Calibration of flap overspeed airswitch

The Vfe of the Sling 2 is 85kts

An ASW1 is used with the TCW IFC-10 intelligent flap controller

The ASW is designed to close at 100kts.

Allowing a 2 kt margin, I want the ASW1 to close at Vsw = 87kts = 44.76m/s

In order to adjust the ASW1 a manometer arrangement is required. I will simulate the dynamic pressure of this speed by creating a head of height h.

The equation is

(Dw-Da)*g*h = 0.5*Da*Vsw^2

Solving for h gives

h = Da*Vsw^2/(2*(Dw-Da)*g)

Da = 1.225kg/m^3 density of air

Dw = 1000kg/m^2 density of fluid in the manometer (water in this case)

g = 9.800m/s^2 is the local net acceleration of gravity at my location

From the above h = 0.1254m which is easily measurable

The close-open (equivalent to transitioning from high to load speed) h value for the ASW1 as supplied is 0.215m. Inserting into the formula

Vsw = sqrt(2*(Dw-Da)*g*h/Da)

gives h = 58.6m/s = 113kts, which is higher than the stated 100kts

I then set the head to 0.125m and then adjusted the trimpot in the ASW1 to the threshold of open-close. The hysteresis appears to be negligible. I also noticed that the trimpot appears to be a multiturn (10 turn?) unit rather than the single turn unit as implied in the information sheet. The plastic case has +/- markings at the trimpot showing the direction of turning to respectively increase/decrease the switch threshold. The diagram is also a little incorrect, however the static port outlet is correctly shown as being closest to the set-point screw

Interestingly, placing the manometer on the static port of the ASW1 and creating a negative head caused the the switch to trip at closer to 115mm, corresponding to 83.5knts. This is a bit unexpected, but could be experimental error.

I have changed the threshold to 90kts, for a 5kt margin over 85kts = 46.3m/s. The head corresponding to this is h = 0.134m. I have moved the manometer back to the pitot input and reset the trimpot

Now moving the hose back to the static input, I get a reversed h = 0.125 at the switch threshold. This corresponds to 87kts.

I have set up a differential manometer arrangement where the pitot and static port pressures can be set individually.

If the static port pressure is ambient (0mm) the pitot head for switching activation is +130mm, so the difference head is 130mm, as set before

If the pitot port pressure is ambient (0mm) the static head for switching activation is -125mm, so the difference head is 132mm

I did some further tests where both the pitot and static pressures differ considerably from ambient, the pressure around which the case of the ASW1

This is of interest because the ASW1 case will be inside the aircraft and this pressure may differ from the static port pressure due to Venturi effect. Online this appears to typically result 40 ft altitude read error - corresponding to 146Pa = 1.46mBar. This corresponds to a head of 15mm in the manometer.

Static head (mm) Pitot head (mm) Difference head for switching (mm)

+15 +144 129

-17 117 134

+235 +375 140

-250 -110 140

+590 +730 140

-603 +465 138

So the case ambient pressure relative to the static port pressure appears to have no significant effect on accuracy under realistic conditions, and has only a slight effect at larger values. There was quite a bit of experimental error for this higher head readings, as these involved opening the taps just enough to bleed down the pressure excesses just slowly enough (very difficult) that reaction time to the ohmmeter sounding a switch opening or closing did not affect the result.

Operation fully as expected, calibrated for 90kts. No strange quirks.