Scale and Wind Speed - What's the Relationship?

[djacob7]

For a model to fly in a 10 mph wind is a tough enterprise while a 747 will fly through 20 mph winds easily.
Is there a relationship of scale between aircraft size and equivalent wind speed?
Put differently, what would be the equivalent wind speed for a 747 of a 5 mph wind speed experienced by a model?

[mhuff]

I have a 1/9th scale corsair. If mine goes 45 MPH then it is scale to the real one going 405 MPH(45*9) which is how fast they could fly. But my overpowered plane can go about 60+ MPH, that is like the real one going 540+ MPH which the real plane can't.

I think that wind is more about mass than size. If my 1/9th corsair weighs 7lbs and the real one weighs 14,000 loaded then my plane is 1/2000th the real one's weight.

For my 1/9th to be scale in weight then it would have to be 1555 lbs (14000 divided by 9)

I have no idea how to answer your question :confused:


will someone answer us :D

I just thought of something If you take a plan for a rc plane you are about to build and 2x it then your new plane will not be 2x the weight but 8x the weight (2x the length, 2X the width, and 2x the heigh) so a 1/9 plane would be 1/729th the real one's weight(9l*9w*9h). My 7lbs plane = a 5103 lbs real one.

I still don't know how to answer the :censored: question. lol

[djacob7]

You raise some good points, mhuff.
Maybe the equivalent wind relationship depends on both the volume and the weight of the aircrafts.
Maybe speed also comes into play, especially the stall speed of the different planes.

[JetPlaneFlyer]

The effect of wind is in proportion to wind speed divided by flying speed.

So if you model comes in to land at 20 mph. The a 10 mph sudden wind gust would represent a 50% change in airspeed. Probably if the gust was in the wrong direcrion this would reduce the model's flying speed below stall and you would drop like a stone. If the gust was in the other direction the airspeed would increase to 30mph and the plane would climb steeply... Either way the gust has a very large effect on the model. The same model would be effected less by the same gust if it was flying faster.

Now look at a 747... It's landing approach speed is about 160mph, so a 10mph gust represents only a 6% change in airspeed; the 745 would hardly notice it. To have the same effect on the 747 as a 10mph gust has on our model the wind gust on the 747 would have to be 80mph!

Steve

[MCarlton]

A model will fly in a 10mph in the same way that it does in no wind, the difference is how the flight is perceived on the ground. Turbulence, Wind Shear and Groundspeed are the real issues in flying models in the wind.

A 747 will land at, say 150mph? So into a headwind of 30mph, that reduces to 120Mph groundspeed. The visual difference from the outside between 150 and 120mph groundspeed isn't that great, so it appears that the wind isn't affecting the 747. It is, it affects it just as much as it does any other flying machine.

Lets look at the model. Landing speed of 30mph, headwind of 30mph. Groundspeed at landing 0mph. From the outside, it looks like the model is badly affected. It isn't, if you took the "ground reference" away, the model would appear to be flying just the same as it would in no wind. Because it is.

Turbulence has a similar scale effect, if you look at gust velocity and wind velocity as a percentage of airspeed.

So a 30mph gusting to 50mph as a fraction of the 747's speed range;

Takeoff/Landing speed (guessed) around 150 - 250mph

The 30 mph wind is between 12% and 20% of airspeed in that part of the flight.

The "gusts" increase that to between 20% and 33%.

Do the same to the model;

Take off/Landing speed 30mph - 50mph

The 30mph wind is between 75% and 100% of the models airspeed.

The gusts are between 100% and 125%.

If we put those figures backwards and see how a 747 would behave with winds of a similar % of airspeed;

Essentially, to mimic the groundspeed effect and so on that we see on a model, a 747 would have to be flying in winds of

75% - 100% of airspeed at landing speed (150mph) = 112.5 - 150mph

Gusts at 100% - 125% of landing speed = 150 - 187.5mph

Can you imagine a 747 trying to land in a 150mph wind gusting to 187mph??

JPF - Snap, I was writing this before I saw your reply. Great minds perhaps? Or not, in my case...