Aerodynamics

["Corpse"]

Corpse,

Sorry for not making myself clearer. I should have said, “...and equipped with a pitot/static system to measure airspeed.”

Your're cool! Just clarifying lol.

 

["Corpse"]

I think you're both correct, just getting tangled up in the terminology.

If I fly 15kts into a headwind of 15kts my plane is stationary. The plane could care less about the ground speed as far as the aerodynamic forces, but it does care about F = M × V. When it turns to come downwind the mass of the plane has to go from stationary to 30kts. The plane in the air reacting to the wind is not attached to the ground but it is still subject to mass and acceleration forces.

Of course, it's not exactly that simple, but it is one of the factors that makes the, "Impossible turn" so dangerous. Many pilots, even ones who are experienced and should know better, have tried to make it back to the runway after a problem on climb out.

With model planes, unless you are flying something pretty heavy and scale, you wont really notice this effect. If you are cruising at normal power for your model (say 50 - 60%) you can turn from upwind to downwind in a good breeze and not notice much change other than visually seeing your plane go from fast to slow. I think that causes more problems for new RC pilots in wind than anything. They see the plane turn and rocket downwind and they start cutting power instinctively. In this sense, Josh is more or less correct as it relates to model planes. Most of the time to you can fly your plane as though it doesn't care about the wind but you may notice a little pitch up turning into the wind and down when turning downwind.

Maybe this would be a good one to do on my smart duck that has GPS and a pitot tube...

SCIENCE!

 

[PianoGuy]

Science indeed. Momentum applies to calm as well as wind conditions. If you're flying north on a calm day at 15kts and turn 180 degrees, your velocity (and hence your momentum) has changed by 30kts. If you're pointing into a 15kt north wind, your groundspeed starts out at zero. As you turn 180 degrees your groundspeed picks up to 30kts, so the change in momentum is the same in both situations. In both cases it takes 2.48 seconds for the airplane to turn 180 degrees. If the airplane can reverse its velocity by 30kts in 2.48 seconds in calm winds, why is this suddenly a problem when the wind is blowing??? I used an online tool to plot out each of these situations using 15kts airspeed and a 15kt wind from the north. The top half shows the airplane turning in calm conditions. The path is circular with a radius of 6.1 meters. The bottom shows the path the plane would take in a 15kt north wind. The changes in the northerly component of momentum as a function of time are the same in both cases. Since force is equal to the rate of change of momentum (F = dP/dt), the forces acting on the airplane are the same in both cases. Yes, Hondo, PLEASE do the experiment with your Smart Duck and post the results.

 

[Monte.C]

Ok then.
In zero wind, let's place a helium balloon in the air. It has a weight attached such that it has neutral buoyancy in the air. It's just sitting there stationary. You can fly circles around it easily enough at 10 mph.

Now let's place that balloon in a 10 mph breeze. It's now moving 10 mph sideways. You can still fly perfectly level 10 mph circles around it. The balloon and the plane are doing their dance together within a "mass of air" that's moving at 10 mph sideways. You - on the ground - are watching your plane go 0 mph to 20 mph repeatedly. Upwind and downwind. But THE PLANE DON'T CARE. The plane is flying 10 mph in a continuous turn. And they all lived happily ever after. The end.

 

[Hondo76251]

And like @PianoGuy said, and this is where the confusion comes in, it takes more force (power) to change direction than to continue flying straight and level but that has little to do with the wind. It's the visual cues that deceive the pilot. When you turn into the wind you see the plane slow down so you add power, when you turn downwind you see the plane speed up so you think you should cut power... nose up, nose down...

I think this will be a fun one, I'm on it! I have GPS, Airspeed, and data tracking on the plane. I also have loiter mode, where it circles a set point, and "circle mode" where it flies a perfect circle but does not try to maintain position, exactly like the balloon mentioned earlier...

 

[JasonK]

If I fly 15kts into a headwind of 15kts my plane is stationary. The plane could care less about the ground speed as far as the aerodynamic forces, but it does care about F = M × V. When it turns to come downwind the mass of the plane has to go from stationary to 30kts. The plane in the air reacting to the wind is not attached to the ground but it is still subject to mass and acceleration forces.

yes, that exactly is the miss conception here. when you turn your plane isn't going from 0 to 30 relative to the air, it is going from 15 to -15 (all relative to the air). the plane cares not about the ground at all, all of the aerodynamics forces are relative to the air, not the ground.

