OkadaKeisuke

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Hey All,

I recently got an Alfa P-47D ARF and I have been flying it without it's stock gear due to lack of smooth surface runway. Until, now I have had little problems with it. However, I have accrued quite a bit of scuffs, dings, and scratches in the foam on the underside of the wings. Although I have been belly landing it very gently on packed snow (the surface some what rough) I cant seem to avoid damaging the surface of the foam while landing. So I decided to try to fashion a set of skis to prevent any new damage to the belly and underside.

I took the stock struts and fastened homemade skis to them. The skis are made of two pieces of plastic laminated together with 30 min epoxy and formed around some pegs I placed in a piece of ply. I used small servo screws to fasten a block of wood to the top of the ski in which the axle sets. The ski is held taught with a small extension spring connected from the toe of the ski to the strut, and a length of fishing line from the heel of the ski to the strut. From my limited knowledge I believe they are quite well built and sturdy. Also, the pair only weighs 45.5 grams! I think thats good.

Ok, so now for the problem. I field tested them and I am having trouble. When I throttle up about 5-6 clicks on the Tx the tail lifts and the plane proceeds to roll over onto its nose resulting in prop strike. Heck, it doesn't even move forward more than a few inches before tipping forward. The CG is spot on.

Is this just the nature of warbirds? Is it impossible to retro-fit skis on a warbird taildragger? What can I do to solve the problem?

More info about the Setup:

Motor: Rimfire 400 Brushless
ESC : Silverseries 25A
Prop: APC 9x6 Slo-flyer

Another thought...
Could the prop just be to big for this application (with skis)? And if I where to get a smaller prop what would you recommend? The plane flew great, hand launched, with the 9x6, and I'd rather avoid changing the flight characteristics if possible. I believe I read on here that if you REDUCE the length of the prop by an inch, you should INCREASE the pitch by an inch, Is this correct?

I look forward to any responses.

~Noah

proptop

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Looks like you did a nice job on the skis...but perhaps they should be a bit longer? (the front part, ahead of the axle )
Or...maybe move them fwd. a little bit?

OkadaKeisuke

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Thanks proptop!
I thougt about the length as well, but can't see how that would help seeing how the axel or fulcrum isn't fixed but free rotating. If it was fixed it may indeed prevent the plane from rotating forward, but I need it to, right? However, moving the struts forward seems like it would definately help. It would move more of the weight behind the struts. Unfortunately I would probably do a hack job on the wings to move the strut mounts forward and I really wish to avoid further damage to the wing undersides. It's funny, because I taxied it around on my street one day with the wheels on it and I never had the problem I'm having now. I wonder if there is too much friction causing the plane to rotate before it moves forward, as if the skis where pinned to the ground. I wonder if wax would help, or somthing to make them more slick?

hmmm..... I certainly enjoy a good puzzle.

proptop

My Feedback: (8)

Actually, what I was talking about was to move the ski itself fwd....not the whole strut.

I think maybe if you adjust the travel limiters you have on the rear of the ski (what is it made of...looks like fishing line? )...make them a little shorter so that the tail can only come up so far...and not nose over until the prop hits the snow?

Waxing can definitely help...I use spray on Turtle wax...but anything similar will work.

CGRetired

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Yeah, my first impression of the original with the follow up post by the OP was that perhaps the ski's don't "break friction" soon enough and perhaps they need to be more "slippery". I am no expert with ski's by any stretch of imagination, but it seems plausable. That along with a slower move of the throttle to keep the power down as it does break friction???

CGr.

JohnBuckner

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It is just like many of the warbirds on wheels and the fellows must learn to always keep full up elevator on the start of a takeoff roll and on rollout after landing or you get a prop strike. Also as already mentioned a little parafin wax on the ski's would be a wonderful idea.

Now having mentioned the piloting part of the equation a very simple rerig as Proptop described that will simply disallow over rotation to the point of a prop strike is the obvious next step and that is what the point of the ski rigging is in the first place i.e. not allowing over rotation either way. Actually a full over rotation of a ski to ninety degrees down is a horrfic thought full scale or model.

