I am going to be building a hovercraft full scale ( and it will be a wig craft.. it flies in a sense ) but want to build a scale model first so I can use it as a test bed instead of my body. I have never tried to fly a scale craft so my question prob will seem pretty dumb BUT I am reducing the scale 1/4 so over all length will be be 4 feet and will use small weed eater engines for lift and thrust.. the question is, besides the size scale, is the weight reduced to the same scale. I am looking at a wing surface area of 96 square feet on the full size and reduction to scale reduces it way down to 6 square feet. The full size will be 650 lbs in flight.. so what scale weight do I shoot for? Some how 162 pounds scale with a 6 sq foot wing does not seem right.. Is my whole concept of scale reduction messed up? My whole idea is not to make a model fly, but make a model fly that will show me what full size will do. Any help would be great ! Thanks
Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:
If you go to Allen's page
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about
1/4 of the way down are several articles on design and about 3/4 of the way down are several articles on hovercraft. These may be of some help to you.
No. Area and volume (therefore weight) do not reduce on the same scale.
Let's imagine a simple case: you're going to reduce a 40' x 8' wing to 1/4 scale. The surface area of the original is 320 square feet. Your model wing will be 10' x 2' with 20 square feet of surface--16 times less than the full-scale version!
Now let's do a really simple calculation of the volume of the wing. Imagine that the original wing is 8" deep. That will give a volume of about 214 cubic feet. The model wing will be 2" deep and will have a volume of about 3.3 cubic feet--64 times smaller than the full scale!
So here's the formula: area will diminish by the square of the reduction factor and volume will diminish by the cube of the reduction factor. That just gives a very rough idea of what will happen because the materials necessary to make a sufficiently strong airframe at small scale are so much lighter than the materials needed for full scale. If all of the materials stayed the same, I guess the weight would come out at 1/64th, but it is very rare that a model needs such strong materials.
This skews the power-to-weight ratio that the model possesses. Folks can make pizza boxes fly because our little engines make so much horsepower.
There is a whole branch of science devoted to how to predict from the behavior of models what will happen with full-scale surfaces. The problem is that air (or more generally, fluid) molecules do not scale down along with the surfaces in question. Scientists cope with this variable by the use of Reynolds numbers. I've read a lot about them and all I know is that the relationship between the 1/4 model and the 1:1 surface is not linear. Nor do I know what difference a difference in Reynolds numbers makes.
That doesn't mean that you can't learn something by the use of your model. If your fan shroud and control systems work on the model, they will probably work on the full-scale vehicle. If you've got problems controlling the model, you will probably have problems controlling the full-scale machine.
You may be able to answer questions about the value of the wing design by the application of mathematical formulas for lift and drag at the airspeeds you expect. I don't know how to work the formulas, but there are people who do. My bet is that the mathematical formulas will be more valuable for you than the scale model--they can answer questions that just watching a model perform would not.
Well, that is a start and help to know I am not nuts about the math.. I had done the math on the full scale and numbers say it will work but just wanted a test first.. The design is not from scratch as the hovercraft model I will be using is a modified uh 13 pt from universal The part that was tricky is the design of the wing for max lift made from fabric. I will come back and post a how it went. :) Yep, I still will build the 1/4 scale as it will make sure I save on an expensive boo boo when something is not right.
It sure is not. A Cessna 172 full-scale weighs 2300 lbs at gross, and with a 150 HP engine, we have 15.3 lb/hp. A model 172 with a .40 might weigh seven pounds, and that .40 turns out around 3/4 hp, for a ratio of about 9.3 lb/hp. And that's a relatively underpowered model. I wish I had the sort of performance we get from models when I'm flying my full-scale Jodel. I'd have to have about 400 hp instead of the 65 that's in it.
Full scale thrust : weight (bhp is confusing...) you'd find appx 1.0
3.0 appx for a Cessna 172 if you tied a huge fish scale to the back an had the pilot run it up to take-off power.
We consider a plane unable to ROG with les than 1.0 : 2.0 (or 0.5 Thrust : weight
BHP is a measure of power output at the crankshaft... a lot of which i LOST at the prop tips. That and scale effects of the prop disc size make the BHP ratio of the model to the full scale almost meaningless.
