Forum: F-35 Spotting & Photography

This .DOC was downloadable but with a change in DTIC website it seems to have disappeared. Maybe I'll find it there again. From a PDF made from the .DOC file (which messed up the equations) here is some text to explain not about 'roll posts' but pitch control.

The Shaft Driven Lift Fan Propulsion System for the Joint Strike Fighter
Paul M. Bevilaqua ASTOVL Program Manager

http://www.dtic.mil/dticasd/sbir/sbir032/n184.doc

"...The primary objectives of these tests were to prove the feasibility of changing the operating point of the turbine section to provide power for driving the lift fan, and the ability to rapidly transfer thrust from the cruise engine to the lift fan and back, in order to provide pitch control power. The dual cycle operation of the engine was successfully demonstrated by connecting the lift fan to the engine, and then increasing engine speed to full power along the STOVL operating line [4].

Pitch control is obtained by coordinating the area change on the cruise engine nozzle with the movement of the lift fan inlet guide vanes. At constant engine speed, increasing the nozzle area produces more turbine power and reduces the engine thrust. Opening the inlet guide vanes produces more lift fan thrust. If the movement is coordinated, thrust is transferred from the aft nozzle to the lift fan, while the total thrust remains constant. This provides a large pitching moment which can be used to control the aircraft in hover. Since thrust transfer is accomplished without changes in engine speed, high response rates are achieved. In addition, the pitch control loop is decoupled from the total thrust control loop which is used to command changes in sink rate...."

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Great "Job" on this thread, very enjoyable with lots of info. What attracted me to the photos was the underview of fan discharge doors and a "passing thought" of how much clearance is there between said doors and the attachable centerline gun pod that recently went into production? That said, the Bee is demonstrating flight with the doors open and gear up. If this is a normal operating mode, then can one assume that in this "near hover" mode, gun strafing or precision strike is surgically capable for those of us interested in it's CAS capabilities and tactics? Smile

Last edited by spazsinbad on Feb 10, 2012 - 09:27 AM; edited 1 time in total

Yep interesting question. Here is a bit more about the 'roll post nozzles'.

Rolls-Royce's LiftSystem for the Joint Strike Fighter
28 March 2011 | By Ellie Zolfagharifard

http://www.theengineer.co.uk/in-depth/r ... 08.article

"...Stabilising this force are two small ’roll posts’ in the wings. These ducts each direct 2,000lb of thrust during short take-off and vertical landing. Compact flap mechanics regulate the amount of thrust produced by each of the roll posts. During short take-off, the roll posts are opened..."

Super stuff guys. So now brace for the dumb question 1 what is the difference between roll control and lateral stability?

From the CodeOneMagazine article:
Butterfly Valves
The X-35B had butterfly valves at the interface between the engine and the ducts leading to the roll nozzles. These valves were removed to save weight. The roll nozzles themselves now serve as the cutoff valves for the roll control air from the engine. The performance requirements for the F-35B roll nozzles are also greater because they have to deal with asymmetric external store loads.

Dumb question 2, isn't it curious that moving the roll control nozzle hydraulic shutoff valves and their control wiring and fluid filters and sumps all the way out to the end of the ducts (and thereby ensuring that a hot air tube will be a constant presence throughout the center of the wing with a direct path to the engine compressor) could possibly be just to save weight?

I am going to make a wild guess that there was airframe space conflict near the location where they were and/or it reduced the engine/airframe commonality too much to keep them there.

One answer from online: "Lateral stability (rolling). The stability about the longitudinal axis of an aircraft. Rolling stability or the ability of an airplane to return to level flight due to a disturbance that causes one of the wings to drop." Whereas 'roll control' is just that the ability to control rolling movements - however the aircraft has a 'stability' (necessary because by now it is completely "jet borne") that will work against that (with the computer controls deciding what to do with any pilot inputs). Apparently at 100 feet in a hover the F-35B will not move, then the pilot has minute control over where to place the aircraft in small increments (in a special mode).

