Weathervaning - what actually turns an aircraft in flight.

Is the "rocket on the front of the plane" facing in such a way as to push the plane backwards?

Or is is out on a pole as if pulling the plane but offset so that the thrust doesn't burn the aircraft. It is going to lower the resale value if it is blasting the airframe.

Maybe a series of ropes way out in front.
 
alfadog said:
Very good. And the name for air making something turn by striking the side of it is called weathervaning.
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Weathervaning

Process by which a floating structure passively varies its heading in response to time-varying environmental actions.


I still don't like this term for anything other than fixed objects on the ground which have the ability to pivot and align themselves with the earth's wind. It does not help the cause of explaining aircraft turning, IMO.
 
SixPapaCharlie said:
Is the "rocket on the front of the plane" facing in such a way as to push the plane backwards?

Or is is out on a pole as if pulling the plane but offset so that the thrust doesn't burn the aircraft. It is going to lower the resale value if it is blasting the airframe.

Maybe a series of ropes way out in front.
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The latter, on a pedestal. Also, since we do not have to worry about drag, we shut that puppy off once we reach Vne but the rocket on the wing has to keep burning and that is the one we are talking about.
 
I'm pretty sure I posted this once in some old thread re "weathervaning,"but... I prepared this diagram to pretty up my school's explanation that planes don't just fly off at an angle, but turn thanks to the fact that they are designed to be "directionally stable," i.e. if the relative wind blows on the nose from the side, the airplane is going to try to turn into it:

turns.jpg
 
MAKG1 said:
You're missing the point. There is no "it."

The aircraft responds to all its forces at once. Never one at a time.
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"It" is the force other than the horizontal component of lift and is the force that causes the airplane to turn at cruise speed in a nearly co-ordinated turn without adding any rudder. Breaking complex systems down and treating factors independently is the normal way we solve problems in mechanics; it is why we use the term "horizontal component of lift" at all.
 
MAKG1 said:
Control inputs != control surfaces.

<snip>
That's the fault. Reduction of the airplane to parts that don't make sense in isolation. An airplane that can't control pitch, roll, AND yaw at a minimum, all at the same time, cannot fly for any length of time.
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Somebody forgot to tell the Ultralight and RC guys that....

https://en.wikipedia.org/wiki/Eipper_Quicksilver

Brian
 
alfadog said:
holding the lift vector at an angle to the horizontal can never produce curving flight where you can turn completely around.
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Yes it can as shown in a nice diagram #44. It is enough to have a horizontal force acting perpendicular to aircraft's path.
 
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brcase said:
Somebody forgot to tell the Ultralight and RC guys that....

https://en.wikipedia.org/wiki/Eipper_Quicksilver

Brian
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Umm, I guarantee you that all three axes are controlled SOMEhow, even if it's passive.

Roll is often controlled with dihedral. That doesn't mean it's uncontrolled. It means the control is the design of the airframe.

You MUST control it somehow. No, you do not necessarily have to have a lever labeled "roll" or "aileron" to do that. That quicksilver has obvious and enormous dihedral. Take it out, and it will not fly.
 
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olasek said:
Yes it can as shown in a nice diagram #44. It is enough to have a horizontal force acting perpendicular to aircraft's path.
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That drawing is pretty but incorrect as it shows the plane turning but does not show the force causing the turn. He mentions it, calling it "the airplane's stability", but he does not show it.
 
Money is a secondary force of turning. Lift turns the plane, money creates lift.

A plane will 'weathervane' in a gust, but it is because the whole air mass realigns in the gust, not because it blows harder.
 
alfadog said:
My point is that simply holding the lift vector at an angle to the horizontal can never produce curving flight where you can turn completely around. It cannot yaw or rotate the airplane. You need a second and opposing force to create rotation. As pilots, we know that instintively but folks are still taught only one half of the story as if that is the whole story.
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"Certs is a breath mint. No, Certs is a candy mint." Oy. I always wanted to shove a Certs up both nostrils of each of those bubbleheads arguing about it.

If you bank the plane while holding the rudder dead centered (that's centered rudder, not rudder for centered ball), the plane will turn, albeit in a slipped mode. Of course, if you displace the rudder while holding the wings level with aileron, it will also turn, albeit in a skidded mode. At the end of the day, it's not just one force that makes the plane turn. As Dr. Denker says, what makes a plane turn is a combination of forces which we try to balance using bank angle and rudder displacement to create a coordinated turn of the desired rate.
 
I agree with alfadog here. The aircraft's vertical stabilizer, and fuselage surface area behind the CG, are responsible for providing directional stability--preventing slipping when the aircraft is banked by weathervaning the aircraft into the relative wind. Barry Schiff has noted that he flew the Spirit of St. Louis (well, a replica at least) which has a very small tail. Banking that aircraft and neutralizing the controls, he said, results in a perfect sideslip with no turn whatsoever.
 
To every thing.....turn, turn, turn.
There is a season......turn, turn, turn.

so, the Byrds make you turn.
 
OK, Cessna in space, with rockets pulling from the front but tethered, A rocket in the center of lift beneath, and 2 rockets on the ailerons.

We're all on the same page now right?

So how does an airplane turn?

999926261797.jpg
 
The pilot makes aircraft turn in flight that and airpockets. You people think too hard, the simple answer from the test is correct. Aircraft with no tail, rudder or fuselage turn just fine, you know by using the horizontal component of lift.
PS thinking like this is why some of you were beat up in school.
 
the stuff in my bong makes that thing turn.
SixPapaCharlie said:
OK, Cessna in space, with rockets pulling from the front but tethered, A rocket in the center of lift beneath, and 2 rockets on the ailerons.

We're all on the same page now right?

So how does an airplane turn?

