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Old Sep 23, 2011, 10:44 AM
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Landing skids - pros/cons

Hello,

I'm currently in the process of building a very small, light micro RC plane with my group. We are planning to enter it into a competition in which it must carry a weight. The magnitude of the weight depends on the weight of the aircraft.

In our case, the plane is planned to weigh just under 2 lbs.

Since the plane is so small, I want to use simple landing skids made from a bent, thin steel tube with a diameter of about 1/8". I would like to know what you all think of this idea.

What are any glaring disadvantages or advantages of a landing skid assuming the aircraft is otherwise conventional (i.e. straight wing, 3-panel empennage, tractor engine, carbon fibre stick as a fuselage)?

Thanks for the help!
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Old Sep 23, 2011, 10:54 AM
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I'm having a hard time envisioning a light micro RC plane that comes in at 2 pounds. Could you help us out with a little more detail and some pics?
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Old Sep 23, 2011, 11:17 AM
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How are you intending to get the model off the ground? 2Lbs + a payload isn't going to be easy to handlaunch.
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Old Sep 23, 2011, 11:29 AM
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tread?

I use the type of tread that looks like sandpaper and has an adhesive backing to put on steps. It adds strength, protects and brakes, but unless you use just a little it won't save weight in your case. Can't have too many ideas to choose from eh?!
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Old Sep 23, 2011, 11:36 AM
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Skids don't help much in most cases because they don't allow a takeoff from the ground. They'd need a hand launch or bungee launch. There are some very light wheels out there so you can use conventional gear with very little weight penalty, or (if you're flying off grass) put some packing tape on the belly of the plane, do without gear at all and have a belly lander.

Steel tubing is usually mild steel and doesn't spring back after impact. For either skids or conventional gear, you would have better luck with music wire.

In today's RC world, "micro" means about 2 ounces, not 2 lbs. 2 lbs is a biggish parkflyer. That's good because there's lots of good, inexpensive equipment out there for planes in that size range.
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Old Sep 23, 2011, 12:28 PM
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Sorry, I have no pics at this time, but the aircraft is intended to be hand-launched. I can't believe I forgot to mention that. The payload is going to be very small. I can't remember exactly what it would be for a 2 lb plane right now. If I were at home with the desktop I could give far more info. Our team is actually subdivided so that each component has a team (i.e. wing team, powerplant team, and my landing gear team).

The aircraft fuselage is going to be a basic tube (likely carbon fibre). We'll be mounting a straight wing (can't remember the dimensions or airfoil). At the nose will be a motor (still figuring out which motor to mount).

Yes, and I suppose 2 lb is no longer considered micro, but that is the weight the team wishes to be below.

Thanks for the quick replies though. I'm contemplating using a heavy gauge wire rather than tubing or rod.
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Old Sep 23, 2011, 12:45 PM
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Hi Pappy,

The powerplant team can't even begin to start designing until the wing team delivers them a wing.
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Old Sep 23, 2011, 12:49 PM
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This is true, which is why they are currently designing a simple mounting which we will custom build as soon as the wing team has their wing design physically built. They've already got their airfoil with Horner tips ready on CATIA. They're going to focus on figuring out the best materials/internal mountings now.
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Old Sep 25, 2011, 12:04 PM
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Sgt Pappy, 2 lbs even for an all up weight can cover a LOT of ground for model size. For example a 56'ish inch span free flight model for Nostalgia free flight has to weigh in at 19 and a bit oz when powered with a .19ci engine. So that's about 1.6 lbs for a 56 inch span model. Not many would consider that to be a small model. So just saying "2 lbs" doesn't tell us diddly.

In any event the ONLY thing a skid is useful for on a model is to prevent the prop from snapping when landing. Otherwise it serves no purpose at all. Frankly it's easier to leave it off and just have the pilot blip the throttle until the prop stops in a near horizontal position. Using an ESC, I'm assuming it'll be electric powered, with a brake function makes this easier to do.

Now if you're flying from pavement then metal wire skids perform an anti abrasive function. In that case you'll want wire whiskers at any and all points of contact to avoid abrasion. The whiskers should be a few inches long and slightly swept back and curved to also provide a slight spring suspension aspect as well to cover you off against hard arrivals.

Something your group also needs to consider in terms of size to weight is to get the wing loading down to a low enough value that allows for hand launching. If the model isn't up to at least the minimum flying speed by the time it leaves the launcher's hands then bad things will happen. Launching into wind helps as it's like "free" velocity. But if the meet is held on a still day then the wing loading becomes far more important. Generally you want to keep a hand launched model down to 16 to 18 oz/sq foot or less. And even at 16 oz/sq ft you'll want the guy on the team that most closely approaches being a linebacker to do the launching.... More ideally for dead air launches by mere mortals 12 to 14 oz/sq foot is better.
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Old Sep 25, 2011, 12:20 PM
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Additionally, if the weight is the design spec rather than the size, then I would think that an electric glider with a folding prop would be the best idea. Possibly a good way to land on tarmac would be a semi-recessed monowheel under the fuselage and a couple of plastic skids on the wingtips.

Build it as light as possible, with a geared motor or possibly a low Kv outrunner to run a big prop slowly, keep the wing loading down to <8oz/ft2 and you'll be able to cruise at altitude with very low current draw, which will reduce the capacity of battery (ie weight) that you'll need to haul aloft, depending if your task has a duration limit?
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Old Sep 28, 2011, 01:30 PM
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Thanks for the comments again.

Turns out the aircraft part weight goals are as follows:

Fuselage - 0.1 lb
Wings - 0.1 lb each
Tail/empennage - 0.1 lb
motor + batteries/controller - 0.25 lb
Landing gear/skids - 0.1 lb

I believe we're aiming for a 18-24" sing span with a chord of 5". This plane is going to be very small. Based on last year's design, our team was indeed able to meet most of these weight goals, so I see it as possible now.

The plane is supposed to carry 1.75 lb in payload weights. We are planning to put this in a fuselage cargo hold.
The multiple wire idea is interesting, and I may have to pitch that to the team.

Thanks!
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Old Sep 28, 2011, 02:46 PM
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24" span and 5" chord gives you about 0.85 square foot of area.

If your AUW is 2.25lbs or 36oz thats a wing loading of 42oz/ft2 which is WAY too heavy for that size of model, it'll have to fly ridiculously quickly to stay up, and the stall speed will probably be not far short of the maximum speed. You need to rethink this a little.

What are the size limitations for the model?
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Old Sep 28, 2011, 03:04 PM
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Stall speed will be over 32 mph.

Agree, the plane is going to fly like a brick.
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Old Sep 28, 2011, 09:09 PM
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So even mentioning that we'll be using an Eppler 420 airfoil won't help?

Our plane was a little heavier last year and had a 6" chord with a 32" wing span. It weighed little over 1 lb. I suppose I could be wrong...
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Old Sep 28, 2011, 09:20 PM
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Quote:
Originally Posted by SgtPappy View Post
So even mentioning that we'll be using an Eppler 420 airfoil won't help?

Our plane was a little heavier last year and had a 6" chord with a 32" wing span. It weighed little over 1 lb. I suppose I could be wrong...
Hi Pappy,

No matter what kind of airfoil you use, 42 ounces per square foot of wing loading is far, far away from the type of wing loading that will be conducive to what you're after.
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