OF course in this hover mode if anything malfunctions then an automatic ejection sequence kicks in before the pilot can ever realise what is happening. So the jet borne hover flight must be stable but influenced in a small way for controlability to make small movements. The aircraft will not make large movements in hover mode.
__________

"I am going to make a wild guess that there was airframe space conflict near the location where they were and/or it reduced the engine/airframe commonality too much to keep them there." Are you suggesting that the 'roll post nozzles' were moved? I'll have to get back to that - sorry run out of time.

Found the compilation pics: Going Vertical http://www.codeonemagazine.com/gallery_ ... lery_id=14

http://www.codeonemagazine.com/images/m ... 7_9911.jpg

Test Flying The Joint Strike Fighter by Graham Tomlinson

http://myweb.tiscali.co.uk/hawkerassoci ... ghter.html

"...The STOVL mode control system is derived from ‘Unified’ developed by the ‘RAE’ on the VAAC Harrier. The throttle commands acceleration and deceleration (or thrust on the ground and in the STO mode, and in all conventional modes); in the hover the stick moved backwards/forwards commands upwards/downwards vertical velocity (or pitch rate elsewhere); in the hover the stick moved from side to side commands bank angle (or roll rate elsewhere) and if released returns the aircraft to wings level; in the hover the pedals command yaw rate (or sideslip elsewhere).

Future development will clear full envelope autopilot/auto throttle, automatic deceleration to a spot, and TRC (translational rate command) which in the hover allows the pilot to make small positional corrections easily, and will then bring the aircraft to a standstill if the pilot releases the controls....

...The F-35B fly-by-wire system gives angle-of-attack and sideslip control, and departure protection. Further pilot workload reduction is given by performance deficit protection, conversion speed window protection and FOD protection warning; and flight test has a watching brief on the requirement for possible tail strike protection during slow landings (currently not considered necessary). Pilot cognitive errors (of trying to control thrust with the throttle) have been mitigated in the design. In the unlikely event of the lift fan failing catastrophically the aircraft would pitch inverted in 0.6 seconds, and the pilot is protected by auto-ejection signalled by pitch rate and attitude (derived from the YAK 38 & 141 systems).

The flying controls are powered by electro-hydraulic actuators (electric power to hydraulic pumps at the control surfaces). The IPP (integrated power pack) is a combined gas turbine and electric starter/generator. After starting the main engine, bleed air keeps the IPP spinning all the time to provide ECS and cooling air and standby power generation. Should the bleed air fail the IPP reverts to a gas turbine mode. To convert from the CTOL to STOVL mode a button push opens the necessary intake doors etc (13 in all), prepares the engine and engages the lift fan clutch which transmits 28,000 shaft hp...."
&
"...Problems in the early development testing, which are addressed in the production aircraft, included: clutch drag in conventional flight, driveshaft length issues due to expansion/contraction, intake door structure, roll post heating (it is a continuous bleed system), sideslip in wind-up turns, nose high attitude in land-aheads from hovers, and HMD vibration and latency issues.

In the following areas where problems might have been expected there were none; hot gas ingestion, ground effects, weight-on-wheels operation (gives signals to aircraft control systems), conversion dynamics, performance, deficit protection and help from mission control...."

A good discussion of the roll posts by Graham Warwick this past December --

http://www.aviationweek.com/aw/blogs/av ... 925abcbdfa

Thanks, previous page on this thread has that info as well as a ZOOM graphic of the top of the 'roll post nozzle' from graphic on that Warwick page PLUS some text quotes.

Only thing to clarify and reiterate is that F-35 RPN air is bypass air. Google around and there is lotsa semantic reference to 'bleed' and 'continuous bleed' etc, but F-35 RPN air is bypass air -- i.e. down stream from the LPC (not HPC bleed). Harrier RCS is 8th stage HPC bleed and much hotter than bypass air temps measured at the F-35 RPN exhaust point.

Forum: F-35 Spotting & Photography

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