999926261797.jpg
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SixPapaCharlie said:
OK, Cessna in space, with rockets pulling from the front but tethered, A rocket in the center of lift beneath, and 2 rockets on the ailerons.

We're all on the same page now right?

So how does an airplane turn?

999926261797.jpg
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You, Sir, are awesome!
 
Ron Levy said:
... At the end of the day, it's not just one force that makes the plane turn. As Dr. Denker says, what makes a plane turn is a combination of forces which we try to balance using bank angle and rudder displacement to create a coordinated turn of the desired rate.
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Absolutely, it is a combination of, basically, two forces that act more or less at the same time to cause what we all consider a turn in flight. My point is that students are presented this erroneous and oversimplified concept that just one force, the horizontal component of lift, is responsible. And they will defend that misconception.
 
alfadog said:
And they will defend that misconception.
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No, it isn't a misconception. A single force is enough whether it is a satellite orbiting the Earth or aircraft turning. All other forces may assist in 'coordination' and have a role in a turn but are of secondary importance.
 
olasek said:
No, it isn't a misconception. A single force is enough whether it is a satellite orbiting the Earth or aircraft turning. All other forces may assist in 'coordination' and have a role in a turn but are of secondary importance.
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No. There is a force from each and every surface on the aircraft. And they all interact. There is even some (modest) lift from the body, especially at high angle of attack.

You can describe center of mass motion by a single net force, but the utility is very limited. In this case, you will get the wrong answer. You don't even get torques, so you can't describe attitude at all. At a minimum, you need net force, and pitch roll and yaw moment (or torque) to even describe the aircraft state.

These "extra" forces are not secondary. Ignore them, and you get wrong behavior. Those of us who work with flight simulators (that is, making them, not playing games) must include models for all of the exterior surfaces on the aircraft. Including things like trim tabs and gear.
 
I say no. Just knowing the horizontal lift component is enough to compute the turn radius with a fairly good accuracy. All other forces will change this number just slightly. So as mathematicians would say for 'first degree approximation' horizontal lift component is good enough and everything else will just add a small refinement if you need 2-nd degree approxim, etc.
 
olasek said:
I say no. Just knowing the horizontal lift component is enough to compute the turn radius with a fairly good accuracy. All other forces will change this number just slightly. So as mathematicians would say for 'first degree approximation' horizontal lift component is good enough and everything else will just add a small refinement if you need 2-nd degree approxim, etc.
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That's first ORDER approximation.

And it can't tell the difference between a constant rate climb and a loop.

It is wrong.

You can't just use an approximation because you feel like it. You have to evaluate it. The entire world is not first order.
 
MAKG1 said:
The entire world is not first order.
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No, not the entire world but the "turn" we are discussing here. First order is enough to have a good grip what turn radius it is going to be and all you need is horizontal force. And turn R = v*v*W/g/LH (v = aircraft speed, LH = horizontal force, W = aircraft weight).
 
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olasek said:
No, not the entire world but the "turn" we are discussing here. First order is enough to have a good grip what turn radius it is going to be and all you need is horizontal force. And turn R = v*v*W/g/LH (v = aircraft speed, LH = horizontal force, W = aircraft weight).
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Answer the "Lunar Cessna" question I posed, so nicely illustrated by 6PC, and you will see that horizontal force is far from enough.
 
Just because you need to do silly things to make a simulator turn like a real aircraft it doesn't mean a real aircraft needs to do silly things to turn.
 
SixPapaCharlie said:
See posts 37, 41, and 61
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Got it!

When your lunar Cessna banks presumably its main engine thrust (the one that supports weight) goes slightly sideways, the horizontal component acts to turn aircraft, assuming the vertical component can still support aircraft weight or you have to increase engine thrust. Again, there is a horizontal force and it is enough to cause a turn (or change orbit).

You could remove wings from your Lunar Cessna (they are useless anyway) and simply mount those two small engines sideways and this way control direction of flight.
 
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olasek said:
Got it!

When your lunar Cessna banks presumably its main engine thrust (the one that supports weight) goes slightly sideways, the horizontal component acts to turn aircraft, assuming the vertical component can still support aircraft weight or you have to increase engine thrust. Again, there is a horizontal force and it is enough to cause a turn (or change orbit).

You could remove wings from your Lunar Cessna (they are useless anyway) and simply mount those two small engines sideways and this way control direction of flight.
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Yes, we increase thrust in the lift rocket as needed to maintain level flight. So do you think if we held the 30° bank, the Lunar Cessna would fly in circles?

6PC, care to venture a guess?
 
In space no. In space, the plane would start moving up like an elevator opposite the direction of thrust on the main rocket.

Now add an elevator rocket that changes the pitch and you got yourself a turn
 
No rudder, not much dihedral that I can see - this dang thing couldn't possibly turn:

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SixPapaCharlie said:
In space no. In space, the plane would start moving up like an elevator opposite the direction of thrust on the main rocket.

Now add an elevator rocket that changes the pitch and you got yourself a turn
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You are close. Remember, we are not in space, we are on a celestial body that has appreciable gravity. The only unbalanced force in our Cessna is the horizontal component of lift, the exact force that many believe is sufficient to cause an airplane to fly in circles. However, to your point, there would be no upward motion.
 
alfadog said:
So do you think if we held the 30° bank, the Lunar Cessna would fly in circles?
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Would be flying and turning constantly but definitely not flying in circles. It would be a complicated curve - one would have to write down equations of motions and solve them.
 
olasek said:
Would be flying and turning constantly but definitely not flying in circles. It would be a complicated curve - one would have to write down equations of motions and solve them.
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But at some point, the nose of the airplane would be pointed back from where it started, right? I mean if we started heading north, at some point we would be heading south in your scenario. The problem there is that we have not introduced any force that would cause such a effect.
 
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