Go by static thrust.
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The hovercraft... if set up correctly you can lift a man using a .61 c engine... All you have to do is make the hovercraft skirt "seal" to th ground well enough, and closely shroud the prop of the engine... the not have air leaks. You'll get off to the point where the skirt start leaking air.
Its actually very easy to get a hovercraft to "float" on its ai cushion... its maintaining the air cushion at forward speed (an driving the thing forward at speed...) that makes you need more power
Model IC planes fly with similar power to weight to WWII warbirds
Electric parkflyers fly at similar power to weight to todays lightplanes.
Yoiir examp,le above proves that its ONLY a factor of 2:1 for going from full size to say 1/6 scale. Not anything like teh way teh size scales or the weight scales at all.
15 lb per horsepower is almost exactly 50W/lb which is the guideline for an average electric flight setup.
The fact that IC models are overweight and overpowered for 'scale' performance is neither here nor there.
If you want a model that flies like your Jodel flies, get a low powered electric parkflyer :-)
| Full scale thrust : weight (bhp is confusing...) you'd find appx 1.0 : | 3.0 appx for a Cessna 172 if you tied a huge fish scale to the back and | had the pilot run it up to take-off power. | | We consider a plane unable to ROG with les than 1.0 : 2.0 (or 0.5) | Thrust : weight
Are you talking about models or full scale planes?
Since you say `full scale thrust' and `huge fish scale', I suspect you're talking about full scale planes, though what you're saying isn't remotely true for either one.
A full scale powered glider may have a weight/static thrust ratio of around 30:1, yet they can still rise off the ground.
I can't seem to find any figures for the Cessna 172, but I believe for a J-3 Cub the weight/static thrust ratio is more like 7:1.
A full scale plane with a 2:1 weight/static thrust ratio would a very good performing (and expensive!) airplane indeed.
As for models, yes, they certainly do usually have lower weight/static thrust ratios than full scale planes (due to the Reynolds numbers differences, being hard to be precise about the angle of attack in a model, and just that an underpowered plane is not much fun to fly,) though I don't doubt that you'll find ones that can easily rise off the ground with a ratio of 10:1.
10:1 may be a bit extreme although I am sure there may be an example somewhere. Most good .46 sport engines can generate 60 oz of thrust with some even as good as 80 oz. It might be hard to imagine getting the 800 oz plane off the ground!
| 10:1 may be a bit extreme although I am sure there may be an example | somewhere. Most good .46 sport engines can generate 60 oz of thrust | with some even as good as 80 oz.
Really? I've got a Tower Hobbies 46 ABC and a Thunder Tiger 46 Pro and I'd say the most I get out of them is perhaps 40 oz of thrust. Maybe I don't have the best props for static thrust (ok, almost certainly) but 80 oz of thrust sounds crazy without a huge prop and a gear box or something.
| It might be hard to imagine getting the 800 oz plane off the ground!
I would think that this would be a good example --
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but I'm not aware of the static thrust figures on any of these planes. The winner in that case lifted 7.5 kg, or 16.5 lbs.
An Irving 40 isn't very powerful. And the limit on the wingspan is rather arbitrary -- they could do better with a larger wingspan. They might just hit the 10:1 ratio.
| > | Full scale thrust : weight (bhp is confusing...) you'd find appx 1.0 : | > | 3.0 appx for a Cessna 172 if you tied a huge fish scale to the back and | > | had the pilot run it up to take-off power. | > | | > | We consider a plane unable to ROG with les than 1.0 : 2.0 (or 0.5) | > | Thrust : weight
In any event, the idea of a plane being `unable to ROG with less than a 1:2 static thrust/weight ratio' is crazy, model or full scale.
Most good .46 engines can take a 4 lb plane straight up. That would be better than 64oz of thrust.
Even at a lowly 40 oz thrust, 10:1 equates to a 400 oz plane. That's a bit over 20 pounds! I think this highlights some of the differences that scaling up or down does. ;^)
I remember reading about a design contest that was won by a plane that took off, circled the field, and landed in a prescribed area carrying a payload of 19 pounds of ballast.
