up-mar8a.htm Small-Pogo Project


Small-Pogo Project


Status Updates


March 20-31, 1998

March 20, 1998

Been a real rough week. The last couple of days I've been working on various ideas for fuel pressure regulators, float bowls and such. There are quite a number of ways of doing this that I hadn't thought of before. I've done some kitchen experiments with diaphragms of neoprene and latex, plastic tubing and such. I believe I know enough to build a 1-2 oz regulator that will keep the pressure very constant.

Today at work one of the folks told me about how the model helicopters maintain fuel pressure. They use two tanks, a large one below the engine and a small one nearer the engine. They use a small fuel pump (where I've been trying to do this with muffler pressure) to keep the upper tank full. Excess flow is shunted back to the main tank keeping the level in the upper tank constant without excess pressure. I had thought of using two tanks but couldn't figure how to do it with muffler pressure. I've borrowed such a fuel pump and will look at incorporating it tomorrow along with modifying the rudders on the Small-Pogo.

Here's a short list of what I've come up with in controlling the the fuel pressure:

Three major categories of providing control:

  1. Restrict the fuel flow
  2. Control the fuel pressurization
  3. Provide a constant level source of fuel

Some examples of how to do the above are:

March 21, 1998

Performed a couple more kitchen experiments on pressure regulators, still a little ways off from being able to make one with confidence. Pulled the engine and mount from the Small-Pogo model in preparation of testing the fuel pump/dual-tank method. Tomorrow, Sunday, I hope to test this.

Tomorrow I also hope to add trim-tabs made of aluminum sheet to try to control the rotation better. This should be easier than building rudders and adjustable rudder controls. The aluminum I can bend easy enough. Today I picked up some piano wire to replace the soft wire I was using between the servos and the rudders. The soft wire was good for making quick and easy adjustments but it didn't hold up well against the force of the exhaust stream. With luck I should get in a couple of runs Sunday afternoon or Monday after work.

March 22, 1998

I tried setting off the Model Estes rocket igniters on a 9v battery to see if they would work. My intent was to verify that they would work off the battery I would use to operate a timer. Of all the luck it takes about two seconds for them to actually ignite, exactly the time delay I've been needing. I won't have to develop any delay circuits after all. I just have to release the rocket with a switch that closes at separation and two seconds later I get rocket ignition. I'll lose my $.79 battery each time but that's cheap.

I also performed a test of a fuel pump that attaches to the engine and uses the vibration of the engine. The best I could get was about 3-4 inches of head. This may have been due to the fact that the engine was mounted to a fairly substantial 2x4 rather than a lightweight model aircraft structure. In any case I've decided not to use it.

March 23, 1998

I had a few minutes during the day while waiting for my computer at work to be repaired and think I have finalized on a simple design for a pressure regulator. From the engine runs last night I realized that the engine will draw fuel at least three inches without tank pressure. Therefore I will build my regulator to provide zero pressure but all the flow the engine wants to draw. This makes the design and assembly easy because I can eliminate springs and adjustments. The in-laws came by tonight and we talked computers and family so didn't get to build or experiment. Hopefully tomorrow I'll get to the servo control rods and build the pressure regulator.

March 25, 1998

Yesterday I stopped by the model shop and picked up some fuel line elbows and such for making a regulator. While there I saw a pump/regulator that looked interesting. Today I called the manufacturer to get more information on the specs. He wasn't sure of the pressure tolerance but said that he had set up an 0.60 engine about a meter off the ground with a hose to a jug of fuel on the ground. The pump/regulator pulled fuel from the jug which he lifted from the floor to well above the engine with almost no variation in the engine speed.

Today I bought one and did some preliminary testing on it. Using water I found that it drew from more than half a meter below the pump to half a meter above it with little variation in output head. Set from the factory the head pressure was about 15 cm give or take about 3 cm with wide variation in the draw reservoir height. I then hooked it up to the engine on a test stand to pump fuel but only got in a couple of runs. The first run I got some surging but the second with a different mixture setting the engine ran very smoothly and even idled smooth, which it hadn't done before. Tomorrow I hope to get in some more runs while varying the height of the tank.

