skywagon8a
MEMBER
SE Mass MA6
Courierguy's post in the 2018 Texas STOL thread triggered a memory. So I thought that my response would be better brought up in a separate thread.
Long ago I recall seeing a drag vs speed curve which I have not been able to locate to help with my response. This curve showed the rate of total drag increase from zero speed up through supersonic. Basically there is a gradual drag increase up through just above 100 mph when the rate of drag abruptly goes upwards to maybe 120 mph where the rate again returns to a gradual increase until reaching the speed of sound where there is an abrupt increase in drag until above the sound barrier where the increasing rate again becomes gradual. If anyone can locate this graph, please post it.
Long ago I owned a Lake LA-4 which has a pair of very aerodynamically clean floats/sponsons hanging under the wings. I hit a bird with one sponson which produced a dent. So the sponson was removed for repair followed by a test flight to see how it flew with just one. What do you know, the cruise speed was 5 mph slower with just one. So me being the curious type, I removed the other sponson and went flying. Guess what, the cruise speed went back to the same as it was with both sponsons installed. My conclusion was that the amount of rudder displacement to offset the drag of one float was equal to a 5 mph speed loss. With both installed the rudder was not offset. The cruise speed of the Lake is approximately 118 mph. This is within that first abrupt drag increase on the curve which I mentioned.
How and why does this differ from courierguy's observation? I don't know his cruise speed though I suspect it to be in the 80-90 mph range which is well below the abrupt rise in drag above 100 mph. His drag is in the lower range so the the rudder offset with one pod would be minimal. At least small enough so a speed loss with one pod would not be noticeable. In his case I would be concerned if the airplane entered a spin. It will spin at a higher rate in one direction vs the other. This could result in a nonrecoverable situation. I flew the spin tests for Lake when the sponsons were certified to carry fuel. The testing was done with one full and the other empty.
Now as far as some Cubs going maybe a little faster with a belly tank installed. The tank is directly behind the landing gear V/shock strut drag mess. The tank streamlines the turbulence which is generated thus reducing drag a bit. The tank is essentially a landing gear fairing.
Here is the punch line: as near as Joel and I could determine, the damn plane flies BETTER with the pods then without. It got off a bit quicker, and landed a bit shorter, not a lot, but for sure not slower to get off, or any longer to land. We reluctantly came to the conclusion that just possibly, some kind of black aero magic was going on, similar I think maybe to a lot of you Cub guys reporting a bit faster cruise with a belly tank. Maybe they act like little wings, or at least produce enough lift to offset their drag and weight, beats me. NO discernible difference in cruise.
Based on my earlier testing last year when I flew at one point with just one pod, (no yawing problems, couldn't tell any difference) when I bought recently I bought just one, and have about 25 hours on it so far. Maybe I'm just clueless but I can't tell it's there in flight, (including stalls) the only disadvantage seems to be it looks weird, having two would look better, but I really don't need all that extra baggage area two would offer.
Long ago I recall seeing a drag vs speed curve which I have not been able to locate to help with my response. This curve showed the rate of total drag increase from zero speed up through supersonic. Basically there is a gradual drag increase up through just above 100 mph when the rate of drag abruptly goes upwards to maybe 120 mph where the rate again returns to a gradual increase until reaching the speed of sound where there is an abrupt increase in drag until above the sound barrier where the increasing rate again becomes gradual. If anyone can locate this graph, please post it.
Long ago I owned a Lake LA-4 which has a pair of very aerodynamically clean floats/sponsons hanging under the wings. I hit a bird with one sponson which produced a dent. So the sponson was removed for repair followed by a test flight to see how it flew with just one. What do you know, the cruise speed was 5 mph slower with just one. So me being the curious type, I removed the other sponson and went flying. Guess what, the cruise speed went back to the same as it was with both sponsons installed. My conclusion was that the amount of rudder displacement to offset the drag of one float was equal to a 5 mph speed loss. With both installed the rudder was not offset. The cruise speed of the Lake is approximately 118 mph. This is within that first abrupt drag increase on the curve which I mentioned.
How and why does this differ from courierguy's observation? I don't know his cruise speed though I suspect it to be in the 80-90 mph range which is well below the abrupt rise in drag above 100 mph. His drag is in the lower range so the the rudder offset with one pod would be minimal. At least small enough so a speed loss with one pod would not be noticeable. In his case I would be concerned if the airplane entered a spin. It will spin at a higher rate in one direction vs the other. This could result in a nonrecoverable situation. I flew the spin tests for Lake when the sponsons were certified to carry fuel. The testing was done with one full and the other empty.
Now as far as some Cubs going maybe a little faster with a belly tank installed. The tank is directly behind the landing gear V/shock strut drag mess. The tank streamlines the turbulence which is generated thus reducing drag a bit. The tank is essentially a landing gear fairing.