I was talking to a guy at the field the other day about a second plane and he suggested a big stik 60. He said larger planes handle wind better. I don't understand why this is so. I've heard other people say this too. I know they are heavier but they also have a greater surface area. It looks to me like a 1/8th scale or any fraction model of a plane would behave the same as the full scale model in the same wind. What am I missing here?

Renyolds number. (sp)

Ok that's what you are "missing" Basically, you scale a plane down, but the air molecules remain the same size.

That aside, in a perfectly steady state wind, all planes handle it just fine as they don't even know the wind is there. So if you are in a flat area and high enough off the ground, you can fly anything in any wind. You might not be able to fly it back to the field, it might be going backwards out of sight at 30mph, but all the plane knows about is airspeed, it doesn't know about the ground.

However, in the real world we have turbulance. Edies, up and down drafts, etc. Some of these are smaller than others. If you have a small plane, it is more bothered by smaller bits of turbulance than a larger plane would be. For example, a Cessna 152 has to worry more about the wake turbulance of a 747 than another 747 would. The size of the turbulance compared to the size of the plane does matter, so with models and the small bits of turbulance we run in to when low and slow, a bigger plane is less bothered.

Imagine a mosquito and a hawk flying in the same wind. Make sense?

Don't forget wing loading.....maybe the statement should be 'heavy' planes fly better. Or 'heavy planes handle wind better'.

Light goes up better, but heavy penetrates a breeze more easily.....

You know, I'm not at all sure that is true. I've never heard a solid, aerodyanmics-based explaination of why a heavier plane would "handle" wind better.

We all know it's usually easier for us to fly a heavier plane in the wind, but I think that is because of where we are standing, outside the plane, than anything else.

A heavier wingloading will result in a faster glide, but the actual glide angle will be the same, since the glide angle is the L/D ratio, which doesn't change directly with weight. (It does change a little, since at a higher weight, you have to operate at a higher AOA, which means a different L/D for most airfoils, but that effect is generally minimal).

Anyway, a lighter plane and a heavier plane will glide though the airmass exactly the same, following the same path. However, a ligher plane will do so at a lower airspeed. So, if the wind is high compared to the airspeed, a lighter plane will fly very slowly against the wind, and will show a larger apparent change in speed when it turns from downwind to upwind or vice versa.

So, a heavier plane will be faster flying, which will make a given wind speed a smaller percentage of the ground speed, which will make the airplane appear to fly at a more constant speed (airspeed remaining constant, but ground speed varying due to wind), and the higher speed will make covering ground against the wind easier.

However, all the above is mostly talking about planes with out engines, where the airspeed is set totally by the rate of descent and the L/D. Put an engine in there, and the situation changes.

Your airspeed is no longer controled mostly by weight, a lightly loaded plane can fly at the same airspeed (or usually faster) than a heavily loaded plane. So wind penetration with an engine is less of an issue. A lighter powered plane can throttle up to get better wind penetration and match the heavier plane.

On landing, and intersting thing happens. A pilot flying a lighter plane is going to be used to slowing the plane way down, and using that light wingloading for a nice slow glideslope. However, a pilot of a heavier plane is going to be used to a faster descent. In effect, a landing approach turns a powered plane in to a glider as above. So the pilot of the lighter plane will, out of habit, slow down farther, causing the ground speed to drop way down, and making the landing seem "harder". The pilot of the heavier plane is not going to see the plane slow down nearly as much (as a percentage), so the plane will look and feel closer to "normal" in a high wind.

If the pilot of the lightly loaded plane simply holds more power all the way to the ground, and uses the engine to establish an approach with a higher airspeed and a "normal" ground speed, they will find the lightly loaded plane lands just fine in the wind.

The above is all assuming a steady wind.

In turbulance, lightly loaded planes seem to bounce around more, but part of that is because of the lower airspeed. If you power up and fly the same airspeed as a heavier plane, the bouncing reduces quite a bit. I haven't flown side-by-side comparisons to see if the bouncing around becomes equal at equal airspeeds or not, but again, I can't see any aerodynamic reason why a lighter loaded plane would bounce more.

