If a given RC airplane (say 1/5) scale flies relatively well in a 20mph wind , can one just multiply this wind velocity by 5 and conclude that a full scale airplane will fly about the same in a 100 mph wind?

In wind tunnel testing, by what factor is the wind speed reduced by?

The short answer to your question is No. There is no direct correlation between the way a model handles in the wind and a full scale aircraft of similar design.

To understand, it is important to realize that an airplane is not affected at all by a steady wind of any velocity. It only responds to sudden changes in wind direction/velocity as may be due to turbulence, windshear, etc. The main problem with wind occurs when landing and taking off with a significant crosswind. There the problem becomes one of maneuverability as the pilot flies relative to the runway. The maximum crosswind is frequently limited more by pilot skill than airplane characteristics. The maximim crosswind component for full scale aircraft is usually in the range of 1/4 to 1/6 of the approach speed, and is determined by flight test. That may be a little much for models since the model pilot is not in as good a position to judge drift as a pilot riding inside the aircraft.

Some of the same factors that make an airplane more maneuverable make crosswind landings easier. These are less stability (more rearward cg, less dihedral, etc.). Also a higher wing loading and the resulting faster approach speed helps. These same factors make an airplane less disturbed by turbulence.

In wind tunnel testing a sigificant factor called "Reynolds number" is used to relate test data to actual aircraft. Reynolds number is basically a ratio of inertia to viscous forces. Generally data taken at a known Reynolds number can be extrapulated mathematically to another. The process is only approximate thought and can lead to error if carried too far.

I cannot resist stepping up to this business about stable flight in steady wind. Where is this steady wind about which I have read so much? Does it ever occur in RC model airspace?

Yesterday, against my better judgment, I finally put on a flying demonstration for my niece & nephew and a couple other family members who have been asking for a couple years when were they going to see me fly. It was a windy day. Trees were bouncing all over the place. But having driven a few hours to visit them, I said we'd go to the field and decide then whether it was too windy. Got to the field, checked the windsock, about 12' up from ground level, it was streaming about 45 degrees out, didn't look all that bad, and of course it was straight across the runway.

So I took off my pretty little Venus. As soon as I reached about 20' altitude, it was like flying inside a washing machine. Hairiest flight I've ever made or seen. I really thought somebody must have de-trimmed everything on my plane. Half the time I had zero recognition of change in flight corresponding to change in controls. The only stable attitude seemed to be knife-edge, blowing away from me (far away, fast!), just like a boat overpowered by strong wind and heavy seas.

I was really proud of myself for saving a low-level inverted nosedive that came out of somewhere upwind. The peanut gallery was cheering me on, thinking all this was part of my demonstration. Three times I brought it around and tried to crash it not too hard. The third time I got it a few feet off the ground, upright, headed down the cross-wind runway, and somehow put it on the ground in one piece. I looked the plane over carefully, expecting to see all the surfaces out of trim. But they were all nice and neutral.

The only stable parts of this flight were within a few feet of the ground. Above let's say ten feet there was no stability whatever. I like to fly with soft and smooth inputs. Practically every move I made on this flight was full stick one way or full the other, and even then it wouldn't move the way I wanted, or if it did it would snap over and go the wrong way in the other direction.

When I came back to the fence, I got raving compliments about the great show, but how come the flight didn't last longer? In the outward calmness of my desperate concentration on flying, as far as they could see or imagine, I was in total control.

I suppose there is a valid theoretical notion about steady wind of any velocity supporting stable flight in any attitude. But the practical value of this concept for flying real models in real wind, in my experience, is close to zilch. If you want to land safely, I will say less than zilch. I hope the theoreticians will pardon my getting this off my chest.

I share your frustrationwith the gusty day conditions. I have had such days flying full scale aircraft where a mishap could be even more costly than crashing a model airplane. My comments are not so much theory as actual experience holding a stick or wheel in my sweaty hands. I think you misunderstand the term "stability". It doesn't mean tranquil calmness in the midst of turmoil. It merely means the "tendency" to restore original conditions after an upset. As far as aircraft go I stand by my remarks. An airplane with less longitudinal stability will not be affected as much by sudden gusts of wind as a more stable one, and with less lateral stability (dihedral effect) won't be affected as much either. High wing loading with a faster approach speed helps also. The more maneuverable aircraft responds to control inputs correcting upsets faster and with less trouble than a less maneuverable one.