Were the problems comes in, is that us humans will make the above 'thinking' that the airspeed is relative to the ground, not relative to the air. When we do this, we miss judge how much speed/etc is needed coming around that turn (or what to expect coming out of that turn).

[this is all assuming a constant air speed, if there are gusts/etc, then the changes in the airspeed are relevant]


Perhaps another way to help with this. Why do we take off into the wind and land into the wind? if what matters for flight is your ground speed, it wouldn't matter at all. but what matters is your airspeed, so by taking off/landing into the wind, you get an airspeed greater then your ground speed (which means less runway, better slopes compared to the _ground_, etc).

 

[Hondo76251]

@PianoGuy your graph perfectly represents what I mentioned earlier about the maneuver commonly called, "the impossible turn" neat to see it with the math along with the visual...

I'm sure most of you have heard of this but I'll go into it for those who maybe haven't:
If that plane in the graph was on climb out after take off and had engine trouble the pilot may think he has enough altitude to make it back. He is expecting his plane to turn like the first plane in the graph with no wind, but if he has any head wind at all this will put him in a situation like the 2nd plane. Add to that the fact that now, on climb out, he's trying to make a turn which will require energy from the plane, an increase in power, and since he may not have that power the only way to get that would be to sacrifice altitude, not something a pilot in trouble is going to instinctively do. As the pilot attempts the turn his visual cues are telling him his speed is good, if not increasing. This leads to a stall. Now you are not just in trouble, you've increased the speed at which you are going to impact the ground because you have a tail wind. The plane in the wind doesn't care about the wind, but the pilot about to hit the ground does.

I think training, at least in GA, on this issue is where people get the sense that the wind affects the plane when in reality its just the pilots sense of spacial orientation causing poor judgement. The plane doesn't stall because it turns downwind, it stalls because the pilot lets the plane slow down in the turn and does not properly correct for it because his visual cues tell him he is ok, right up until its too late.

 

[Monte.C]

@PianoGuy your graph perfectly represents what I mentioned earlier about the maneuver commonly called, "the impossible turn" neat to see it with the math along with the visual...

I'm sure most of you have heard of this but I'll go into it for those who maybe haven't:
If that plane in the graph was on climb out after take off and had engine trouble the pilot may think he has enough altitude to make it back. He is expecting his plane to turn like the first plane in the graph with no wind, but if he has any head wind at all this will put him in a situation like the 2nd plane. Add to that the fact that now, on climb out, he's trying to make a turn which will require energy from the plane, an increase in power, and since he may not have that power the only way to get that would be to sacrifice altitude, not something a pilot in trouble is going to instinctively do. As the pilot attempts the turn his visual cues are telling him his speed is good, if not increasing. This leads to a stall. Now you are not just in trouble, you've increased the speed at which you are going to impact the ground because you have a tail wind. The plane in the wind doesn't care about the wind, but the pilot about to hit the ground does.

I think training, at least in GA, on this issue is where people get the sense that the wind affects the plane when in reality its just the pilots sense of spacial orientation causing poor judgement. The plane doesn't stall because it turns downwind, it stalls because the pilot lets the plane slow down in the turn and does not properly correct for it because his visual cues tell him he is ok, right up until its too late.

 

[Tench745]

I think where the confusion really comes into play is when you have gusty conditions. It's all well and good to have a theoretical constant airspeed which you're flying in. But, if you have gusty conditions momentum will come into play. This is why full scale pilots have to include a gust-factor in their landing speed, so that if suddenly the wind drops they won't be caught below stall speed while the aircraft accelerates into the now stationary air.

 

[PianoGuy]

And like @PianoGuy said, and this is where the confusion comes in, it takes more force (power) to change direction than to continue flying straight and level but that has little to do with the wind. It's the visual cues that deceive the pilot. When you turn into the wind you see the plane slow down so you add power, when you turn downwind you see the plane speed up so you think you should cut power... nose up, nose down...

I think this will be a fun one, I'm on it! I have GPS, Airspeed, and data tracking on the plane. I also have loiter mode, where it circles a set point, and "circle mode" where it flies a perfect circle but does not try to maintain position, exactly like the balloon mentioned earlier...

Have you performed the test flights yet? I’m planning to write to Josh with a suggestion to perform their own experiment. It would be great to have some actual flight data for them to duplicate.

 

[Hondo76251]

Have you performed the test flights yet? I’m planning to write to Josh with a suggestion to perform their own experiment. It would be great to have some actual flight data for them to duplicate.

Got a few flights in but had a video card go bad. Also need a better way to show the data... working on it but ive been so busy lately havent had much time time...