I can see one reason, and that would be inertia. I'd be willing to believe that a heavier plane (even with a lighter wingloading) would bounce less in the same turbulance, since the turbulant air acts as a force trying to accelerate the airplane in a direction. A heavier plane will take more force than a lighter one. But that's mass, not wingloading that matters.

Anyone see where I got something wrong in this? It does match the experiences I do have flying lightly loaded and small planes in high winds compared to flying larger, heavier planes in the same wind.

Also consider: a 3 foot (1/8 scale) airplane facing into a 20 mph (29 ft per sec) wind is experiencing a scale headwind of what a full sized airplane would with a 160 mph wind (232 ft per sec).

I know this doesn't work because most models are flying at far-greater-than-scale speeds and, as Montague pointed out, the relative air is denser for the model. A full sized plane also weighs considerably more than 8x the weight of a 1/8 scale model. Like pounds to the nth power instead of pounds times n scale ratio. Just another fly in the ointment of model aerodynamics.

Remember, as Mark Twain stated, there's three kinds of lies: Lies, Damn Lies, and Statistics.

Actually, you can account for the "scale speed" issue though the use of Renylds number (I can't spell it to save my life and I'm too lazy to look it up).

Re is a function of wing chord and airspeed, so a smaller wing flying at a higher speed has a simular Re of a larger wing moving slower (if I remember that right, I might have it backwards)

Weight and wingloading effects, however, are much messier. You can approximate wingloading effects using a volume loading instead of a area loading. You can calculate the volume loading the hard way, using the true volume of the wing, or with a shorthand way that only works as long as you compare simular airfoils at simular Re values, but even the short hand ways are pretty good for most messing around.

But the general statement that scale models usually have lighter wingloadings and higher power loadings than larger counterparts is acurate. Though when you keep it in the model realm, larger models usually have lighter loadings than small ones. So it's not a linear "smaller is lighter loaded" thing at all.

Kirk,
You raise some valid points, but I only know one thing.... Big flys better.
Dennis-

Yeah, I agree, big does fly better. I just wish I knew "why"

Though you'd be surprised at how well some small planes fly. A lot of smaller planes aren't well designed.

I'm thinking of taking my above rambleings over to the aerodynamics forum and see how many holes people find in it.

Big planes fly better...

You are forgetting one very impotant thing. Inertia!
A heavier plane must be hit by a stronger turbulance, or the same turbulance for a longer duration to be affected. The airfoils may not care about air speed, and the size of the air molecules may not matter, but shear mass does...

Put a marble and a bowling ball on the kitchen table and try to blow them both off...
Same aerodynamics, different mass..


Just my semi-educated guess.

Turbulence also causes angular disturbances (roll, pitch, yaw) which are smaller on a large aircraft because the moments of inertia grow faster with size than the forces/moments exerted by the turbulence (moment of inertia grows as length^2 while moment due to gust on tail, for example, grows as length).

I use a basic rule for flying in the wind . Some planes I will leave at home . I quess to sum it up there are several planes in my hanger I dont like to fly in the wind and some it doesnt matter they range from a .40 size trainer to a 1/4 scale . As the marble and bowling ball go which won is easier to stop with an Opposite reaction. I would Quess the marble.There is a whole lot of factors to consider in what plane to fly in the wind. But after all the science mumbo jumbo, and reading the forums that have discussed this topic before, it comes down to this. Everyone has there own "opinion" on what plane is better in the wind. So it seems its all "personal preferance".[quote]ORIGINAL: daver29



Put a marble and a bowling ball on the kitchen table and try to blow them both off...
Same aerodynamics, different mass..

Actually, I did mention intertia up there somewhere

The moment arm point is a good one. I think I did comment (maybe not this thread, I forget) on the relitive size of the plane to the turbulant air, but taking that a step farther to moments of interia and the torque required of the turbulance to rotate an airplane is a good one I hadn't thought of, and hadn't heard before.