Flight Control Modeling...

[Cobra412]

I've just got a curious question about how the flight controls were on WWII aircraft.  Now I don't know if this is just something that newer fighter aircraft have but the ailerons seem to deflect differently.

When going to the forward stick position then coming left or right around a box pattern the ailerons stay at full deflection.  As you follow the box pattern around they constantly stay fully deflected.  Is this normal?

I'm just going off experience with the F-15 and when you come around the box pattern the aileron deflection decreases some in the forward right/left and aft right/left positions and starts to increase again at the full left and right positions.  This is due to how they are routed through a mixer assembly and offer better roll coordination.  Now the ailerons are a non boosted item to an extent on the F-15.  They do get limited as airspeed increases but even on the ground the difference is highly noticable.  Once the roll ratio system has gotten to a certain point they aileron movement is close to nil due to lack of boost from a Pitch and Roll Channel Assembly which is hydro mechanically connected to the push pull rods.  We are talking aircraft that are doing Mach speeds not subsonic speeds such as the WWII fighters so the boost function wouldn't be completely needed.

Now I doubt they have an aileron rudder interconnect in the WWII fighters to allow for better turn coordination but wouldn't atleast the horizontals and ailerons be coordinated in such a manner?  I would just think that this in itself would make the flight model more realistic if it is infact true.

[Ecliptik]

I can't imagine the control mechanism for any WWII era plane was even close to as complex as the computer aided hydraulics in an F-15.  I know a few aircraft (at least, for example, later model P38's) had hydraulically boosted ailerons, but for most, it was fairly standard cables, pulleys and rods with no special control systems, powered by nothing but muscle on the stick/yoke and pedals.

[HoHun]

Hi Cobra,

The only control cross-influences I'm aware of for WW2 aircraft are connected with the use of flaps. The Me 109 for example deflected both ailerons downwards when the flaps were deployed while the A6M reversed the Flettner tab linkage.

I don't think there was any connection between aileron and elevator circuits. It probably was up to the pilot to avoid large aileron deflections near the critical angle of attack.

Regards,

Henning (HoHun)

[Cobra412]

Ecliptik the thing about it is that while the F-15 is not being ratio controlled the output is pretty much pure mechanically driven until it gets to the actuators themselves.  The horizontal and vertical controls are the only ones that are hydraulically actuated.  

The hydraulic control over the rudders is the boost function which is a single control rod between the Pitch and Roll Channel Assembly (PRCA) and the Aileron Rudder Interconnect (ARI) assembly and is ratio controlled depending on airspeed and altitude by the PRCA.  They also have actuators right at the surface itself but will only output what it is able to if in the event they are being ratio controlled. The ailerons and horizontal stabilators are also ratio controlled directly at the PRCA and there output is limited in the same manner as the rudders.

The automatic flight controls system (AFCS) only has a very small part in the whole process.  When the aircraft is airborne the AFCS is only fault monitoring the PRCA and ARI.  All airspeed and altitude inputs are direct inputs to the PRCA.  

If in the event the AFCS fails itself the control augmentation system (CAS) will be shut down and leaving a very primitive hydro-mechanical flight controls system.  Again even in this configuration the ailerons will still deflect as normal but will be ratio controlled through the boost function of the PRCA.  It is how they are routed through mechanical mixer assembly that causes the decrease in deflection depending on where the stick is positioned in the box pattern and not by some external electronic  or hydraulic control.  

If the airspeed and altitude weren't a major factor for the F-15 and how much the system needed to be limited then in essence the boost control could be removed leaving only the basic mechanical inputs with hydraulic actuators at the rudders and horizontal stabilators.  It's kind of hard to explain since you can't actually see the mechanical inputs and where they are placed in reference to the PRCA and ARI.  But basically the mechanical controls that allow for this type of deflection are actually aft of the PRCA and ARI. You have one lateral control rod and one longitudinal control rod through the PRCA to the mixer assembly and then you have one control rod for the directional control through the ARI.

The whole hydraulic control shouldn't make a difference to be honest.  It's actually how the mechanical controls themselves are setup that allow for this type of deflection.  Since ofcourse the only reason for the hydraulics is to easy the pilot load.

[Ecliptik]

Thanks for the explanation of the F-15 control assembly, Cobra.  I knew the Eagle wasn't fly by wire or anything close to it, but I thought it would have been at least one of the first planes with some external electrical sensor input affecting control output.  I guess I was incorrect in that assumption.

Anyway I'm not aware of any WWII vintage aircraft that had ratio assemblies changing control surface deflection on a particular axis based on stick input along different axes.  Fully deflected ailerons would remain so throughout the full range of elevator or rudder movement.

[Cobra412]

There is some parts that are electronically controlled.  You have the ability to force the system into an emergency mode which limits your deflections to a predetermined amount.  And does have an item built in that can sense g forces.  But the rest is all ratio controlled by impact pressure and static pressures.

I was just curious is all..