What's Up With Downsprings? BY ED KOLANO

A

downspring pulls the control stick forward. Another way to think of it

is pulling the elevator trailing edge

down or the airplane nose down. The downspring adds to the aft (pull) stick

force required to fly slower than trim speed. It also adds to the forward (push) stick force needed to fly faster than trim

speed. Let me get this straight. The downspring pulls the stick forward. Yup. I can accelerate and maintain the faster airspeed by holding forward stick. Yup.

The push force necessary to hold the faster airspeed is higher with the downspring installed than it would be without the downspring. Yup. Whoa... pretty deep. Figure 1 shows a functional diagram

of a downspring incorporated in an airplane's longitudinal control system. The spring is pre-loaded, meaning it is in constant tension. So it tends to pull the stick forward regardless of stick position. (Note that installing a compression spring behind the stick would have the same downspring result.) Springs exert a force proportional to how far they're stretched or compressed. There is a linear relationship between the spring displacement and the spring force.

That is, a spring stretched twice as far exerts twice the force. Three times as far — three times the force. Many downsprings,

however, are "sized" such that the force it exerts on the stick is virtually constant.

via cables, pushrods, etc. As a result, the

force on the stick; airspeed has no effect.

pilot can feel the air load exerted on the surface through the stick and pedals. The

Adding the downspring effect to the airspeed effect on stick force (Figure 2a)

faster an airplane travels, the higher the

results in the curve in Figure 2c. In other

forces exerted on a deflected control sur- words, the curve in Figure 2c is simply the face. Unlike the linear relationship curve in Figure 2a shifted up by the amount between spring force and displacement, however, the force exerted on the control

surface varies with the square of the airspeed (V 2 ). So, an airplane traveling twice as

fast (Vb = 2V a ) has a force exerted on its elevator four

times as much (Ft, = KVb2 = K{2Va}2 = K4Va2), assuming

the elevator has the same deflection in both cases.

FIGURE 1

It doesn't make much sense for the elevator to have the

same deflection at two different airspeeds. The point is, for a simple longitudinal control

system, the faster the airspeed, the greater the change in air

load on the surface and stick force required to maintain the surface deflection. The airplane represented in Figure 2a

More Pull Stick Force More Push

shows this stick force vs. airspeed gradient change. The curve indicates how much pull or push force is required to fly the plane at different airspeeds

Stick

Force

Airspeed (V)

n u

(fixed or constant throttle setting). Trim speed is indicated where the curve crosses the zero force line. For an airspeed change (AV) on the

Curve 2a

faster side of trim speed, the

stick force change is greater than that required for an identical airspeed change on the slow side of trim speed as shown in Figure 3. This is because of the V2 relationship. If a downspring is installed length change, but the preload is there all the time. Neglecting elevator air loads for in the same airplane, and all the moment, this implies a pull force other conditions remain the equal to the downspring force would have same, the pull force the pilot to be held by the pilot to prevent the stick must exert to counter the forward force exerted by the from moving forward. Most sport and general aviation air- downspring is represented in planes have their control surfaces Figure 2b. Notice this downconnected directly to the cockpit controls spring is exerting a constant

This can be done by selecting a long spring and pre-loading it to exert a small force. When the pilot pulls or pushes the stick, the total spring length changes by a small amount. So the force exerted by the spring changes very little as a result of its

of the curve in Figure 2b. If the pilot does not apply backstick, the airplane should ac-

-urve 2b Stick n Force °

Airspeed (V)

