How Does F-35B Not Flip Over During Transition?
$begingroup$
As the engine nozzle rotates, the lift-fan cannot. Wouldn't this imbalance of moments around the center of gravity result in the F-35 pitching up onto its back?
As can be seen at 5:14 in this video:
the F-35B is capable of transitioning from vertical hover to horizontal flight.
It accomplishes this in part by gradually rotating the engine nozzle from the vertical position to the horizontal.
Let us assume that the throttle input stays constant in the following scenario:
As the nozzle rotates, the vertical component of its thrust is reduced (as the horizontal component increases) as
$$thrust_{horizontal} = thrust cdot cos(theta)$$
$theta$ being the angle between the jet exhaust and the vertical axis.
The problem is, the lift fan does not rotate. And since it is attached by shaft to the engine, we assume it always rotates at the same RPM as the engine itself.
Therefore, the vertical component of lift of the lift fan does not change for a given throttle input, while the vertical component of the jet engine nozzle does!
So as the nozzle transition occurs, the thrust of the lift fan (ahead of the center of gravity) stays constant, but the vertical thrust behind the center of gravity (rear engine nozzle) reduces: shouldn’t this cause the aircraft to pitch upwards and flip onto its back?
Yet in the video the aircraft seems to pitch up only ten or so degrees (I assume due to the increased coefficient of lift on the wings) but otherwise stays flat in pitch.
The thrust of the lift fan must be somehow modulated to accomplish this, right? If so how is this done if its shaft speed is tied to that of the engine?
Possible solutions:
1. As the aircraft’s forward velocity increases, the lift of the wings compensates for the reduced vertical lift of the rear engine nozzle.
2. The aircraft pitches up so as to deflect the angle of the lift fan from the vertical to match the angle between the engine nozzle and vertical.
aircraft-design takeoff aircraft-physics vtol thrust
edited 4 hours ago
Federico♦
25.4k16101152
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
$endgroup$
add a comment |
$begingroup$
As the engine nozzle rotates, the lift-fan cannot. Wouldn't this imbalance of moments around the center of gravity result in the F-35 pitching up onto its back?
As can be seen at 5:14 in this video:
the F-35B is capable of transitioning from vertical hover to horizontal flight.
It accomplishes this in part by gradually rotating the engine nozzle from the vertical position to the horizontal.
Let us assume that the throttle input stays constant in the following scenario:
As the nozzle rotates, the vertical component of its thrust is reduced (as the horizontal component increases) as
$$thrust_{horizontal} = thrust cdot cos(theta)$$
$theta$ being the angle between the jet exhaust and the vertical axis.
The problem is, the lift fan does not rotate. And since it is attached by shaft to the engine, we assume it always rotates at the same RPM as the engine itself.
Therefore, the vertical component of lift of the lift fan does not change for a given throttle input, while the vertical component of the jet engine nozzle does!
So as the nozzle transition occurs, the thrust of the lift fan (ahead of the center of gravity) stays constant, but the vertical thrust behind the center of gravity (rear engine nozzle) reduces: shouldn’t this cause the aircraft to pitch upwards and flip onto its back?
Yet in the video the aircraft seems to pitch up only ten or so degrees (I assume due to the increased coefficient of lift on the wings) but otherwise stays flat in pitch.
The thrust of the lift fan must be somehow modulated to accomplish this, right? If so how is this done if its shaft speed is tied to that of the engine?
Possible solutions:
1. As the aircraft’s forward velocity increases, the lift of the wings compensates for the reduced vertical lift of the rear engine nozzle.
2. The aircraft pitches up so as to deflect the angle of the lift fan from the vertical to match the angle between the engine nozzle and vertical.
aircraft-design takeoff aircraft-physics vtol thrust
edited 4 hours ago
Federico♦
25.4k16101152
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
$endgroup$
5
$begingroup$
"Let us assume that the throttle input stays constant in the following scenario:" That is a really bad assumption to make on such an advanced aircraft. Obviously, the nozzle thrust is adjusted to compensate for the changing nozzle angle.
$endgroup$
– abelenky
4 hours ago
$begingroup$
That assumption was just made to simplify the formulation of the question. Of course I actual flight the throttle would be adjusted.
$endgroup$
– AllTradesJack
3 hours ago
add a comment |
$begingroup$
As the engine nozzle rotates, the lift-fan cannot. Wouldn't this imbalance of moments around the center of gravity result in the F-35 pitching up onto its back?