The power plant was an OS FP .40.
It can be done. It may not be pretty, but it can be done.
Think about Maynard Hill's plane, too--11 pounds flown by an OS .61 four-stroke turning a prop at around 3500 RPM. I think the true airspeed was 40 knots or so. I'll bet it didn't have a very good weight-to-static thrust ratio!
That said, these are definitely extreme cases. I much prefer having too much power available than too little.
No, it highlights the extraordinary silly power to weight ratios that some glo flyers employ.
It is possible to fly a 60" span plane of about 3lb weight on the electric equivalent of an 049. If an 049 were geared to swing an 11" prop.
Most glo flyers would consider this is 40-60 sized territory. TEN TIMES greater power than is actually NECESSARY to fly like a lightplane does.
Most glo planes seem to have the sorts of power to weight ratios, and performances, of WWII warplanes. Or even more.
The original spitfire was about 6000lb and developed 1000bhp roughly.
6lb per bhp.
That's like putting a tuned 40 in a 6lb aeroplane. Nothing unusual for glo flyers. In fact many would go larger.
with a bhp at 750 watts, its a shade over 125watts per lb at the prop - something I can also achieve with a £5 motor and lithium batteries, and yes, the performance is in the spitfire class. Over 2000feet per minute (spitfire could do nearly 4000 fpm)
Although the top speed of the spitfire is a lot higher, and its climb ANGLE therefore lower.
Its not that the scale factor changes the *required* power to weight, its that modellers build uncessesarily heavy overpowered planes.
Because its easier. They fly in totally unscalelike manners as a result, but unless you fly vintage or scale, or parkflyers, no one seems to care.
And since youi can't scale the air or the wind, they have better wind handling ability than more true scale weight and speed planes would.
I means, take a 747 at 200 foot wingspan that cruises at what- 450mph? and lands at 130mph.
These things are powered sailplanes. To get a good 'sit' in the air, of say a 5ft span model it would need to land at 3.5mph and fly at around
11mph.
Even if you go for time dilation scaling and knock the scale factor on speed from linear to root scale, its still a speed scale factor of about
6:1 which means this thing then lands at 20mph or so and flies at 55mph or so. That gets the angle of the banks correct etc, but totally spoils the illusion of a large plane far away. Its simply looks far too fast.
However whatever you do, the power to weight of the model will not be far off the power to weight of the full size.
A 747-200 has a max cruise of 602 mph. It weighs 802,000 lb at gross, and its engines put out 50,000 lb thrust apiece for a total of
200,000 lb. It's thrust-to-weight ratio is just a hair better than a Cessna 172's.
My son's .40-powered, 7-lb model has a power-to-weight ratio of about 9 to one, and will outclimb a 172 in real numbers, going to 500 feet in maybe 20 seconds. And that's a tame airplane in the modelling world. It's not just climb angle or scale effect; IT'S REAL NUMBERS. You still don't get it?
| A 747-200 has a max cruise of 602 mph. It weighs 802,000 lb at | gross, and its engines put out 50,000 lb thrust apiece for a total of | 200,000 lb. It's thrust-to-weight ratio is just a hair better than a | Cessna 172's.
weight:static thrust ratio of 4:1? Wow, a lot lower than I'd expected.
| My son's .40-powered, 7-lb model has a power-to-weight ratio of | about 9 to one, and will outclimb a 172 in real numbers
Power to weight? The units don't work. Did you mean static thrust to weight ratio? (9 is way too high for that.) Horsepower per lb? Wing loading in lb/sq ft?
Where did I ever say that graceful flying and finesses don't enter into it?
I am saying that if you pick up any mainstream magazine or modeling catalog, or if you go to most flying sites not devoted to only on type of flying, the .46-class sport plane will be the dominant model.
Paul to be fair did not say that: He said that was not HIS prime motivation.
I have found to an extent, it is part of mine. Which is why scale and vintage style planes are more prominnet in my hangar.
But I suspect the reason 46 type planes are popular is simply because that is atually the cheapest way to go flying with an IC engine, just as speed 400 is the cheapest way with an electric.
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