Cowl-flap as speed brake?

[bwilson4web]

Thanks Ron,

Bob: Love the drawings. It looks like the nose is being extended a lot. Have you looked at directional stability? All cross-sectional area ahead of the CG is destabilizing. To put it in perspective, the destabilizing effects of air data nose booms can be easily measured.

Yes but I'm not at that stage. I may wind up adding a dorsal to the rudder fin after completing getting the exact, firewall-forward configuration.

The planned, 128 lb engine replaces a 172 lb engine. But I won't know the final placement until I have the BSR (35 lbs), battery, engine (128 lbs), Ivoprop flight adjustable prop, and radiator. With the exact weights (including coolant and hoses,) I'll be able to calculate their final placements using similar weight from the existing cowl to estimate this part. Then I can fabricate the engine mount, and foam-and-fiber glass a cowl and radiator duct. I'll plan several mount locations for the battery so as it serves as movable ballast to get the final CG right at 58".

The speed brake also looks like it will be ahead of the CG ... another destabilizing surface. You would be surprised what a turned nose gear does to directional stability ... and it's on the center line of the airplane (yes, except the A-10). Perforating the brake will have a tendency to stabilize the surface and not change the (+/-) lift in that area.

Have you considered splitting the rudder?

Thanks for tips. I do have a 'loose' rudder but I haven't taken it apart yet to find out what it will take to make it tight. If I have to rebuild the rudder, adding area would be fairly easy at that time. But if I don't have to replace the rudder, a dorsal fin might provide a simpler answer. But funny you mentioned split rudder.

The original build had streamlined foam over the legs and wheel pants. I had thought about a 'split flairing' around the gear to use as speed brakes. Activated with the brakes, it would be complex but another approach potentially giving more yaw control. But angled in-ward, they might passively add yaw stability.

Angled slightly inward, 2-3 degrees, the leg flairings should act like 'toe', adding yaw stability. The forward leg would have more 'lift-drag' to turn the nose forward. The trailing leg would be at a lower angle, in a lower drag-lift profile. Located ~48" to either side of the CG, the force differential should help. This will be fairly easy to test.

Bob Wilson