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Post by starboy on Feb 6, 2011 21:50:59 GMT 1
I epoxied a largish delrin mounting platform just forward of the battery hatch on the sloping section of the fuse and screwed an aluminum landing gear from a GP Venus 40 to it. To the landing gear I've mounted some Dubro skis, which I hope to use on snow as well as on grass. Not only is the new gear forward of the original position but the slope of the mounting plate puts the gear at a forward angle and has the effect of putting the skis on the ground even farther forward than they'd be otherwise. (I made sure to keep about an inch and a half of clearance between the tips of the skis and the prop.) I have two questions about my Franken-cub, which I'm hoping someone with experience might provide some wisdom regarding: 1) Should the fact that the gear will now contact the ground forward of their original position over the CG have any unexpected adverse effects, e.g., will it interfere with proper rotation on takeoff, etc? I'm hoping the forward position will forestall a tendency of the SC to nose over on landing, but it will be good to know whether it means I'm limited to hand launching. 2) The CG has now moved forward to a point 1.5" from the wing's leading edge. What will be the effects of this? If this is too far forward, can it be sufficiently compensated for with trim or do I need to counterbalance? Here's some images showing the new gear, skis and the new position of the CG as indicated by my CG hanger. The small pen mark on the wing indicates the 2.25" factory CG point.     |
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Post by killioughtta on Feb 6, 2011 22:52:32 GMT 1
I love it I love it! Like you guessed, by moving forward the touch-down point (not an official term by any means) of the landing gear, you effectively cut your chances at nose-overs. The CG, if it's off a bit, can be balanced by moving the battery or adding a little bit of weight to the fuse aft. Don't just trim, move the CG back to 2.36 (2 3/8 inches) by balancing otherwise you'll have a tendency to nose-up during flight.
I love the mount, the landing struts and the skis. Nice-looking SC.
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Post by starboy on Feb 7, 2011 3:27:56 GMT 1
I love it I love it! Like you guessed, by moving forward the touch-down point (not an official term by any means) of the landing gear, you effectively cut your chances at nose-overs. The CG, if it's off a bit, can be balanced by moving the battery or adding a little bit of weight to the fuse aft. Don't just trim, move the CG back to 2.36 (2 3/8 inches) by balancing otherwise you'll have a tendency to nose-up during flight. I love the mount, the landing struts and the skis. Nice-looking SC. Thanks for the kind words. Means a lot coming from an old hand. ;D I'll plan on adding some weight aft. Any idea whether the forward gear position will have any effect on takeoff? Since it's no longer balanced over the forward gear I wonder if it will be able to lift its ass-end off the ground. |
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Post by killioughtta on Feb 7, 2011 5:09:28 GMT 1
It should do just fine on take-off. A friend flew a HK Cessna with modified landing gear and the front gear was far forward too. It took off with no problem.
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Post by starboy on Feb 7, 2011 16:53:42 GMT 1
When I get home tonight I'll post some pics on how I control the angle of the skis, preventing them from tilting too far up, which would cause them to act like air brakes, or tilting nose down, which would allow the tips to dig into the ground on landing. I read somewhere that skis should ride at about 10 degrees up angle when the aircraft is in level flight but it didn't give any tips on how to accomplish this, so I came up with my own solution.
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Post by starboy on Feb 8, 2011 1:49:58 GMT 1
The problem with any of the skis I've seen is that they don't seem to provide the means to control their angle, other than a spring to ensure that they're tilted up. The first time I tried these skis last year it was a disaster because under the pressure of the air they tilted up vertically and acted like air brakes. What I did this time was to use some large (really cool aluminum) paper clips (I was going to use plain steel ones but these were in my wife's paperclip box instead, oh joy!) to create a rotation limiter. I epoxied them to the struts, after bending them roughly into the shape I wanted, and then I fine tuned the shape to stop the skis from moving farther in either direction than I wanted them to. It was just a matter of taking some protractor measurements of the fuse in it's two natural positions - on the ground and in the air - and deciding what maximum position the skis should be allowed to be in, in those two positions and adding that to the forward angle of the struts. Since I knew 10 degrees was the maximum up angle during flight based on this article: www.stenulson.net/rcflight/skikits.htm, and that I didn't want a negative deflection of more than 0 degrees on the ground, I pre-bent the paper clips using the calculated angles and epoxied them in place. In these images I don't have the axle collars installed yet. (They're in the mail.) The rubber bands replace the springs that come with the skis, which I've since lost, and work just as well at keeping the skis tilted up at their 10 degree angle and not flopping around when they're not on the ground.       |
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Post by ginginho on Feb 8, 2011 10:30:17 GMT 1
That's a very nice job you've done there. I like!
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Post by alaskabliss on Feb 19, 2011 0:24:30 GMT 1
What I did with my cub was to install small fishing swivels to the two bottom cowling screws. I used black elastic cord from the sewing shop and and hooked it up to the front of the ski so the angle was perfect in the air. This eliminates the problem of the wrong ski angle while sitting on the ground since the cord acts like a bungee and has plenty of give. The cord should be under tension when sitting parked on the ground. I used the back two battery box screws and tied dacron fishing line from the and to the back of the ski's to act as a limiter strap to keep the ski's from angling up farther than I want. This adds realism since I got the idea off a real cub on ski's and gives the cub some suspension too.
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Post by alaskabliss on Feb 19, 2011 0:26:44 GMT 1
Here is a picture sitting on the lake. Another thing to think about is how well will ski's work on grass. I personally don't think they are going to work unless the grass is wet to slick them up. The forward landing gear might help to keep it from tipping but I would try and taxi it around on the grass and get a feel for what its going to do. Good luck and let us know. If they will work on the grass can you imagine how much fun it would be to skip it on the water in the summer! Attachments:
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