F-5 Pitch trim question.

F-5 Pitch trim question.

I came a crossed this thread just out of boredom, but in my experience with airliners, I know apples and oranges, but airliners set the trim to certain values not just for proper rotation rates/control forces but also for Incase of engine failure at rotation the aircraft is already trimmed for single engine flight. So even the big fat jets have to retrim. Some even have airspeed limits to retrim because the aerodynamic loads will stall the motor to move the stabilizer.

Just a thought.

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I have never noticed the stick movement before but I started up on the ramp and moved the trim and my stick in fact moves with it. And my trim is working normally, so I believe the stick movement is not relevant.

 

That mouse/keyboard thing is weird...did you try clearing the keyboard binding and setting it to something else?

 

You don't have to do anything to make the trim indicator display, it is a small gauge directly above your ADI. It is the gauge you use to set your elevator trim before takeoff. If this gauge is moving back and forth from stop to stop you indeed have a trim problem, I would submit a track file of this. If this gauge isn't moving, I suspect the issue is actually an elevator axis binding problem masquerading as a trim problem.

 

 

 

EDIT: Did some research on the stick movement. USAF TO 1F-5E-1 is very thin on the flight control section, but it describes the horizontal as "an all moveable horizontal tail" and it says the pitch trim indicator "indicates trim position of the horizontal tail." It also says "Artificial "feel" is built into the system, and electrical trim actuators change the relationship of the "feel" springs to the control stick." This is all on page 1-57. The DCS manual says the trim gauge indicates "pitch trim position (control stick position and horizontal tail position accordingly)." That is page 106. This leads me to believe movement of the stick when the trim is moved is a correct feature. Maybe someone with RL F5 time could speak to that though...

This is common on aircraft with hydraulically controlled surfaces, the flight controls move actuators which in turn move servos. there's no physical connection between flight controls and control surfaces. To account for the lack of input feedback, feel springs are used. the stick moving with trim is part of the system. I've flown 3 different aircraft (10 variants) that have systems like this.

Interesting. I thought most of these systems have a manual reversion, in case of hydraulic failure. Might as well since you still have a cable/pulley, going to the actuator itself. Unless it’s FBW System, now that is indeed no mechanical cable.

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there's no physical connection between flight controls and control surfaces.

Well I think that is not entirely correct. My experience is limited to the F-104 but the basic setup of flight control and hydraulic systems are always the same. I have no experience with the F-5 in any means, so I could be wrong. It is of course true that the flight controls system works indirectly as the pilots inputs are translated via hydraulic power. But still it is a reversible system as the control surfaces and the controls in the cockpit are all interconnected. In the 104 and the MiG-21 you can move the control surfaces without hydraulic power, I am quite sure the F-5 is no different. In a normal hydraulic Power Control Unit the linkages from the control stick actuate a control valve which directs hydraulic fluid to either side of the actuating cylinder that in turn moves the related surface. But you must keep in mind that it is not enough to set the control surface in a new position it must be locked in that position or the airloads will simply push it back. Therefore, the control surface is also connected with the input linkage from the stick and as it moves to the position associated with the stick position the control slide valve is returned to it's blocking position. Now the actuator is hydraulically blocked and the surface locked. That is how manual hydromechanical flight control systems in military jet powered aircraft work, simplified of course. So there is a connection between cockpit and controls. Such a system is often called irreversible as no forces are transduced to the pilot but in the sense of a technician it is reversible as there is a mechanical linkage. That is not the case if the actuating cylinders are servo controlled which is the case in aircraft with electronic flight control systems (fly by wire).

As I said all of that must not apply to any airframe and I have no clue if it's case with the F-5. But I see no reason why the setup of the system should be completely different in that specific aircraft. I did not follow this discussion from the beginning, so I hope this is not redundant knowledge.

Some aircraft have manual reversion capability, I'd think if the F-5 had it, you should have hydraulic pump switches in the cockpit. Some aircraft, like you said, use cables to control PCU's, one particular airframe I've flown had cable actuated PCU's and triple redundant hydraulic systems, but no manual reversion capability. Another had manual reversion capability but is like stirring molasses with the hydraulics off. I'd imagine the control forces on the F-5 would be significant. I've got a buddy that flew 38's, I'll pimp him for some clarity.

I'd imagine the control forces on the F-5 would be significant.

Yeah, by no means you would be able to move the controls in flight without hydraulic power. In the F-104 you can move the controls on the ground but even without any air loads it requires a lot of strength. And you can barely move the big stabilator as it simply is too heavy. Would be very interesting to hear about the 38!

I'd really like to know which aircraft can be flown without any hydraulics operating.

Those with a mechanical control linkage system. You're welcome :D

So, My buddy that flew '38's (was also an F-22 Test Pilot) seems to remember that if you lost both hydraulic systems, you were S.O.L., as an aside, the 737 is one of the last (if not the last) commercial aircraft that has manual reversion capabilities (the a/c I currently fly).

Total hydraulic loss on a modern airliner is extremely unlikely.

On fighters like the F5 albeit the probability of a hit causing a total hydraulic failure was very much real I don’t think a fail safe was in the designers agenda anyway. After such a devastating hit you where expected to bail anyway.

I remember on 1.5 on the Cold War server my F5 took a few rounds causing an engine fire on the left engine. I could perform an emergency landing as I was literally next to a friendly airfield but the hydraulic loss on the remaining hydraulic made the approach an eye opening experience.:pilotfly:

I guess I was lucky I had the flight controls hydraulic working long enough to get me down.

They can’t fly without hydraulics because it’s impossible to provide the control forces needed anyway else. Still unlikely is not the same as impossible.

Same happened to the DC10 in Sioux City.

The A-10 has an emergency Manual Reversion Flight Control System. It is the same: cables moving the trim tabs.

From memory, so I could be wrong, back in the 1991 Gulf War it was not allowed to land the A-10 in manual reversion but several pilots going back with damage tried it, some with better luck than others.

A guy died in 2003 trying to fly an A10 back like that.

Little reason to have such a system except perhaps to give a pilot a few precious miles before bailing to distance himself from the guys he just bombed...

Well, the 737 is a different story because the ailerons and elevators have balance or servo tabs.

 

Without these, you couldn't move the flight controls in case of a double hydraulic system failure.

 

Aircraft like the F-5, F-15 etc, don't have such tabs.

There are no balance or servo tabs on the 737, that's why it is so difficult to fly in manual reversion.

F-5 Pitch trim question.

That would make sense why RJs small turboprops have manual reversion and jumbos don’t. Nice discussion guys!

So no tabs would make a reversionary system ‘unable’. Therefore you can’t design an aircraft to have transonic capabilities without the flying tail design. Hence F5 doesn’t have a backup flight control.

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It’s been years since initial ground school, I guess I need some remedial training!

That would make sense why RJs small turboprops have manual reversion and jumbos don’t. Nice discussion guys!

 

So no tabs would make a reversionary system ‘unable’. Therefore you can’t design an aircraft to have transonic capabilities without the flying tail design. Hence F5 doesn’t have a backup flight control.

 

 

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Anything certified under part 25 requires some sort of redundancy. Multiple hydraulic systems etc.