BFF Control Loader (Force Feedback) - Selected Behaviours

 

Aileron Stiffness and varying Airspeed

 

"At low airspeeds, the controls usually feel soft and sluggish, and the airplane responds slowly to control applications. At high speeds, the controls feel firm and the response is more rapid." (Pilot's Encyclopedia of Aeronautical Knowledge, FAA, 2007).

 

The spring-like feel of flight controls is mainly due to the force required to displace a control surface into the moving airstream – the larger the displacement the larger the force required. A neutral force position exists to which the surface is returned by the airstream when it is released.

 

Further, as airspeed increases, the force required to displace the control surface into the airstream by a set amount also increases. This ratio of force to displacement is the control "stiffness". In flight controls stiffness usually increases with increasing airspeed – hence the FAA statement above.

 

In conventional sprung joysticks the spring stiffness is constant – ie they behave like real flight controls which are operating at the same airspeed all the time. There is no difference in control feel as airspeed changes.

 

In an aircraft in which the flight controls are connected directly to the control surfaces (such as a light GA) stiffness usually varies with airspeed squared. If the aircraft is a fly-by-wire type then there is no direct mechanical connection between the controls and the control surfaces and the relationship between stiffness and airspeed can be anything the manufacturer wants it to be. Here stiffness might be set to vary linearly with airspeed for example. Both situations are definable in the BFF CL system (and anything in between).

 

The graph left shows typical BFF CL system aileron control stiffness variation with airspeed. Each line on the graph plots aileron force (or torque) against control displacement at a given airspeed. As the airspeed increases, the force response lines steepen. Here force is set proportional to airspeed squared – it is from the BFF CL Software with FSX flying it's default Beech Baron. If the CL configuration was set for stiffness to vary linearly with airspeed there would be a more gradual steepening of the force lines – as in a heavy jet for example.

The overall effect is soft control feel at low speeds such as on landing approach, and tighter control feel when the aircraft is at cruising or higher speed.

 

Level Flight Elevator Forces at varying Airspeed (cf Aircraft Positive Longitudinal Stability)

 

The above control stiffness effects are also seen in the elevator axis off course. An additional elevator axis force effect in the BFF CL System is a further way in which elevator forces can vary with airspeed - this time when the aircraft is in level flight.

The longitudinal stability characteristics of an aircraft tend to induce nose-up pitching when airspeed increases and nose-down pitching when airspeed reduces.

 

In contrast maintenance of level flight as airspeed increases generally requires nose-down adjustment to reduce the wing angle of attack and so hold the aerodynamic lift at the same level in the face of the rising airspeed. Conversely nose-up adjustment is needed when airspeed reduces – to increase AoA and maintain the same overall lift at the lower airspeed.

These pitch adjustments are made using the elevator controls off course, and the net effect in level flight is to require increasing push force on the elevator controls as airspeed rises above the trimmed speed. And, increasing pull force to hold level flight with airspeed dropping below the trimmed speed.

 

This control behaviour is a requirement of the FAA eg FAR 23.173 "A pull must be required to obtain and maintain speeds below the specified trim speed and a push required to obtain and maintain speeds above the specified trim speed."

 

Empirical and estimated data illustrating the effect can be seen here – the graph is for a B-35 Bonanza. It shows elevator stick force at varying airspeeds – trim was at 120 mph (zero stick force). You can see on the 1G line that to maintain the level flight of the aircraft below 120mph the stick has to be pulled with forces up to about 10lbf, and above the trim speed it has to be pushed with forces up to 30lbf. (Full article here.)

 

This important elevator force effect is implemented within the BFF CL System. The graph on the right shows BFF CL System elevator force against airspeed for a light GA aircraft trimmed at 130 knots in level flight. To maintain level flight below 130 knots the stick requires pull force, above 130 knots it needs to be pushed. The data was for the FFB Flight Yoke with brushless drives, running FSX and its default Beech Baron aircraft.

 

The force capacity of the DIY yoke is lower than the fairly large forces generated by the real aircraft, however the underlying behaviour is correct. Normally off course the elevator forces are trimmed out so that you are not left fighting the controls just to hold level flight – the BFF CL System allows this realistic trimming also.

 

The overall effect when using the BFF CL System is that the pilot feels changing elevator stick forces as the aircraft speed increases or decreases in level flight consistent with the normal positive longitudinal static stability of an aircraft. These variations require trimming and are a useful reminder of the actual behaviours that would be experienced in a real aircraft.

 

Effect on Longitudinal Stability of Rearward CG

 

A characteristic effect of loading an aircraft so that its centre of gravity (CG) moves rearwards is to reduce its longitudinal (pitch) stability. Eventually, if the CG is moved far enough rearwards, the aircraft may suffer from severe pitch oscillations and become very difficult to control - the speed of response of the pilot's control actions becomes insufficient to control the unstable aircraft. In extreme cases the elevator stick forces required to control pitch can actually reverse in direction.

 

The longitudinal stability reduction associated with a more modest rearward shift of CG shows by the elevator stick return forces increasing less steeply when the airspeed moves away from the trimmed airspeed. Effectively the curve shown in the above graph becomes flatter.

 

The graph on the left shows this effect with the BFF CL System - it shows elevator stick force v true airspeed for a light GA aircraft in FSX trimmed at 130 knots and in level (1G) flight. The steeper curve shows the response with the CG at a relatively forward position of approx 20% of wing chord, the shallower curve shows the effect of moving the CG rearwards to about 45% of the wing chord. The overall aircraft weight is the same for both flights.

 

You can see that the CG = 45% out-of-trim forces are much less than those at CG = 20%. The stick returning forces in the CG = 45% chord case are about half the strength of those in the CG = 20% chord case for a given difference between actual and trimmed airspeed. In the CG = 45% case the simulated aircraft was prone to strong pitch oscillations and required much more concentration to fly.

 

The BFF CL System accommodates these behaviours for aircraft with reversible controls - the pilot will feel the correct characteristic changes to the control feel associated with rearward shift of the aircraft CG. Movement in airspeed away from the trimmed airspeed will become less noticeable through the elevator stick force when aircraft CG is moved rearward.

 

Although the force levels may not match exactly those of the real aircraft the simmer will feel the correct general behaviours in the force feel which adds further to the realism of the flight simulation when flight controls using the BFF CL system are in use.

 

 

 

 

 

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