Ryan Smith

My Feedback: (25)

Hello all,

Back when I was youn(ger than I am now) the barber shop that my dad and I would go to used to be stocked with RCM issues, presumably from another client. I seem to recall in one of those magazines, an article for proper scaling of airplanes to retain flying characteristics. Granted, it's probably been close to 15 years ago putting me around age ten, but there were several formulas and rules of thumb that provided guidelines for weight, wing loading, power loading, and a host of other information that would allow you to scale an airplane down or up and retain the "same" feel.

Does anyone here remember this, or am I a victim of an overactive imagination? If someone does remember this, could you please point me in the direction of the article, either by means of an electronic copy or a place to get a hard copy? I hope it was RCM that the article was in, and I'm almost positive it was.

If anyone out there has any help at all, your assistance would be GREATLY appreciated.

Best regards,

Ryan Smith

Jet_Plane

This article is good: http://www.astroflight.com/pdfs/ScaleSpeed.pdf

jeaton01

My Feedback: (1)

I agree that is a good resource. One other factor he did not address is mass distribution. In full scale airplanes the major weights are generally located closer to the CG than in models. In other words the moment of inertia is greater relatively in models than in full scale and this makes them less maneuverable and less stable. It's better to build a light model and ballast it at the CG to get the proper scale weight than it is to build heavier wings and tails and have to add weight in the nose to balance the model.

pimmnz

And very difficult to retain the same 'feel'. Typical model trannys with centering springs do not 'feel' like the full size control feedback. Models fly in the same air, so do not 'influence' the same mass of air to do the job, and have much lower inertias than full size, so need much less 'bank angle' to turn at the same rate, etc, etc, etc. The best you can do is to fly the thing with respect to the full size airplanes capabilities, and do it as smoothly as you can, almost as though you were in the cockpit...there was a spate of 'scaling' formulae a while back, but I can't help thinking it was just an excuse for various modellers inability to design/build scale models light enough to fly at scale speeds, but that is only a personal opinion, of course.
Evan, WB #12.

rmh

scale feeling - dream on -
as close as you will get is in a $$$$imulator
a model?
it won't happen

Jet_Plane

Actually if you follow the 'rules' in the article i linked to in post #2 you will end up with a plane that flies in a perfectly scale way in regard to bank angles, turn radius, climb angle, take off distance,etc etc.... However the model will appear to fly faster than the linear scale would dictate because, as the article describes, you would need toallow for'scale time'.. that is time for a scale model runs faster than full scale, by the square root of the scale factor.

Steve

pimmnz

The only thing that does not change, model to full size, is time. Forget 'scale time'. If the full size covers its own length in one second, so will the model. The second is the same second. As for bank angles...if the full size takes 180 seconds to to turn 360 degrees, so will the model, but the physics will still show a much reduced bank angle for the model. You can try it yourself, but a 3 minute 360 takes a long time with a model, and it is a nearly flat turn...If you want to fly 'just like the real thing' then you have to scale the air to whatever scale the model is. And I can't think of anyone who has been able to do that yet...As stated, the 'fudge factors' for 'scale flight' are just that, and the best of them can be used to predict some full size performance data from models (wind tunnel stuff) but ain't no way to model models from the full size data.
Evan, WB #12.

BMatthews

I've seen this discussion come up before.

The outcome of the past threads has been if we scale the model so that it travels the same number of fuselage lengths per unit of time between the small and big ones then the model becomes almost silly light and slow flying. You end up with a P-51 that is flying at speeds which make it look like a J3 Cub. If the model is scaled for weight such that it mimics the behaviour in turns and other maneuvers of the big ones then it's flying too fast to look scale. So the J3 Cub ends up looking like a P-51 for flying speed.

The happy middle ground seems to be make it light enough to fly in between the "scale speed" and the "scale behaviour speed" and just doctor the maneuvers with a bit of control input to make them look "right". Which side of this you run closest to depends on your own tastes and the sort of model you've built. Fast flying full size designs tend to look better in the air when they operate closer to the scale behaviour weight and speed while slow flying full size craft when modeled tend to look better when kept closer to the scale speed weight.

jeaton01

My Feedback: (1)

I've gotten so used to watching models fly that I guess I just accept the speed most model Mustangs fly like as the norm. I wasn't really talking about scale speed in my earlier post, but that the airplane has the same general pitch and roll response, and for instance the stall recovery is like the full scale. Its hard to describe, but I know what a full scale Cub flies like because I have a fair bit of time in them, and when I set up my 1/6 scale Cub as I described it seems very similar to me, much more satisfying to fly for me than one with heavy airframe extremities. If the wind comes up it needs to be ballasted, but then when the gust spread was above 15 knots I didn't much care to go fly my "real" Cub.

I used to have a Hangar 9 1.20 size Mustang, and it bothered me because when I pointed the nose down it didn't feel as "slippery" as a clean airplane like a Mustang should feel. Too much drag in that fat wing, I guess. It was a great flying model, it just wasn't a great Mustang model in my hands. What other people want in a model is up to them, I guess, and that's fine with me.

pimmnz

Like BMatthews says, and build the model as big as you can, then the 'scale factor' starts to work for you. Just fly it like you are in it, and you don't want to lose your breakfast. Scale speed (and therefore performance) will always be a contencious issue, consider, for instance, that the whole performance envelope of a full size WW1 fighter will be done at an airspeed less than the landing speed of most WW2 fighters and you can begin to see the difficulties for models, and what effect any wind over the strip could have...
Evan.

