Go Back  PPRuNe Forums > Aircrew Forums > Military Aviation
Reload this Page >

The F-35 thread, Mk II

Military Aviation A forum for the professionals who fly military hardware. Also for the backroom boys and girls who support the flying and maintain the equipment, and without whom nothing would ever leave the ground. All armies, navies and air forces of the world equally welcome here.

The F-35 thread, Mk II

Old 6th Mar 2020, 02:47
  #1 (permalink)  
Thread Starter
 
Join Date: Apr 2004
Location: Planet Earth
Posts: 1,844
F35B rolling vertical landing

Saw this demonstrated for the first time on a documentary


Very impressive, the demonstrated level of stability and almost rock steady flight path precision on a very slow landing and extremely short roll out to a braked stop


The approach speed is so slow it brings up the obvious question though, I think it’s about a 30 knot overtake, assuming 20 knots wind over the deck that’s around 50 knots IAS


At such a low speed I don’t see how you’re generating any worthwhile, additional wing lift so what’s the point if the idea is to allow higher landing weights ?


At the moment it looks like an interesting demonstration of slow flight enabled by the aircraft’s vertical lift system


Unless they’re planning higher speed RVL’ s ?


stilton is offline  
Old 6th Mar 2020, 03:04
  #2 (permalink)  
 
Join Date: May 2005
Location: QLD - where drivers are yet to realise that the left lane goes to their destination too.
Posts: 2,258
If they are moving forward, does that not make it a short landing, rather than a vertical one?
Traffic_Is_Er_Was is offline  
Old 6th Mar 2020, 05:11
  #3 (permalink)  
 
Join Date: Jul 2013
Location: California
Age: 55
Posts: 13
Why do you think the wing would not be generating any lift at 50 knots?
BrogulT is offline  
Old 6th Mar 2020, 07:57
  #4 (permalink)  
 
Join Date: Oct 1999
Location: Nigeria
Age: 53
Posts: 4,640
Originally Posted by BrogulT View Post
Why do you think the wing would not be generating any lift at 50 knots?
Yes, plus we don't know the ship and windspeed, so the actual airspeed is probably significantly higher than the groundspeed visible.

If they are moving forward, does that not make it a short landing, rather than a vertical one?
My first thoughts too, but searching online this does seem to be the accepted terminology. Mind you, all landings have a vertical component or else you'd never touch down....
212man is offline  
Old 6th Mar 2020, 08:15
  #5 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,635
I don’t know about allowing a significantly higher landing weight but the main advantages of an “RVL” on any aircraft designed for vertical landings are that the pilot has a far better view of the touchdown point and as the OP noted, the aircraft is easier to fly in the lateral sense because there are more visual cues. It may also retain some natural aerodynamic stability until touchdown.
ShyTorque is offline  
Old 6th Mar 2020, 08:22
  #6 (permalink)  
 
Join Date: Nov 2000
Location: UK
Age: 65
Posts: 1,093
Interesting to consider the balance of thrust vectors vertical and horizontal vs weight, lift and drag. Could you end up using higher power settings if the wing is not producing sufficient lift?
beardy is offline  
Old 6th Mar 2020, 08:44
  #7 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,635
Originally Posted by beardy View Post
Interesting to consider the balance of thrust vectors vertical and horizontal vs weight, lift and drag. Could you end up using higher power settings if the wing is not producing sufficient lift?
No, because the aircraft is in a constant descent. However, as the IAS decreases and aerodynamic lift reduces, power will be increased to compensate.

Another factor is that in a fully vertical descent, the engine may ingest hot air from its own exhaust and possibly dirt. Neither are good.
ShyTorque is offline  
Old 6th Mar 2020, 10:33
  #8 (permalink)  
 
Join Date: Nov 2000
Location: UK
Age: 65
Posts: 1,093
Originally Posted by ShyTorque View Post
No, because the aircraft is in a constant descent. However, as the IAS decreases and aerodynamic lift reduces, power will be increased to compensate.

Another factor is that in a fully vertical descent, the engine may ingest hot air from its own exhaust and possibly dirt. Neither are good.
Why no?
Constant descent is a red herring, if it is constant there is no acceleration so vertical lift vector = weight. Vertical lift vector is the sum of vertical thrust vector and vertical lift vector. As you say as speed decreases lift from wing decreases and engine thrust must compensate although the airframe drag will decrease which will require less engines thrust to overcome and may compensate. Unless the Rod in the approach is sufficient for landing then that too must be slowed by a further increase in thrust.
It is all speculation on my part, the truth is out there!
beardy is offline  
Old 6th Mar 2020, 11:07
  #9 (permalink)  
 
Join Date: Apr 2005
Location: UK
Posts: 1,918
Gravity is an acceleration. If your vertical lift vector = weight then you are not descending towards the herrings.
Just This Once... is offline  
Old 6th Mar 2020, 11:09
  #10 (permalink)  
 
Join Date: May 2009
Location: Here
Posts: 1,275
RVLs on land were done on the Harrier, they let you use shorter strips amongst other things. What the F35B brings to the game is the ability to do them on a ship reliably. An example of use is bringing back weapons in hot conditions when you don't have enough thrust for the vertical landing (It's expensive jettisoning £100K wepaons so you can land)

Whilst Dave Morgan probably did the first SBRVL in 1982, the Indians have done them in SHARs too.



