increasing inflow of cooling air. These are all tricks that were suggested to solve the problem WITHOUT building cowl flaps. The general consensus was that cowl flaps would be a last resort, somewhat complicated to build and install, and would add a little weight. Everyone felt that a manual push-pull cable was the way to go to have some control over the cowl flaps for various flight regimes.

I am planning to install a Mazda rotary powerplant, and heat rejection is a fact of life for these engines, so I paid attention to the conversations. Should it happen that I need additional cooling, I am toying with the idea of enlarging the lower aft air exhaust area by cutting out a strip section, then securing it with piano hinges, and weighting the trailing edge internally with tire balancing weights, so that they will "free-fall" to the open position when speeds fall below a certain level, and migrate to the closed position as airspeed and external airloads increase above a certain speed. Shock cooling is not an issue here, but ground cooling and climb cooling may be. Not being an aerodynamicist or schooled in the fluid dynamics discipline, I envision getting precisely the right amount of weights to make it work through numerous trial-and-error test flights, possibly with an external video camera to speed up the process. But cowl flaps are certainly feasible, and several builders have added them. You might check the RV-7 Matronics list archives.
Regards,
J.W.