spazsinbad wrote:Interesting tidbit dit about VLing the F-35B with weapon bay doors opening only just before touch down by Steve George.
Info on the F-35B STOVL flight controls01 Oct 2018 'Engines' [Steve George]"Going back to the start of the programme, the customers were very focussed on reducing through life cost and enhancing safety. The lessons of the USMC AV-8A fleet and the UK's Harrier experience were uppermost in their minds. They were also looking at reducing the pilot's flying workload as much as possible to allow him/her to focus on managing and winning in combat. They also wanted an aircraft that required less pilot training. These desires all came together in looking very hard at how to best control a STOVL aircraft in the transition, takeoff and landing.
These were crystallised out in the JORD in two ways. First, they demanded very good handling characteristics. secondly, they mandated that the flying controls that were operated by hand had to be limited to two 'inceptors'. The led to many studies and trials, and out of these came the F-35B system. I should note the huge contributions made by the BAE STOVL test pilots throughout the early years of the programme - their inputs, suggestions, criticisms and teamwork laid the foundation for what the team has achieved. A special mention should also go to the pioneering VAAC Harrier test bed, designed at Cranfield and put to hard work at Boscombe and also at sea, helping to develop the new STOVL flight control laws. It should be understood that two of the highest risk areas of the F-35 development programme were the integration of the STOVL propulsion system with the airframe, and the development of the STOVL flight control laws and flight control system. Both of these were led and executed by BAE Systems. Personally, I don't think they get anywhere near the amount of credit they are due.
The end result is a 'powered flight mode', which the pilot selects by a single action switch on the left hand 'inceptor'. This initiates the change from fully wing borne flight, and it can be selected over quite a wide speed range. In this mode, as I've posted earlier, the controls change from a 'throttle' (more like an energy demand) and 'stick' (more of a flight path demand) to a 'fore and aft rate' demand via the left hand inceptor, and a vertical rate and lateral rate demand via the right hand inceptor. This was a very controversial decision at the time, but was supported by extensive work in both conventional sims and the vertical motion simulator at NASA Ames, I believe. One aspect of this new flight control system is that the pitch of the aircraft is not controlled by the pilot.
However, the transition from 'normal wing borne' to 'full stop powered lift' is managed via an automated 'blend' programme which, I understand, is mainly driven by aircraft airspeed. So, the excellent pictures Gums posted up very probably (alert - I'm now assuming stuff) show an F-35B in the high speed end of the 'powered lift' mode. The landing gear is still up (this is selected independently from the powered lift mode selection), the lift fan door is in the 30 degree position for higher speeds, and the 3BSM is down at around 10 to 15 degrees or so. All the various doors are opened at this stage,
except the inner weapon bay doors that deploy automatically just before touch down. In the powered lift mode, thrust can be moved between the front lift fan and the aft nozzle - it's not a fixed 50/50 spilt. So, in these pics, the lift fan IGVs will have shut down, reducing the power demand from the fan, and allowing more 'grunt' to come out of the rear....”
Source: https://www.pprune.org/military-aviatio ... st10263295
A couple fine points --
"Powered-lift mode" (what the guys flying it refer to as 'Mode 4') is initiated by actuation of a small, roughly 1"x1" square button on the left side of the forward instrument panel. In the 'A' or the "C' the same button deploys the hook. Once the jet has transitioned to Mode 4, there are HOTAS functions that allow control of airspeed in one-knot or 10-knot increments via the left inceptor -- if so-desired. The alternative is movement of the left inceptor (conventionally speaking, 'the throttle') in/out of the detent position that is commanded with selection of Mode 4. We talked about this at length in the long thread.
In Mode 4, thrust split between the main engine amd the lift fan is completely controlled by the integrated flight-propulsion control system. The pilot's input to the system is his or her airspeed command via the left inceptor/HOTAS function. As the jet flies slower, the pilot has progressively less control of aircraft attitude in pitch; at some airspeed, control in that axis is completely controlled by the jet. The pilot can effect roll, yaw and height function (Z-axis) via the right inceptor. As one poster at pprune astutely noted, if one thinks about the controls in Mode 4 like formation work, the mental model for what to do with the jet is very easy.
A better semantic description of what the lift fan VIGVs do (as opposed to 'shut down') is to vary mass flow to the fan while the vane box nozzle (VBN; in some places VAVBN, as in 'variable area...') controls the output vector along the longitudinal axis of the jet (I've forgotten the entire range/degree of movement). Of course, the smart engine guys can describe the pressure in/pressure out thing that is being managed there as well.
Thrust split is an under-appreciated part of what the flight propulsion control system does. One of the coolest examples is the use of thrust split and differential vectors to pitch the jet during short takeoffs. There are some ship STO videos that illustrate (that are around here for the price of a search).