John

proptop

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"Actually a full over rotation of a ski to ninety degrees down is a horrfic thought full scale or model."

John



Funny you should mention that John...
Last winter, one of our flyin' buddies had that happen...just one ski...[X(]
Talk about "out of trim"!
He had a heck of a time getting 'er back down in one piece...but he did make it safely back.


FWIW I would just cover the belly of the plane w/ clear protective tape, and get rid of the landing gear all together...just hand launch it and belly land on the snow.
We used to do that quite often with our Ace Upstart .049 racers...[8D]
(but since you did such a nice job on the skis, I can see where you want to keep using them / get the bugs out )

JohnBuckner

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Hmm Yes we do have to go to extremes here, where we seldom get a sufficient suppy of snow or water in usable quantities. In this case a little aluminum duct tape on the keels and wipe of parafin wax from the canning department at the grocery:[X(]

http://www.youtube.com/user/JohnBuck.../9/cYiD5_AKjFY

John

OkadaKeisuke

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Proptop, I totally misunderstood you![X(] That makes so much sence now! Also your comment about the limiters definately shed a whole new light on my problem. The rear limiter was REALLY slack when the plane sat on the skis at rest. Shoreing them up would most definately aid in preventing over rotation.

All of those who posted, helped shed new light on the problem and for that I am super thankful!
Thanks everyone!

I will report back with results after I make some changes based on you folks advice.

Now time for the fun part!! Modification!!

Thanks again all!!

Noah

OkadaKeisuke

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Ok... so,

I shored up the line from the strut to the heel of the ski and now, when I place the plane on the floor, there is very little slack in the line.
I took the plane out into the driveway, (which is covered in nice packed snow) and gave it a go.
It was a sucess!! Just enough rotation occured to bring the wings parallel with the ground without the prop strike!! I am extremely elated!!
Now, the only problem I have is the plane tends to veer to the left. I have read that tail draggers will do this due to motor torque (??). I have a hunch, that when I bring it out into a wide open feild, and after I gain a little more speed, it will straighten out, but I was unable to confirm this hunch in the limited space of my driveway.
Well, now I just need to wait until daylight (some day this weekend hopefully) to confirm or deny this hunch.
Until then!

I will report back with more results.

Any feedback is welcome!

Thanks again all!

~Noah

proptop

My Feedback: (8)

Glad to hear that the modifications helped![8D]

Do you have rudder control on that airplane? It is common for an airplane to veer to the left on application of power...if you feed in a bit of right rudder as you apply power, you can make your take-off run straight(er )

OkadaKeisuke

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Yes, I do have rudder control indeed! I can't wait to get out into the field and try it out.

~Noah

OkadaKeisuke

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Well I flew the Alfa with skis today. It Flew and landed quite nicely. It was a little slower, and more "Scale-like" With the extra weight from the ski's which made me happy as it was not unrealistically darting around through the sky. As for taking off, I need a bit more practice. In order for me to keep the plane tracking straight, I have to start out giving it around 80-90% right rudder, and about 90% up elevator. I learned that I must ease off right rudder as well as and most importantly up elevator as the Alfa gains speed. I had a few premature lift-offs causing the plane to bank sharply to the left and cartwheel. Thank goodness there was plenty of powdery snow off the edges of the 30ft wide snowmobile trail I used as a runway. The plane suffered no damage.

Now I just need to work on some muscle memory. I need to advance the throttle slowly and constantly to WOT, and as I pick up speed I need to relax the right rudder input as well as the elevator input all simultaneously. This sounds like a job for the simulator!

I have to say, though, it is so much more fun flying this bird with skis as opposed to handlaunching/bellylanding. It comes in nice and level and touches down smoothly and glides along the ground for about 30 feet or so before it stops. After I get more practice I would love to do some touch-and-go's! AFTER more practice, that is!

~Noah

proptop

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Excellent![8D]
It sounds like you really are getting the hang of it...your experience with this bird will help you with just about every other airplane you fly...expecially tail draggers.
The blending of rudder / throttle / elevator (and maybe some aileron too ) is a tricky thing to learn...so yes, practice until it becomes automatic...a reflex action...sounds like you are well on the way![sm=thumbup.gif]