maxpower1954
Posts: 187
Joined: 11/17/2005
From: Rock Hill,
SC, USA
Status: offline
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Flew the Skylark 56 twin three more times today. I hope it isn't boring you guys, but since it is a twin and therefore unusual for a vintage model I'll probably be blathering on a bit...
Since last Sunday, I made the following changes. I went from a 2" nose wheel and 2 1/4 mains to 2 1/4 nose and 2 1/2 mains, which is what the plans called for in the first place. Big improvement on my grass field! I changed out from 8-6 props to 8-4s, which turned it into a sparkling performer. Take-off roll is now on the order of 50 - 75 feet, and a 30 degree deck angle on climb-out, much better than before.
It's still a little nose heavy, with two ounces of ballast forward. On the second flight, I took it to altitude for spin testing, where it stubbornly refused to do so,
in spite of every combination of rudder, elevator, ailerons and power that I could think of. It finally did one full turn, then broke into a spiral dive. I flew it inverted for a while, moderate down elevator required, nearly full down in the turns; again a function of being nose heavy.
After some more gyrations trying to get it to spin, the sound changed - I just lost an engine, but which one? I was about half-power, so I slowly powered up - it wanted to turn to the left... number one was out. I put in full right rudder trim, and since I had lots of altitude decided to see what single-engine flight was all about!
The left engine is considered to be the critical one on an airplane with clockwise turning props (as seen from the cockpit.) The reason being, at higher angles of attack the descending blade on the running right engine is further from the centerline, exerting more yaw force (p-factor.) If the right engine was out, the descending blade on the left engine is between the engine and the centerline of the fuselage, which has a smaller arm. In practice, it really doesn't make much difference. Most full-size twins don't have counter-rotating props, which eliminates the crictical engine. A few modern light twins, like the Piper Seminole, Beech Duchess and some Twin Comanches have C-R, and notably the famous P-38 Lightning, but the Barons, 310s and DC-3s of the world don't.
The Skylark is an absolute kiddie car with an engine out. I slowly went to full throttle, using right rudder to hold the heading...as I pulled the nose up into a steep climb, it finally ran out of rudder, which meant a slow turn to the left. Aproaching the stall, it attained about a 45 degree bank, which a little right aileron
easily took care of. Bringing the power back some immediately restored full control. This basically is a Vmc demonstration in a full-size twin (velocity minimum control.) In a scale Martin B-26 this would be much more exciting!
Even on one engine, I could still climb quite well. I let the speed build up, then did a loop; at the top you have to throttle back or you'll lose heading control; it wasn't exactly gracefull! I did a number of aileron rolls, suprisingly it rolled a little better into the live engine (right) than to the left - I think I have a little more right throw on my servos, since I'm using an old Heathkit transmitter with modern servos. The single-engine landing was uneventful.
On the third flight, I did about six or seven touch and goes. A little power just before the landing flare greases it right on. Overall, I enjoy flying this immensely, and building it was the most work I've ever put into a non-scale model. I'll keep moving the CG back until I find the sweet spot where it will spin and snap-roll consistently, and also help out the landings as it does tend to run out of elevator without power. My daughter has volunteered to take some video that hopefully we can upload to U-tube. Russ Farris
< Message edited by maxpower1954 -- 6/16/2008 7:43:06 AM >
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Goldberg Skylark 56 twin flies! - 