Last edited by Davef68; 6th Mar 2020 at 11:20.
Davef68 is offline  
Old 6th Mar 2020, 11:20
  #11 (permalink)  
 
Join Date: Jun 2016
Location: Norfolk
Posts: 249
They must be practising them at Marham, 'cos they are very NOISY!
57mm is offline  
Old 6th Mar 2020, 11:29
  #12 (permalink)  
 
Join Date: Nov 2000
Location: UK
Age: 65
Posts: 1,093
Originally Posted by Just This Once... View Post
Gravity is an acceleration. If your vertical lift vector = weight then you are not descending towards the herrings.
Constant descent is not an acceleration. Watch the 'g' meter.

An aircraft does not stand still when thrust =drag, now rotate that concept through 90°

Last edited by beardy; 6th Mar 2020 at 11:39.
beardy is offline  
Old 6th Mar 2020, 11:55
  #13 (permalink)  
 
Join Date: Apr 2005
Location: UK
Posts: 1,918
So lowering the nose at a given level of thrust does not cause the IAS to increase or that to maintain a given speed whilst lowering the nose I don't need to reduce thrust?

Last time I rotated your concept and pointed the nose at the nadir my G meter seemed pegged at zero. Maybe it was broken.
Just This Once... is offline  
Old 6th Mar 2020, 12:28
  #14 (permalink)  
 
Join Date: Nov 2000
Location: UK
Age: 65
Posts: 1,093
Originally Posted by Just This Once... View Post
So lowering the nose at a given level of thrust does not cause the IAS to increase or that to maintain a given speed whilst lowering the nose I don't need to reduce thrust?

Last time I rotated your concept and pointed the nose at the nadir my G meter seemed pegged at zero. Maybe it was broken.
OK, I'll write slowly.
When thrust =drag vectors in the horizontal plane one can have a constant speed in the horizontal plane, if you want you can express it in feet per minute.
Rotate the concept through 90°
When weight=lift vectors in the vertical plane one can have a constant speed in the vertical plane, if you want you can express it in feet per minute (rate of descent)

​​​​​​If you have access to O level physics notes or AP 3456 it may be worth having a look at them.
beardy is offline  
Old 6th Mar 2020, 14:06
  #15 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 520
Originally Posted by Just This Once... View Post
Last time I rotated your concept and pointed the nose at the nadir my G meter seemed pegged at zero. Maybe it was broken.
Effectively it was broken for the task at hand, as in that attitude it no longer shows vertical acceleration. If you swivelled it 90 degrees so it shows acceleration along nose-tail (i.e., vertically) and flew a constant TAS downward, it would show 1G.

Same as if you stand on a bathroom scale in an elevator (or, lift) travelling up or down at a constant rate, it will show the same as when you're standing on it in the bathroom.

You say (correctly) that "gravity is an acceleration," which generates a force, and if we were subject to only that force and no other, then we would therefore be accelerating. But the part you're missing is that we are also subject to other forces at the same time, such as the floor of the bathroom (or the lift of an aircraft in steady flight) pushing up on us. Because that force balances the gravitational force, the sum is zero and we do not accelerate.
Vessbot is offline  
Old 6th Mar 2020, 14:12
  #16 (permalink)  
 
Join Date: Apr 2005
Location: UK
Posts: 1,918
My big skool stuff is in there somewhere but still need your teaching on why a constant rate of descent requires the same thrust as for level flight of how a g-meter can be used in establishing a rate of descent before you expand your teachings further. Heck, until you offered to help I thought I could set and maintain a constant RoD and speed with no thrust at all.
Just This Once... is offline  
Old 6th Mar 2020, 14:18
  #17 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 520
Originally Posted by Just This Once... View Post
My big skool stuff is in there somewhere but still need your teaching on why a constant rate of descent requires the same thrust as for level flight of how a g-meter can be used in establishing a rate of descent before you expand your teachings further. Heck, until you offered to help I thought I could set and maintain a constant RoD and speed with no thrust at all.
I edited my above post to add more, as you were typing this. I won't do that anymore to avoid cross-wiring.
Vessbot is offline  
Old 6th Mar 2020, 14:21
  #18 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,635
Beardy, So from a constant IAS in straight and level flight, please explain how you begin a descent while maintaining that same IAS?
ShyTorque is offline  
Old 6th Mar 2020, 14:28
  #19 (permalink)  
 
Join Date: Sep 2011
Location: Belgium
Age: 61
Posts: 139
shy torque Take this example.
Take off and go to cruise altitude.
Trim the aircraft to cruise speed, and let go of the yoke. (Keep heading with rudder only).

Reduce power. What happens? Speed stays at the trimmed speed but you descent.
Increase power. What happens? Speed stays at trimmed speed but the aircraft climbs.

Trim sets the speed.
Power controls climb or descent.

Vilters is offline  
Old 6th Mar 2020, 14:29
  #20 (permalink)  
 
Join Date: Apr 2005
Location: UK
Posts: 1,918
Originally Posted by beardy View Post
When weight=lift vectors in the vertical plane one can have a constant speed in the vertical plane, if you want you can express it in feet per minute (rate of descent)
Zero feet per minute.
Just This Once... is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.