Full scale or model, there are obviously some days that you shouldn't fly. On days with not quite as violent "turbulence" as you experienced, it is possible to fly and have fun. None of this alters the fact that an airplane is not affected by a steady wind, and yes that condition sometimes exist when flying model airplanes.

Thanks for telling us about your experience and I'm glad you got it down with no damage.

And now, my apologies to Paul Harvey, the rest of the story: after barely escaping with my plane intact yesterday, this afternoon I take it out to the field, where the conditions are definitely breezy, but nothing like yesterday. Off I go, and in no time at all I am having visions of yesterday. As soon as one wheel is off the ground, it wants to roll into the wind. I am not having as bad a day as yesterday, but again I have the strong impression that this plane is way out of trim. But I checked the trim yesterday (by looking at it) and again today before coming to the field, and everything looked OK, and planes just don't go out of trim by themselves.

So I get the plane down safely again, bring it to the pits and have a closer look. This time I see the ailerons are off significantly. I hit the trim button and run all the way to the end, and I am still not back at neutral. I pull the wheels off the aileron servos, rotate them a notch, and now I am close to neutral setting at neutral trim button. Put it up, trim it out and now I'm flying in the manner to which I am accustomed.

My theory is, during yesterday's drive to the field, nephew is curious about my radio, picks it up, starts fiddling with switches. I ask him not to do that please, but see that the power switch is off, so not to worry. (These are electronic trim buttons, not mechanically driven pots.) I have to conclude that in a moment when I was not paying attention, nephew somehow moved my trim settings. So now I have to eat half the words of my previous post. As Emily Latella used to say, Never mind.

Winds that affect models are usually a significant percentage..(. over 50 to over 100 %) of the normal flying speed.
Full-scales seldom have to encounter winds of this relative magnitude, unless they're trying to land in a hurricane.
Example.... you can't taxi a model in a 20 knot crosswind, but you can taxi a C-172 in such.
It's all relative, since the air can't be scaled, the model is at a distinct disadvantage when a wind is blowing.
That's when taking off and landing across the runway works. Or a "harrier" style hover-up and down..
Takes practice.
When it's too windy at the field on the flat, go to the slope! You will become a better pilot!
This is my CR Renegade 60" sloper in 45 mph winds... And it penetrates!

[8D] "In wind tunnel testing, by what factor is the wind speed reduced by?"

Back in 1957 to 1959 I was personally involved in the design of the nozzle control for the wind tunnel at the Arnold Research center in Tullahoma, TN. (the nozzle control was designed at Rochester, NY by General Dynamics Electronics). The nozzle itself was 120 feet long, and built by the Electric Boat Division of G.D. The steel walls of the nozzle were warped by the nozzle control system to produce various venturi configurations as needed. The input air to the nozzle was supplied by HUGE electric motors (GE) fitted with propellors. The venturi effect in the nozzle accelerated the airflow to as much as 600mph. An interesting side effect of the acceleration of the air is that the temperature of the air drops dramatically as it passes through the venturi tube. This is the old rule of conservation of energy. The increase in velocity is paid for with a drop in temperature. This is my kind of long way to say that the wind speed is is not neccessarily reduced in wind tunnel testing.
Puff

i think the best thing you can do if you really want to fly on a windy day is move the CG forward. this will allow you fly with a lot more authority. i had a World Models Zen 50 pattern plane with a tower hobbies 75 that would easily slice through heavy winds that ground just about everyone else. it doesn't solve all the problems but does make for much more pleasureable flying in less than pleasant winds.

"In wind tunnel testing, by what factor is the wind speed reduced by?"
There may be times in tunnel testing a scale model that one might change the airspeed to compensate for Reynolds numbers. However in that case you would INCREASE the airspeed for the small model, in proportion to its scale. For instance, when I was involved in automotive tests of 1/4 scale models, we would run the tunnel at about 4 times the speed (i.e. 50mph full scale > 200mph for the one quarter scale model).

Allan

Speaking of AEDC's 16T and 16S, I have spent many hours testing down there. Great wind tunnels, not much to do that was interesting around there for fun. And they always ran all night so we crashed at the motel during the day.

We would select a Mach number which was indeed the same Mach number as the full scale airplane and and let the tunnel guys work out what was needed. We always wanted the highest Re that they were willing to run the tunnel to. They shook the earth when running.