I also picked up some plywood and balsa to start making the upper body (ref: drawing in Daily Updates, January 17-31). I intend to make use only four vanes to simplify assembly and operations. These vanes will be larger than needed for strength but will provide extra lateral drag above the CG to help stabilize the vehicle when it tries to translate and rotate. These four vanes will be attached to the outside of the engine shroud near the top by way of lugs. I won't start building the upper body until next week.

This Saturday I should have the fuel problem fixed, the flight control wires changed, and the trim tabs installed. Weather permitting I should be able to get in another tethered flight or two.

March 26, 1998

I ran the engine again tonight with very good results. At first it didn't want to start then I read the instructions on the pump. I mounted the pump according to instructions and turned the engine stand so the engine was prop-up, like I intend to use it. The engine fired up real easy and ran very well. The fuel line was drawing from a jug of fuel which I moved from half a meter below the engine to level with the engine with less than 5% change in engine rpm. After about 5 minutes I shut the engine down and started it a couple more times while warm and it started very easily. Then I waited about an hour to let it cool off and repeated the tests. The results were the same as before, easy start, steady run, no impact of changing the fuel location. I'm ready to put the engine back into the vehicle.

I also replaced the soft wire control rods on the flight control rudders with stiff piano wire. The rudders seem to move move much more positively. If the weather doesn't get too bad I should be able to restart tethered testing this weekend.

March 27, 1998

I didn't have time to put the engine back into the vehicle but did start it a couple more times. Both times it started easily and ran well. I cut down my 12" paddle prop to 11 1/4 and ran it for a few seconds. It seemed to be balanced but at some RPMs one blade was ahead of the other by about 1/4 inch. I don't know if I cut it wrong or not.

March 28, 1998

(AM) The weather this morning is stormy. It rained most of last night and the clouds are still heavy with strong gusty winds. I don't know if I'll get in any flying today.

(PM) The weather stayed unfavorable for flying. I framed in the horse shelter which I can use for tethered flying. Once I have the sides and top on it will block most of the wind. Hopefully the weather tomorrow will be more cooperative.

I moved the engine back into the vehicle and hooked up all the fuel lines and electrical. The process seems too cumbersome so the next time I pull the engine I'll make some modifications. For example, the ground wire for the remote glow plug is a spade connector which goes under one of the engine mount screws. To loosen and tighten the screw I have to use a 1/4" ratchet and a screwdriver bit, hold the wire just right with one hand, and operate the ratchet with the other. This is difficult because of the confined area. The modification will include a quick disconnect somewhere easy to get to.

The lessons from this effort are a) modularize and b) design/build for supportability.

March 30, 1998

I flew the vehicle on vertical (top and bottom) tethers only. Here are two pictures from tonight a) the tethered vehicle, and b) the rudders.

Flying it was more benign than I had thought. The spinning was not as agressive as before. The spinning that I attributed to gyroscopic nutation seemed to be more a function of the way it was supported than part of the inherent design. By applying rotational commands as it started spinning I was able to counter the rotation and keep it fairly stable much of the time. I don't believe I will have much trouble once it's actually in the air and free flying.

Lateral movement was not well demonstrated but that could have been a function of the tethering. As built it will probably never have a large lateral force, but then, it was never intended to. I don't think this will be a problem.

I've decided to replace the lower landing strut with a simpler design. Currently there is a heavy balsa cone and spring-loaded strut on the bottom and plans for balance rods coming down from the shroud. I now see a flat plate on the bottom with horizontal piano wire stabilizers, sort of like a spider smashed flat with its legs out straight. This should be lighter and provide better stability on the ground.

I've gone almost as far as I think I need to with the tethered flying. It's now time to focus on the landing system, upper body section, and parachute. Free flight is getting near, possibly in a 10 to 14 days.

March 31, 1998

Painted half the vehicle red so I can tell which way it's facing and how fast it's rotating from a distance.



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