Spoilers. Speed Brakes or Both?

I patterned my system after the. Cessna Citation dive ... momentum theory shows that, in this case, an incremental ... response is possible with the. Ifl. Photo 2 ...
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By DON WALL 5471 Lake Howell Rd., Ste. 202 Winter Park, FL 32792

I have owned two aircraft, a Turbo Mooney and a Turbo Piper. While flying them, I have wished on many occasions that I had speed brakes or spoilers. How often has ATC given you that sudden descent where you have to do 360s to get down? Some people say simply use the flat part of the propeller. The only problem with this is the sudden engine cooling and the resultant cracking of engine cylinders. Believe me, I've cracked a few. I have even heard them say to be careful with enriching the mixture too quickly as the cold fuel will tend to crack the cylinder. Perhaps so. If possible, I simply prefer to keep the engine warm. To do that, one has to keep power on. This sometimes is difficult to do, especially if you are flying a low drag aircraft.

Ifl. split flap, only upside down. The result is that a small stream of air moving about 200 mph is caused to change direction by 60° upward. A simple application of the fluid momentum theory shows that, in this case, an incremental drag force of about 70 pounds can be developed and also an apparent increase in aircraft weight of about 130 Ibs. You may say, well, that is not too much. Let's just compute the drag on my Lycoming 0320 powered RG when using 70% power (112 shaft horsepower) and traveling at 200 mph. The aircraft drag is about 210 Ibs. Hence, when I apply the boards, I increase my theoretical drag by about 33%. Of course, the apparent increase in weight of about 7% must be compensated for by an increased angle of attack if constant altitude is desired. Power Pack

How could we build a power source to drive the boards? I thought about jackscrews, hydraulic power packs, etc. Since this was a one of a kind, I decided to use air actuated cylinders. A 12 VDC auto emergency tire inflator pump motor was selected as the air energy source. The conventional auto packaging was removed and the pump was installed on a small platform with a manifold, a pressure cut off switch, a pressure gage, and a solenoid valve. Photo 2 is a view of the power pack. An accumulator was fabricated from 2" copper tubing with end caps. The accumulator not only allows multiple applications of the boards in a short period of time but also a faster response is possible with the

accumulator. Nylon tubing (NYLOSEAL) is used for the connection from the power pack to the accumulator and the actuation cylinders. Cost of the parts for power pack was less than $100. Operation

A momentary push button was placed on the control stick. Operation of this button opened the solenoid thereby activating the brakes. A DPDT on/standby switch was placed on the panel. When I start up the engine, I usually place the standby power switch in the STOWED position. This activates the power pack and causes about 135 psi of air to be placed in the accumulator. The pump automatically maintains the desired pressure at the value set on the cut-off switch. Same operation as your air compressor in your shop does. When I push the control stick button, the solenoid valve opens and allows air pressure to flow to the activation cylinders in the wing. The cylinders then deploy the boards through the mechanical linkages. The DPDT switch also can be used to deploy the boards should you not wish to hold down the momentary push button . . . for example, during long descents. Flight Tests

operated wing spoilers/brakes located on the top of the wing at mid flap distance near the aft spar. Photo 1 is a view showing the left wing with the boards activated. The boards are 11" long and 5" in width. A portion of the

Since power is force times speed, the engine power must operate against the drag power consumed, which is drag force times aircraft speed in level flight. The theoretical calculations indicate that drag should be increased by 30% when the boards are deployed. A simple flight test scheme was devised. On the first test, the aircraft was flown at a constant speed of 147 KIAS at 1000 MSL and then the boards activated. At the same altitude and not touching the power, the aircraft slowed down to 120 KIAS. Since engine power was maintained,

mechanical linkage which opens the

the

Photo 1 - Speed brake deployed.

When I was building my Glasair RG, I knew that speed brakes would be a must. I patterned my system after the Cessna Citation dive brakes. My Glasair is equipped with pneumatically

speed

ratio

is

inversely

doors when pressurized air is applied

proportional to the drag. In this case,

to the single acting cylinders can be seen. Two hidden hinges were used on each door. The doors have been designed to open 60° thus allowing the air flow to change and flow vertically.

the drag increased 22.5%. During the

One may argue that this system is a 60 JANUARY 1994

Photo 2 - Power Pack

second test, the speed changed from 126 KIAS down to 103 KIAS, again a 22.3% change. These tests were repeated about a week later and the results were essentially the same. Of

course, the incremental drag is a function of the square of the speed. A third test series was to maintain a given airspeed, and then measure the rate of descent. The aircraft was again held at both a constant power setting and at a speed of 145 KIAS. After the boards were deployed, the aircraft stabilized at 900-1100 fpm descent. A review of the rate of climb formula indicates an incremental drag of about 100 pounds was necessary to cause this effect. Since the computed horizontal drag was about 75 Ibs. when the boards were activated, the flight tests also show the effect of the change in direction of the airstream. The overall test results tend to confirm the calculations. During the flight tests, a very slight pitchdown tendency was noticed. In fact, if you had your hand on the stick, you would not even notice it. Since this tendency was so small, the location of the boards can be considered fortuitous and is almost at an optimum position. In other words, you do not have to fight the action of the boards. As the aircraft slows down, of course the pitch is minimized, so is the drag. With full flaps and full boards, there is a slight rumble across the tailplane. The

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action of the flap and the boards tend to shed many vortices in the wake. Do I Like It?

In simple terms, YES. I have another flight control on board. Some uses - (1) this additional control is used to alter the descent rate on ILS, (2) can also be used to descend from altitude without rapid cooling of the engine, (3) can be used to slow my pattern entry speed also without changing pattern altitude, (4) can be used to effect better stops on dry and wet pavement, (5) can be used to aid in preventing floating after touch down on gusty days. What I Don't Like

Full flaps and full boards cause noticeable turbulence. This turbulence is considered unwanted although the boards are mounted outboard of the tailplane. The wake must spread enough to just catch the elevator. Hence, I don't normally use that combination. When landing with full flaps, I wait until I just touch before employing the boards. Although I placed a return spring in

each flap, there is a tendency of the board to fly up about a quarter of an inch at the trailing edge of the board during cruise. If I increase the force of the spring, it will act against the applied force of the air cylinder. At 175 KIAS, the boards are slow (about 5 seconds) to deploy since the accumulator is small. There is a trade off. Just how big an accumulator? My current accumulator has a volume of about 30 cubic inches. If I were to do it over again I would consider a hydraulic system, assuming I could find a lightweight hydraulic pump at a reasonable price. The flight tests also indicate that the boards could be somewhat larger. I think that I would increase their length from 11" to about 16". Some limited flight tests have been conducted using split board conditions. The aircraft was fully controllable at cruise conditions. However, if I had to land with a split board, I would select a long runway and keep my speed up. Otherwise, I would leave everything else alone. Incidentally, if you apply the boards when you are still 5 feet in the air, the touch down you will get is quite an awakening. All in all I am very happy with the system. *

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