As can be seen at 5:14 in this video:
the F-35B is capable of transitioning from vertical hover to horizontal flight.
It accomplishes this in part by gradually rotating the engine nozzle from the vertical position to the horizontal.
Let us assume that the throttle input stays constant in the following scenario:
As the nozzle rotates, the vertical component of its thrust is reduced (as the horizontal component increases) as
$$thrust_{horizontal} = thrust cdot cos(theta)$$
$theta$ being the angle between the jet exhaust and the vertical axis.
The problem is, the lift fan does not rotate. And since it is attached by shaft to the engine, we assume it always rotates at the same RPM as the engine itself.
Therefore, the vertical component of lift of the lift fan does not change for a given throttle input, while the vertical component of the jet engine nozzle does!
So as the nozzle transition occurs, the thrust of the lift fan (ahead of the center of gravity) stays constant, but the vertical thrust behind the center of gravity (rear engine nozzle) reduces: shouldn’t this cause the aircraft to pitch upwards and flip onto its back?
Yet in the video the aircraft seems to pitch up only ten or so degrees (I assume due to the increased coefficient of lift on the wings) but otherwise stays flat in pitch.
The thrust of the lift fan must be somehow modulated to accomplish this, right? If so how is this done if its shaft speed is tied to that of the engine?
Possible solutions:
1. As the aircraft’s forward velocity increases, the lift of the wings compensates for the reduced vertical lift of the rear engine nozzle.
2. The aircraft pitches up so as to deflect the angle of the lift fan from the vertical to match the angle between the engine nozzle and vertical.
aircraft-design takeoff aircraft-physics vtol thrust
edited 4 hours ago
Federico♦
25.4k16101152
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
$endgroup$
As the engine nozzle rotates, the lift-fan cannot. Wouldn't this imbalance of moments around the center of gravity result in the F-35 pitching up onto its back?
As can be seen at 5:14 in this video:
the F-35B is capable of transitioning from vertical hover to horizontal flight.
It accomplishes this in part by gradually rotating the engine nozzle from the vertical position to the horizontal.
Let us assume that the throttle input stays constant in the following scenario:
As the nozzle rotates, the vertical component of its thrust is reduced (as the horizontal component increases) as
$$thrust_{horizontal} = thrust cdot cos(theta)$$
$theta$ being the angle between the jet exhaust and the vertical axis.
The problem is, the lift fan does not rotate. And since it is attached by shaft to the engine, we assume it always rotates at the same RPM as the engine itself.
Therefore, the vertical component of lift of the lift fan does not change for a given throttle input, while the vertical component of the jet engine nozzle does!
So as the nozzle transition occurs, the thrust of the lift fan (ahead of the center of gravity) stays constant, but the vertical thrust behind the center of gravity (rear engine nozzle) reduces: shouldn’t this cause the aircraft to pitch upwards and flip onto its back?
Yet in the video the aircraft seems to pitch up only ten or so degrees (I assume due to the increased coefficient of lift on the wings) but otherwise stays flat in pitch.
The thrust of the lift fan must be somehow modulated to accomplish this, right? If so how is this done if its shaft speed is tied to that of the engine?
Possible solutions:
1. As the aircraft’s forward velocity increases, the lift of the wings compensates for the reduced vertical lift of the rear engine nozzle.
2. The aircraft pitches up so as to deflect the angle of the lift fan from the vertical to match the angle between the engine nozzle and vertical.
aircraft-design takeoff aircraft-physics vtol thrust
aircraft-design takeoff aircraft-physics vtol thrust
edited 4 hours ago
Federico♦
25.4k16101152
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
edited 4 hours ago
Federico♦
25.4k16101152
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
edited 4 hours ago
Federico♦
25.4k16101152
edited 4 hours ago
Federico♦
25.4k16101152
edited 4 hours ago
Federico♦
25.4k16101152
25.4k16101152
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
asked 4 hours ago
AllTradesJackAllTradesJack
7852821
7852821
5
$begingroup$
"Let us assume that the throttle input stays constant in the following scenario:" That is a really bad assumption to make on such an advanced aircraft. Obviously, the nozzle thrust is adjusted to compensate for the changing nozzle angle.
$endgroup$
– abelenky
4 hours ago
$begingroup$
That assumption was just made to simplify the formulation of the question. Of course I actual flight the throttle would be adjusted.
