Fun Cub on Skis

[aesmith]

Hi,

I thought some might be interested in the skis that I knocked up from scrap that I had to hand. This is very much work in progress, so suggestions are welcome as well.

The skis are made from square PVC guttering down pipe and thread onto the u/c wire in place of the wheels. I added a block of foam with wood reinforcement. The first picture should give the general idea. I made the curved up tips by heating with a hairdrier.



Here's the model with them fitted. The skis are free to rotate so they take up their own position in the airflow, hopefully to minimise aerodynamic interference. They are slightly tail heavy to make sure that they touch down without digging the toes in. A bit of thread prevents them hanging down too far, but they look a little ungainly in flight.



The pictures were taken before the test flight, but once I knew they worked I shaped the tips and generally tidied them up. As finished they weigh just over 90g the pair, 50g or so more than the wheels.

I'm wondering about making a smaller set. Any thoughts on the area needed for a 1kg model? Also I wondered whether leaving them free to rotate was the best option. I note that full-size tail draggers tend to have the skis held quite sharply toe up.

My field's covered with 15" of snow at the moment, and I can't see it going anytime soon, so unless there's a sudden change I'll be doing some more flying off skis.

Tony S

[Richard]

Can't wait for the update, that looks like fun

One question: If the thread on the back gets lost can the ski touch the prop?

[aesmith]

Quote:Originally posted by Richard Archdeacon
One question: If the thread on the back gets lost can the ski touch the prop?

They don't extend that far forward.

[David]

These look great and functional. I would think that your surface area is much larger then needed, which no doubt is your reason for smaller.

I would base any new design on a scale appearance and not worry about surface area, I am sure you will still exceed the needed area. Not sure what your "droop" is at the moment but cant think that you need any more then about 5 - 10 degree's or so.

[aesmith]

Scale would be very small I think. I wasn't planning on going as small as these for example ..



I'm sure you're right on the droop, after all at touch-down the model should be pretty nose high even with full flap.

[Brianathayes]

I can't give you any aerodynamic argument against it, I just don't have the know-how, but from a purely logical point of view, the idea of the skis flapping about in the breeze seems horrendous. I would have fixed them with about 1 degree incidence with the plane held level in flying position.

If you look at almost any photo of a floatplane, you will see that, at rest, they sit squarely on the floats, with the tail off the ground, like a plane with a tricycle u/c. This is more or less the attitude at both take-off and landing.

[aesmith]

You're right about float planes of course. For some reason ski planes tend to go for skis that allow the plane to sit tail down, the same attitude as on wheels. This maybe because they are often temporary fitment, using the existing u/c.

I found a lot more info on the web by searching for "airplane ski" rather than "aeroplane". It looks like normal practice is to have a bungy at the front and a fixed cable at the back, so the ski is held in a consistent position once the weight's off it.

[solentlife]

I was led to believe that Ski's should be set-up similar to floats for water use.
In fact many floats can be used for snow / ice use.

Last site I was looking at talked about a +5 deg. angle of attack of the floats / ski's, ie float front is actually down causing plane to nose up .....

I would be inclined (forgive the pun !) to fix the ski's with a small amount of 'float' of angle.

[aesmith]

This site gives a few (full size) examples of what I think I'm going to aim for.

http://www.trickair.com

Scaling down their PA18 skis would make mine 230 x 45mm, but I think I'll make mine a bit narrower as the loading will be less at model scale. Lots of pictures of them flat on the ground with the plane at rest, not much showing the attitude while airborne so I'll have to make a guess.

I can't see any particular benefit from fixed skis to offset the extra mountings needed.