Thread: Discussion PSS airfoils
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Old May 21, 2008, 03:49 PM
MCarlton is offline
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Cromer,Norfolk, UK
Joined Nov 2006
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I'll bite,

First, it depends what performance you need, or expect. Do you want a model which will fly in a fairly realistic manner, or something that will fly like a slope racer, whatever it looks like?

I think some of the thin sections currently used for PSS look a bit daft on a model that is meant to be scale, so I would go with an old school choice, such as E205 or E374 at a loading of about 14oz dry, with ballast up to around 24oz.

If that doesn't appeal, then MH32 is worth a look.

Some rules of thumb:

Inland slopes which are not brick lifters;

I would suggest a wing loading of around 12 - 16oz without ballast, using a moderately cambered section like E205 or 374.

E374 loves ballast and will rip along nicely in good lift when up at around 18 -22oz loading, as will E205, especially if you mod the section to take out the undercamber.

Fuselage construction can be pretty much whatever will do the job, but a good one if your LZ is tame is either lost foam or a carved foam fuselage with balsa sides.

Another method is to make a basic balsa box for RC gear and wing/tail mounting, then clad that with carved blue foam veneered in balsa or 1/64 ply.

http://www.pjmodelclassics.be/

Wing construction can be real basic foam veneer, or balsa ribs and sheeting (again, ply sheeting will give a nice tough finish without mucking about with eposy and FG)

How wing loading affects flying;

Basically, as wing loading increases, so does airspeed, but allied to that, so does stall speed, and by the same amount. So, if you have two identical models, one with a 10 oz loading and one with a 20 oz loading, the 20 oz loading model will punch through the wind better, stall faster, turn wider and land heavier

Wing loading does not equate to good windy weather performance, nor does it ensure a high top end speed.

Example, a Mini Ellipse and a basic slope trainer will fly at the same wing loading, the ME will probably even be lighter. But, the ME will always penetrate better and fly faster because of the wing section and overall drag of the airframe. Adding more ballast to the basic trainer will do nothing except increase the stall speed, overload the structure, kill the manoeverability and make it land with a bigger thunk.

Remember, adding weight adds stress, think of a 10G turn. With a 2Lb model, that model "weighs" 20Lbs during that turn, ie, that is the force (more or less) exerted on the wing and its mountings. Add 2 Lb of lead, and now those same structures are expected to handle 40Lbs. They might not.

Quote:
X-oz of wing loading using Y-airfoil you'll need Z-mph of wind for the ship to fly and here's how it will handle
That won't work, because the rest of the model, ie aspect ratio, fuselage design, incidence, tail area, CG, tail moment etc will all impact on the model.

Wing loading is also proportional to model size. A 4m scale ship might be a floater at a wing loading of 18oz, but a 40" kipper would be overweight and awful. The overall efficiency of a model increases with size.

Also, remember that some sections (such as RG15 and RG14) actually need a higher wing loading than other sections (such as SD7032 or E193) in order to fly at their most efficient speed, load a model too lightly with a fast section and it may not fly fast enough to take advantage of the low drag/high speed polars that such sections offer.

Quote:
I hope that doesn't mean that I can't get into PSS given the lift conditions I have to work with
You can PSS off any slope, you just have to design your models to do what you want them to. Build a nice PSS P51D for example, with a foam wing, balsa fuselage, about 45" span, maybe a loading of 12oz and an E205 section and you'll have a model which will fly lovely off anything like a decent slope.

Go further, how about a PSS U2? Maybe 72" span, give it a SD7032 section, wing loading of about 8 or 9oz, you'll have a PSS model that'll stay up in a whiff.

BUT.

You would be surprised at how well energy retention will keep up a model with a high loading, if you can keep it moving. Here's a hint, why not put a small hook at the nose of the model, hook it onto a bit of bungee tubing (about 10m will do), stretch it back and catapult the model over the slope. Use the high speed to zoom climb, then let the model dive to a decent speed and pull out. Keep your turns fairly wide, use height and turn it into speed and even a fairly heavily loaded model will stay up better than you might expect.

BUT (again)

A low wing loading does not mean a slow model, don't get too hung up on that idea. Yes, lighter models do not retain energy as well, and lose it quicker in a climb, but if you fly within the energy envelope you have, then you won't have a problem.

Far better to have a model which might not fly like a lead sled off Fermin, but will fly well in your local conditions, than to try to imitate that kind of model and fail because of your slope characteristics.

Basic Rules of Thumb:

Big Slope, Big Lift - Thinnish section, low camber, loading around 18oz+

Moderate Slope, OK Lift - Thicker section (maybe 9%), higher camber (maybe 1.5%), loading around 12 - 16oz

Little Slope, Little Lift - Same as above, but loading around 9-14oz

BUT

Some slopes generate only moderate amount of lift, even in high winds. If this is your case, use a thinner section, but keep the loading the same, that'll allow you to penetrate a bit better but will keep the model up in the moderate lift.

I think PSS "Sleds" are a US phenomenon, PSS does not have to be like that, we tend to take a more "scale" approach over here in the UK and thus a range of models become open to us, for any variety of conditions.

My (lengthy and probably ill informed) 2 penneth

Matt
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