$endgroup$
– AllTradesJack
3 hours ago
add a comment |
5
$begingroup$
"Let us assume that the throttle input stays constant in the following scenario:" That is a really bad assumption to make on such an advanced aircraft. Obviously, the nozzle thrust is adjusted to compensate for the changing nozzle angle.
$endgroup$
– abelenky
4 hours ago
$begingroup$
That assumption was just made to simplify the formulation of the question. Of course I actual flight the throttle would be adjusted.
$endgroup$
– AllTradesJack
3 hours ago
5
5
$begingroup$
"Let us assume that the throttle input stays constant in the following scenario:" That is a really bad assumption to make on such an advanced aircraft. Obviously, the nozzle thrust is adjusted to compensate for the changing nozzle angle.
$endgroup$
– abelenky
4 hours ago
$begingroup$
"Let us assume that the throttle input stays constant in the following scenario:" That is a really bad assumption to make on such an advanced aircraft. Obviously, the nozzle thrust is adjusted to compensate for the changing nozzle angle.
$endgroup$
– abelenky
4 hours ago
$begingroup$
That assumption was just made to simplify the formulation of the question. Of course I actual flight the throttle would be adjusted.
$endgroup$
– AllTradesJack
3 hours ago
$begingroup$
That assumption was just made to simplify the formulation of the question. Of course I actual flight the throttle would be adjusted.
$endgroup$
– AllTradesJack
3 hours ago
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Your assumptions (constant thrust for both lift fan and exhaust) are wrong. From wikipedia:
"For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed"
So yes, the thrust is modulated to accomplish this.
answered 3 hours ago
SanchisesSanchises
6,09712555
$endgroup$
add a comment |
$begingroup$
From what is available on the internet, the F35B's lift fan is more than just a simple shrouded fan. There is at least some variable guide vanes functioning as nozzle. So to answer your question, no, the lift from front and back seem to be completely (lift fan by variable guide vanes, engine nozzle by, well, a variable nozzle) adjustable.
edited 1 hour ago
David Richerby
9,83833478
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
$endgroup$
1
$begingroup$
What are VIGN’s?
$endgroup$
– AllTradesJack
3 hours ago
2
$begingroup$
@AllTradesJack sorry i type 'VIGN' out of habit. it should be VGN= variable guide vane.
$endgroup$
– Meatball Princess
3 hours ago
$begingroup$
@MeatballPrincess OK but that's still a pretty obscure abbreviation! (Especially when one would expect that to be VGV.)
$endgroup$
– David Richerby
1 hour ago
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Your assumptions (constant thrust for both lift fan and exhaust) are wrong. From wikipedia:
"For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed"
So yes, the thrust is modulated to accomplish this.
answered 3 hours ago
SanchisesSanchises
6,09712555
$endgroup$
add a comment |
$begingroup$
Your assumptions (constant thrust for both lift fan and exhaust) are wrong. From wikipedia:
"For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed"
So yes, the thrust is modulated to accomplish this.
answered 3 hours ago
SanchisesSanchises
6,09712555
$endgroup$
add a comment |
$begingroup$
Your assumptions (constant thrust for both lift fan and exhaust) are wrong. From wikipedia:
"For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed"
So yes, the thrust is modulated to accomplish this.
answered 3 hours ago
SanchisesSanchises
6,09712555
$endgroup$
Your assumptions (constant thrust for both lift fan and exhaust) are wrong. From wikipedia:
"For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed"
So yes, the thrust is modulated to accomplish this.
answered 3 hours ago
SanchisesSanchises
6,09712555
answered 3 hours ago
SanchisesSanchises
6,09712555
answered 3 hours ago
SanchisesSanchises
6,09712555
answered 3 hours ago
SanchisesSanchises
6,09712555
6,09712555
add a comment |
add a comment |
$begingroup$
From what is available on the internet, the F35B's lift fan is more than just a simple shrouded fan. There is at least some variable guide vanes functioning as nozzle. So to answer your question, no, the lift from front and back seem to be completely (lift fan by variable guide vanes, engine nozzle by, well, a variable nozzle) adjustable.
edited 1 hour ago
David Richerby
9,83833478
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
$endgroup$
1
$begingroup$
What are VIGN’s?
$endgroup$
– AllTradesJack
3 hours ago
2
$begingroup$
@AllTradesJack sorry i type 'VIGN' out of habit. it should be VGN= variable guide vane.
