Hello!

I don't know if you remember or not, but quite awhile ago we participated in an AH post on AH's trim effects upon main flight controls.

This is not a rehash of it but rather a series of questions I wanted to ask as I am now working on a different sim and needed some good information.

1: How much pressure on the stick can you generate with trim? For arguements sake lets use the Elevator trim. At hands off, if you crank the trim wheel all the way back will it put say, 10, 15 20 lbs of pressure on the stick? Keeping in mind the level of the trim tabs on a 109 you had the picture of, do you think a WWII fighter generated more or less than you have had in modern ones?

2: In situations where you are pulling the stick as hard as possible have you ever reached the point where you could not pull any harder on the stick? Do you think pulling back trim would have given you more leverage?

Thanks for answering!

I'm not andy, but I'd say that trim pressure is more dependant on the plane you fly.

Plus, the tabs being on control surfaces, they will move the control surface by similar angle for a similar setting, since the forces induced by the airflow on the tab will be proportionally "equaled" to those induced by the airflow on the control surface.

Besides, the pressure for a certain result depends on speed (the faster you fly, the more pressure you need to apply) unless totally assisted controls (I've read that at full speed you could need over 30 lbs of lateral pressure on the stick to make a bf-109 roll, and with such a small cockpit it was really difficult just to deviate it when making a power dive) hydraulically or electrically.

As far as how much forces were generated before and now, there's one thing that remain constant, the control surfaces (especially tail (rudder)) were undersized in WWII, and this on all planes AFAIK

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coolaero.altajeux.com

Hi Sorrow!

Good subject...I don't mind talking about it at all!

But...it's not a subject that can be reduced to 25 words or less. You have some really good questions. I'll try to provide some answers in the form of generalities.

Let's begin with the idea of maximum stick force as a function of trim. I would say that no aircraft is designed to allow the pilot to trim the aircraft to an extent that the pilot could not manually hold the stick forces in a complete out of trim situation. The stick will be very 'heavy', but not so much as to be beyond typical human capability.

So, having said that, how much is 'heavy'? 10#...20#...100#? Depends on the aircraft type and the situation. High speed or slow speed...it doesn't really matter...the only thing that changes is the direction of stick force. This means that in slow speed situations, when the stick in fully 'out of trim', the pilot feels very heavy back stick force...he has to pull back hard to keep the nose from falling. In a high speed 'out of trim' situation, the pilot feels just the opposite...a very heavy nose 'up' feel...he has to push forward hard to keep the nose down.

I believe this situation would feel much the same in a Me-109 as it does in the DC-9 that I fly...or in the F-4 or A-10 that I used to fly. The actual number of pounds might vary slightly, but the overall effect will be very similar.

As for the effect of trim on stick effectiveness...now we get into a very grey area. We have to differentiate between tailplanes that are one piece (like a F-4) or tailplanes that have elevators (like a Me-109). We also must look at the impact of mach number on the elevator/tailplane.

I do not think that you would gain any more aerodynamic advantage in a Me-109 by using trim once you had the elevator moved to its physical limit. (But I may well be wrong on this...maybe an aero expert can clarify this issue). My understanding is that (as a rule) trim will effect stick 'feel', not stick effectiveness.

But...this is in 'normal' situations, and does not consider the question of all-moving tailplane (like in a F-4) vs a conventional horizontal stabilizer/elevator. As airspeed approaches the mach, pressure changes alter the trim situation beyond what is usually considered 'normal'.

In the early days of airplanes approaching mach 1, it was eventually discovered that trim could provide a control input when 'normal' stick forces were impossible to make. In these instances, aircraft were recovered from high speed dives using trim.

But these are specialized cases. My experience is that trim only affects stick feel, not effectiveness, when we are talking 'in general' situations.

Hope this isn't 'clear as mud'!!

Andy



[This message has been edited by Andy Bush (edited 07-21-2001).]