Jet_Plane

Scale time is a valid concept.. If you andI were scaled down to the same scale as our planes our hearts would beat faster, our whole bodies would 'run faster',our watches would also run faster. To us then the scale plane would now look like it was flying at normal full scale speed, even though to the rest of the 'big world' it would seem to be flying too fast for scale.
if you went further and scaled down our solar system the planets would spin faster and orbit the sun faster, days would be shorter... Time would indeed 'run faster'

Of course when we scale out models we dont scale ourselves or the rest of the world, which is why a true scale model appears to fly too fast to us 'big people'


Steve

pimmnz

As a 'concept' it may be valid, but even an ant on this planet has the same amount of daylight...for him a second is still a second. Your idea is valid if the earth was smaller, but then gravity would be less, and the 'scaled air' would be comparitvely denser, and the 'scale speed' wouldn't be a problem, as the model would be lighter, and have more air to fly in. The problem with a 'faster second' is that it is still subject to the physics of this planet, and judged against everything else around it. As a 'fudge factor' to compare similar sized models against each other, it works, but not when judging full size against models, which is what we are trying to do. This argument is circular, no one can win. Perhaps the best 'simulation' is to imagine your model superimposed over the fullsize, both flying at a distance that makes them APPEAR to be the same size. They then pull into a 360 deg circle, and complete it at the same time. The angles of bank, 'g' loadings etc will be much less for the model, and the airspeeds will be scalar, that is, the speed of the model will be the same as the scale of the model. The problem is, as BMatthews has stated, the smaller the model, in relation to the full size, the lighter it has to be to achieve this, and the less 'real' it will appear because of the low inertia. The best you can do is to fly it like the big thing, within the limits of the models and the full size performance. Heck, you could even run a competition to see who can do this the best...or have they already done this?
Evan, WB #12.

rmh

No simulator?
make a program on a computer using the plane you ike - THEN you can do it all.
Some ready to go programs are very good.

Jet_Plane

In all practical terms i absolutely agree with all you said. We are stuck with the practical everyday physics we have. Sadlywe can never make a scale plane that performs in a trully scale manor in all aspects so we just make compromises and achieve the best we can..

However as a point of academic interest if nothing else, on theissue that 'a second is still a second'.. This would seem like you are saying time is a universal constant, the same for all. Einsteinshowed this not to be the case with his theory of special relativity.. Time just like distance, size and mass is relative to the observer and could be measured differently by different observers.

js3

My Feedback: (3)

Yes, but unless you're traveling at close to the speed of light, or located fairly near a black hole or other super massive object, the difference is negligible and unnoticeable.

BMatthews

Time may indeed be variable. But our models don't fly at near the speed of light...

The idea of scaling the environment is valid but the environmental aspect that needs scaling isn't time but the medium that the model is flying through. If we scaled the density of the medium that we should be able to maintain scale behaviour AND flying speed all at the same time. So all we need is a really big blimp hanger that's pressure rated for some multiple number of atmospheres of pressure. Or if the model was sized suitably to do this in water such that the density of water was at the same scale factor as air is for the full size we could fly the small model in a swimming pool.

Evan, there's already such compeititions. In scale comp's the judges award points for various maneuvers that are announced by the pilot before taking off. These maneuvers are graded firstly on successful completion and then the grading for points becomes higher for flying the model in a way that most closely appears to make the model look like the fullsize craft. Good pilots will learn what the maximum roll rate was on the full size and try to limit the model to rolling that fast. Same with the size of loops and turns.

pimmnz

I think I already got that...
Evan, WB #12.

Jet_Plane

No, that wouldn't work in all respects either.. For example take off distance.The 'scale rules' give you a model that takes off in true scale roll distance. Now you put that same model in your higher density air and it will take off in a much shorter distance; much less rolling distance than 'scale' for the plane in question. The plane will also climb at a much steeper angle than scale and turn loop and generally perform all manoeuvres much tighter than scale.

Nope, the only way to make a plane perform 'truly to scale' is if we could scale time.. but we cant, so the real world is a compromise.

Steve

Mlotek

Your model will not fly as a full scale aircraft would, as we are all flying in the same air (more or less, density, temperature changes, etc., but lets assume sea level).

Because of your scale length (lets say chord length) and difference of operating speeds, your Reynolds numbers would be different. You could maybe mimic some of the effects with reynolds number matching (probably by playing with the viscosity term) where RE = (Density*Vel*Length)/(kinematic viscosity).

With different Reynold's numbers, you have different effects - at our low RE's you would see different phenomena when compared to high RE 'real' aircraft.

Maybe fly your airplane in honey or vegetable oil?

Cheers

Iron Dog

I just happened to discover this earlier tonight, and here you are asking for it. Here you go - the article can be accessed by the link, below.

http://www.rcmplans.com/issues/reque...061972-1-1.pdf


If that wasn't the exact article you were looking for, they had some others; or, if you want to investigate some of the other old RCM articles still available, access this direct link to the main directory:

http://www.rcmplans.com/index.php?main_page=page&id=18


(From what I understand, their plans service is still in operation, too.)

jeaton01

My Feedback: (1)

Iron Dog, the article you linked to is to scale airfoil thickness. If you do have a link to a scale effect related article that would be great, and thanks for the link to the RCM page.
John

Sport_Pilot

Did anyone tell Einstien? Would this mean that a scale rocket ship can go faster than the speed of light?

rmh

Scale effects are impossible to ignore
for example :
I have been trying to scale down 10 ounce models to one ounce - and the results suck-
You have to design for size. No exceptions

jeaton01

My Feedback: (1)

At that size it's all about Reynold's number. But until the wing chord is over 10 inches it always is, in my experience.

da Rock

There really are two major players when the model size moves into the smaller region.

The construction materials used really can't get any lighter.

The Reynolds affect actually starts to make itself so obvious even the average modeler sees the affect, even if he doesn't realize what it's from.