SPORT AVIATION 89

celerate and stabilize at the speed where the new curve (2c) crosses the zero force line. In order for the pilot to fly at the original airspeed, he must hold a pull force equal to the effect of the downspring. The pilot could, of course, trim the airplane so it maintains the original trim speed with no stick force required. Assume the example airplane uses a traditional elevator and trim tab arrangement. The downspring is trying to pull the stick forward and the elevator trailing edge down. By deflecting the trim tab trailing edge down, an aerodynamic moment acting to rotate the elevator trailing edge up can be generated to offset the mechanical moment acting to rotate the elevator trailing edge down caused by the downspring (Figure 4). The real benefit of the downspring is realized when the plot strays from trim speed. Following is an examination of both cases — faster and slower than trim speed. >. Figure 5a shows the elevator and trim tab from Figure 4. Depicted is the situation after the downspring has been added to the example airplane, and the pilot trims the plane to fly at the original (before the downspring was added) trim speed. Also shown are the downspring-generated moment (MQ) and trim-tab-generated moment (Mx). These moments, acting around the elevator hinge line, are equal and opposite. Therefore, the elevator is balanced, and no stick force is required to maintain this elevator position. To fly faster than trim speed, the pilot applies a push force to the stick, deflecting the elevator trailing edge down as shown in Figure 5b. The air load change on the elevator causes a trailing-edge-up moment (M^) which is proportional to V 2 . This is true in airplanes without downsprings as well. The trailing-edgedown moment caused by the downspring (Mrj>) is still there. Since the spring force is not affected by stick position or airspeed, the moment it generates is unchanged. Because the trim tab, which

stream, it causes a greater trailing-edge-up moment than before a (M-^M-p). The pilot must provide the additional trailing-edge-down moment (by pushing harder on the stick) to compensate. If the airplane had no downspring, the trim tab would not have been deflected trailing edge down to hold trim airspeed, and therefore would not be generating the additional moment for the pilot to balance. Considering the fact that the trim tab moment is proportional to \2>tne forward stick force requirements can increase dramatically as faster speeds are flown. A similar explanation applies when the pilot flies slower than the trim airspeed. Figure 5c shows the same, unchanged trailing-edge-down moment supplied by the downspring (Mrj). The moment caused by air loads on the elevator deflected trailing edge up is also a trailing-edge-down one (Mn2). The already deflected trim tab continues to provide an elevator trailing-edge-up moment, because it remains trailing edge down. However, the moment it generates (Mj3) is weaker than the trim airspeed case, because speed is slower and the moment is proportional to V2 (Mj3 < MQ). To fly this slower airspeed, the pilot must apply back stick sufficient to balance the downspring moment and elevator air load moment — both of which are trying to pull the stick forward. He is aided by the trim tab moment trying to pull the stick aft, but it is less effective at this slower speed. With the downspring, the pilot's pull force must not only compensate for the deflected elevator, but must also "make up the difference" caused by the weaker trim tab e f f e c t . . . with more pull force. In this case, if the airplane had no downspring, the trim tab would not have been deflected trailing edge down to hold trim airspeed. Obviously, with no downspring installed, there would be no downspringgenerated moment, Mr> The pilot would

is now protruding further into the air

elevator deflection moment. .. with less

only have to balance the air-load-caused,

was already deflected trailing edge down,

Curve A represents the basic airplane. Same as Figure 2a. Curve B represents the basic airplane with the downspring installed. Same as Figure 2c. Curve C represents the basic airplane with the downspring installed, and the airplane has been retrimmed for the original trim speed. \ ; , ... Curve

Point

A

1

'

Original trim speed.

B

2

Downspring added. Pull force required to hold original trim speed.

B C

3 1

New trim speed if pull force is not held with downspring installed. Retrimmed for original trim speed with downspring installed.

C

4

Flying faster than trim speed with downspring installed.

A

5

Flying faster than trim speed had downspring not been installed.

C A

6 7

Flying slower than trim speed with downspring installed. Flying slower than trim speed had downspring not been installed.

90 1996

Stick Force

FIGURE 4

pull force. Figure 6 is a composite plot with a sequential map of the preceding explanation. The key is in the re-trimming. If the airplane is not re-trimmed to the "predownspring" trim airspeed, there would be no deflected trim tab and no consequent increased hinge moment and their higher stick force requirements. There would merely be the extra aft stick force the pilot must apply to balance the downspring's constant forward pull (Figure 2a + 2b = 2c). The other trim tab deflection contribution is because the further from trim airspeed the airplane is flown, the greater the V2/hinge moment relationship effect. In order to present the preceding explanation of downsprings, several assumptions were made to simplify the examples. The overall downspring effect is not changed by the assumptions. Many factors play a part in the total picture of an airplane's control stick feel characteristics. Downsprings represent only one control feel augmentation device. Is this the kind of stuff EAAers want to read about or is it merely several decades of personal unresolved childhood Slinky anxieties? Your feedback is welcome: [email protected]. 4

MT