$endgroup$
– Meatball Princess
3 hours ago
$begingroup$
@MeatballPrincess OK but that's still a pretty obscure abbreviation! (Especially when one would expect that to be VGV.)
$endgroup$
– David Richerby
1 hour ago
add a comment |
$begingroup$
From what is available on the internet, the F35B's lift fan is more than just a simple shrouded fan. There is at least some variable guide vanes functioning as nozzle. So to answer your question, no, the lift from front and back seem to be completely (lift fan by variable guide vanes, engine nozzle by, well, a variable nozzle) adjustable.
edited 1 hour ago
David Richerby
9,83833478
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
$endgroup$
1
$begingroup$
What are VIGN’s?
$endgroup$
– AllTradesJack
3 hours ago
2
$begingroup$
@AllTradesJack sorry i type 'VIGN' out of habit. it should be VGN= variable guide vane.
$endgroup$
– Meatball Princess
3 hours ago
$begingroup$
@MeatballPrincess OK but that's still a pretty obscure abbreviation! (Especially when one would expect that to be VGV.)
$endgroup$
– David Richerby
1 hour ago
add a comment |
$begingroup$
From what is available on the internet, the F35B's lift fan is more than just a simple shrouded fan. There is at least some variable guide vanes functioning as nozzle. So to answer your question, no, the lift from front and back seem to be completely (lift fan by variable guide vanes, engine nozzle by, well, a variable nozzle) adjustable.
edited 1 hour ago
David Richerby
9,83833478
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
$endgroup$
From what is available on the internet, the F35B's lift fan is more than just a simple shrouded fan. There is at least some variable guide vanes functioning as nozzle. So to answer your question, no, the lift from front and back seem to be completely (lift fan by variable guide vanes, engine nozzle by, well, a variable nozzle) adjustable.
edited 1 hour ago
David Richerby
9,83833478
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
edited 1 hour ago
David Richerby
9,83833478
edited 1 hour ago
David Richerby
9,83833478
edited 1 hour ago
David Richerby
9,83833478
9,83833478
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
answered 3 hours ago
Meatball PrincessMeatball Princess
70813
70813
1
$begingroup$
What are VIGN’s?
$endgroup$
– AllTradesJack
3 hours ago
2
$begingroup$
@AllTradesJack sorry i type 'VIGN' out of habit. it should be VGN= variable guide vane.
$endgroup$
– Meatball Princess
3 hours ago
$begingroup$
@MeatballPrincess OK but that's still a pretty obscure abbreviation! (Especially when one would expect that to be VGV.)
$endgroup$
– David Richerby
1 hour ago
add a comment |
1
$begingroup$
What are VIGN’s?
$endgroup$
– AllTradesJack
3 hours ago
2
$begingroup$
@AllTradesJack sorry i type 'VIGN' out of habit. it should be VGN= variable guide vane.
$endgroup$
– Meatball Princess
3 hours ago
$begingroup$
@MeatballPrincess OK but that's still a pretty obscure abbreviation! (Especially when one would expect that to be VGV.)
$endgroup$
– David Richerby
1 hour ago
1
1
$begingroup$
What are VIGN’s?
$endgroup$
– AllTradesJack
3 hours ago
$begingroup$
What are VIGN’s?
$endgroup$
– AllTradesJack
3 hours ago
2
2
$begingroup$
@AllTradesJack sorry i type 'VIGN' out of habit. it should be VGN= variable guide vane.
$endgroup$
– Meatball Princess
3 hours ago
$begingroup$
@AllTradesJack sorry i type 'VIGN' out of habit. it should be VGN= variable guide vane.
$endgroup$
– Meatball Princess
3 hours ago
$begingroup$
@MeatballPrincess OK but that's still a pretty obscure abbreviation! (Especially when one would expect that to be VGV.)
$endgroup$
– David Richerby
1 hour ago
$begingroup$
@MeatballPrincess OK but that's still a pretty obscure abbreviation! (Especially when one would expect that to be VGV.)
$endgroup$
– David Richerby
1 hour ago
add a comment |
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5
$begingroup$
"Let us assume that the throttle input stays constant in the following scenario:" That is a really bad assumption to make on such an advanced aircraft. Obviously, the nozzle thrust is adjusted to compensate for the changing nozzle angle.
$endgroup$
– abelenky
4 hours ago
$begingroup$
That assumption was just made to simplify the formulation of the question. Of course I actual flight the throttle would be adjusted.
$endgroup$
– AllTradesJack
3 hours ago