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Old 12-25-2016, 08:42 AM   #1
kitbuilder
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Skis on an Apprentice?


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Is there a rule of thumb for ski size/length on a plane. The Apprentice only weighs about 3 1/2 lbs and I wouldn't think it would need very big skis. I want to do some winter flying this year and this plane is very easy to fly (and easily fixable). Looked at GH and they stock both JayDee and CJ skis. JayDee seems too big and CJ seems too short. Any tips/comments appreciated. Merry Christmas to everyone.
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Old 12-25-2016, 08:56 AM   #2
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Re: Skis on an Apprentice?

Like floats you go by weight, the size determined by how much they can support. Some go by 14 sq.in/lb.

Mine are a bit heavier, but along with JayDee skis I have some of these:

https://www.greathobbies.com/product...prod_id=DUB825

These are a quick replacement for wheels. In the case of the Super 'Bolt I have them installed on a spare set of gear so almost as fast with no angle adjustment.


You can easily make some yourself as I did earlier in the sport. Thin ply with a thicker keel, anchor point are very simple to form and bond together and waterproof.

The key is to put the hinge point where it needs to be, but to also prevent the ski from dropping the front end in flight. Not only incredible drag, it becomes a mess on touchdown.

Luckily there are lots of sources out there you can look through. Here's one with some info:

https://www.greathobbies.com/product...prod_id=DUB825

NOTE: Size and design will also be determined by the environment. Hard pack will require skis of much smaller area than if they are working on even a slight depth of powder. Also, you will need some form of runner on hard surfaces to prevent sliding sideways and that works well on the steering ski in any environment when trying to turn.


Good luck and let us know what you come up with.
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Old 12-25-2016, 09:32 AM   #3
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Re: Skis on an Apprentice?

A few formulas:

1. Length should be about 2/3rds the length of the fuselage.
2. Width should be between 1/10th and 1/12th the length.
3. The axel should be about 60% back from the front of the ski.
4. The front restraint cable should be attached to the firewall and
the rear restraint cable should be attached at least 1" back of L/G attachment point.
This prevents the ski from flipping over.
5. Use an elastic band to keep the nose about 10 deg. nose up. You adjust this with the resr cable.
6. Slant the skis so the outside edge is a few degrees higher than the inside and round the bottom corners so they will not dig in when sliding sideways.
7. A small centre strip on one of the main skis and the nose or tail ski will help the aircraft turn.

OR

Try a set of these, Snow floats.
Attached Files
File Type: pdf Snow Floats.pdf (13.6 KB, 86 views)
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Old 12-25-2016, 11:13 AM   #4
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Re: Skis on an Apprentice?

Thanks guys you've given me enough info to go on. it looks like JayDee.

Kip - I looked at that ski/snow float and the length blew me away - 60" - that is really one giant scale plane.
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Old 12-26-2016, 01:05 AM   #5
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Re: Skis on an Apprentice?

Flying off snow with regular floats works quite well of course, especially in soft snow.
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Old 12-26-2016, 08:57 AM   #6
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Re: Skis on an Apprentice?

Yeah Bill M, I designed these for my ultralight, they might be a little long for a model. You will have to adjust the length as per my instructions. Forgot that line!!!
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Old 12-26-2016, 09:56 AM   #7
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Re: Skis on an Apprentice?

Floats work fine, skis look down rite cool!! GH has their own brand of plastic skis I've used them on several planes including the apprentice they worked great.
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Old 12-26-2016, 12:04 PM   #8
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Re: Skis on an Apprentice?

I find the biggest variable is snow/runway conditions.
I've flown in a wide range from falling snow to -31C.
If the snow is hard enough, any size skis work, I've flown from wheels in the Winter as well.
You can get floats for that plane and they work better than skis in some conditions but ice wears them almost like sand.

Wet or frosty grass:
Skis or wheels work well.

Hard crusty snow or ice that you can walk on is the best for the runway because there are no boot holes.
Again, skis or wheels are best.

My favourite surface is walkable with about 1/2” of snow on top. The snow provides some steerability and braking.
I've used wheels on that but skis are best.

Hard or semi-soft snow:
Skis but as it gets softer, floats are better.

Deep powder snow:
The apprentice might be light enough to make skis work, my planes just make a blizzard until enough attempts to take off, then the snow gets tramped down and if you can keep it on the tramped part you can get in the air. Long touch-n-goes are fun in this stuff as long as you keep the speed up.
What really shines is my Flying boat(CL-415 twin) but the snow has to be dry, heavy snow makes it impossible to reach takeoff speed with this plane.

I calculated my ski loads to see how close they are to the provided value:
Alpha 40 - 7.5 in^2 / Lb
LT-40 – 5.2 in^2 / Lb
Both are far shy of the 14 in^2 / Lb, that might explain some of my trouble!

The LT-40 works better probably because I made it a taildragger, the nose up attitude helps a lot in deeper or softer snow.

Well I guess that's my 2 dollars worth

Amusing video of Alpha:

Video of LT-40

Flying boat
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Last edited by Mad_Mike; 12-26-2016 at 12:18 PM. Reason: Corrected Alpha ski loading
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Old 12-26-2016, 01:05 PM   #9
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Re: Skis on an Apprentice?

I will agree tail draggers have an easier time in the snow on skis. I plow my runway at the field so tires are fine, but at home I like skis. My wife's green house already has touch and go ski marks on it.
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Old 12-26-2016, 04:47 PM   #10
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Re: Skis on an Apprentice?

Great videos Mike. You've convinced me that skis that are long and light are the way to go. I'm not going with Jay Dee or CJ as on second thought they're probably too heavy for the Apprentice which is only rated as a 15. However, Ryan mentioned GH has some white plastic skis that 10 1/4" long and 2 1/2" wide and look to be very light. If I use 3.5 LBS. AUW for the plane, 26sq." per ski for 52sq." in total (just using main skis not nosewheel) then 14sq."/LB X 3.5lb gives me a figure of 49 or slightly less then the 52sq." figure. Somebody feel free to correct my assumption ( also known as fudging the math) of using two skis for the calculation. I'm going to give them a try as soon as GH can get them to me and it stops pouring rain.
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