Cookie Monster
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While the canards along with the all moving vertical stabilizers may not necessarily be enough(as speed brakes) to guarantee short landing ability...they would certainly be the biggest(together in terms of surface area) speed brakes on any fighter jet I've seen...including F15, F16, and F22.Airbrake and speedbrake are used interchangeably, but the preferred is 'speedbrake'.
Now...Regarding the video. At timestamp 0:03, we see the J-20's canards are in what seems to be full deflection, but there is a crucial visual clue that most would miss: LEADING EDGE DOWN.
Whether it is leading edge (LE) down or up is important for the flight controls engineer. It depends on the MODE OF OPERATION or to put simply -- what the aircraft is doing at that moment in TRANSITION to what you want the aircraft to do.
What you want the aircraft to do implies a FUTURE mode of operation. So at the time of change, as the pilot changes cockpit switches, the jet will know how to deflect the flight control surfaces to make controls smoothly and safely.
So what is so significant about LE down?
If it is LE up, you would create a nose-up condition, which when you are moving on the ground trying to land in as short a distance as possible, a nose-up condition would be a very bad thing to do.
Further, we do not want this level of deflection while the jet is still flying. Leading Edge (LE) down is nose-down. Full LE down command while still in the air would mean a crash. So we install a safety condition call 'weight-on-wheels' (WOW). All aircrafts has WOW automatic switching. Another word is 'squat switch'.
http://www.askacfi.com/20020/squat-switch.htm
The F-16 has three WOW switches -- one per gear -- and there is a logic to this.
So if the J-20's flight controls engineer want to design a safe speedbrake system using the canards, how would he do this LOGICALLY?
If you have WOW on the main gear, that means the jet has only a PARTIAL touchdown condition. Whether the jet is in a take-off or landing mode, partial WOW means a partial ground condition. You want WOW on all three landing gear struts before the avionics fully reconfigure itself for landing.
So in general principles, the logic would be in this sequence:
- Cockpit switch activation (this essentially prepare the avionics to let the system know that you want to land)
- Main WOW switches active (this tells the avionics that the jet is partially on the ground)
- Nose WOW switch active (this tells the avionics that the jet is fully on the ground)
- Canards LE down
All three WOW switches must be active in order for the canards to deflect LE down. The avionics should not expect first main gear WOW, then nose WOW. The logic should not be a 'first-then-second' or sequential condition. The logic should be a simple 'and' condition because there will be times when a jet could land with all three landing gear making ground contact at the same time. It would not be a smooth landing but it is possible, so we just want to know when the jet is fully on the ground.
Now we come to the vertical stabilators and how they could be used as speedbrakes.
Note the F-18's vertical stabs, especially the rudders. And note that the vertical stabilator and the rudder are not the same thing, even though people uses the two words interchangeably. The stab CONTAINS the rudder. Or the rudder is a component of the stab.
The F-18 is clearly taking off as we do not see the arresting cable anywhere. The rudders are pointing inwards, or in a 'toe-in' condition. This condition assists the horizontal stabs in generating down force, which means nose-up, which assists take-off. This toe-in condition also exists on landing to generate additional aerodynamic drag.
What is the difference between 'toe-in' and 'toe-out' ? Certainly they generate some kind of forces but also certainly those forces are different in directions.
- If the vertical stabs are canted (angled) outward, rudders toe-in would generate downward force to assist nose-up. So to generate drag to slow down the jet, rudders should be toe-out.
- If the vertical stabs are canted (angled) inward, rudders toe-out would generate downward force to assist nose-up. So to generate drag to slow down the jet, rudders should be toe-in.
The above two rules are not absolute as the F-18's avionics uses toe-in and toe-out with different angle-of-attack.
So for the J-20, until someone post a video of the J-20 landing and showing from the rear perspective, we do not know for certain how the J-20's flight controls engineering staff uses the vertical stabs during landing. But we can have a high degree of confidence that the J-20's vertical stabs are used in some ways as speedbrakes. Even though the J-20 do not have rudders, the all-moving stabs can still be used in the same ways as the rudders.
This does not mean the J-20 can exhibit true S/TOL capability. Speedbrakes are used by the F-15, F-16, and F-22 and they do not have true reduced runway length landing capability. For that, we need thrust redirection, aka 'reverser'.
I was just curious after seeing pictures/videos of the canards and vertical stabilizers in positions where they can serve as speed brakes. This in addition to J20 eventually having more powerful engines(for possible shorter take off) led me to think that STOL may be possible. Thanks for ur detailed explanation...much appreciated.