up-mar8a.htm Small-Pogo Project


Small-Pogo Project


Status Updates


March 1-15, 1998

March 1, 1998

Completed the servos and rudders installation. Everything works nominally, I have full rudder control. It looks like the best place for the throttle is attached to a piece of sheet metal off the side of the engine mount. This gets it close to the engine and out of the main airstream.

Ran the engine a few times. It's still getting hot so it's not yet broken in. This time I removed the engine test stand from the work bench and rotated it around a few times. There appears to be no real problem with having the fuel tank so low. Putting the check valve in line with the pressurization causes too much pressure and pumps fuel out the carburetor.

I tried different propellers (it's getting easier to start). The best one was a 12 inch/5 pitch "paddle." This prop has a longer cord than usual. The thrust was enough to just lift the engine and test stand. When I measured the weight of the engine and test stand it was about 2.6 kg. I was hoping to get 2.8 kg of thrust so I'm not off by too much. It could increase once its mounted in the vehicle with the shroud and no 2x4 the hard way behind it. Different props may also help. I'm still expecting the vehicle to weigh in at 1.7 kg. This still gives me a 1.6 T/W ratio and an acceleration of 5 m/sec vertical.

I was unable to pick up the lumber for the tether rig yet. Domestic duties (patching the roof before the next rain) and all that. I hope to start tether flying on Saturday because Sunday I have to mow the lawn and pull weeds. Yes, it's spring in the Mojave desert. The fruit trees are blossoming, the flowers are blooming, the trees are budding out. Living in Southern California is rough duty but someone has to do it. :)

March 3, 1998

Yesterday I crashed out and didn't get much done. Tonight I built the supports for the last control servo, the one for the throttle. The control rod for the throttle will run up the side of the lower body tube within an aluminum tube (already in place). The servo is mounted inside the lower body tube. I also applied the second of three coats of white paint.

Tomorrow I should get the last coat of white paint and the first coat of red (to identify which way it's flying). The painting should be complete Friday in time for tethered flying on Saturday. The fuel lines are ready for installation. I still need to extend the wires for the servos and stuff the avionics but that should take only a couple of hours on otwo evenings.

I won't have time to build a fancy tether rig for Saturday so I will be using the support beams for the lean-to next to the garage. A bit crude but it should work just fine. I'm also trying to borrow a camcorder so I can get it on video. I may have to buy one as a long-term solution.

March 4, 1998

Too cold to paint tonight. Added the tether hooks. Installed the tank and fuel tubes. Made places for the battery and the receiver.

Still need to extend the servo wires and secure them to the structure, mount the switch, install the engine and fuel lines, and a couple of other small items.

I hope to have a camcorder ready. Anticipate tethered flight about 1400 Saturday.

March 5, 1998

The weather went bad. Cold and rainy. Any more painting will have to wait for another time. The avionics are all but completed. Now I just need to bolt the engine and landing strut on, hook up the fuel lines and throttle linkage, and it's ready to fly.

March 7, 1998

Attempted to fly the Small-Pogo on tethers but the engine did not want to run, apparently because the fuel tank is too low. I attached another tank higher up and it ran for a short time. With the little time I got I learned:

It will probably be sometime next week before I can really get back to it.

March 11, 1998

Here is a photo (37k) of me with the Small-Pogo while it was running. I started with many tethers and ended with few. the day was bright and the background dark so the white vehicle was a bit overexposed. The fuzzy stuff at the bottom was plastic wrap I had over the avionics. It will eventually be replaced by a smooth plastic sleeve. The ring above the Small-Pogo is just a rectangular spacer to keep the support lines straight and will not be used in flight.

March 12, 1998

Having reviewed the video a few times I now believe that there is some tendency to rotate with high rpm but that this can be countered with some slight input from the radio control. If not then certainly with additional rudders.

Sunday was domestic duties, Monday was a day of rest, Tuesday I welded an upper deck on the metal rollaround stand I bought (for $5 US). Wednesday I replaced a catalytic converter on a Toyota (I hope I never have that pleasure again). Today I laid in the plywood shelves and built the tilt stand for the Small-Pogo. This field test bench will allow me to start the engine and warm it up at an angle, tilt it straight (normal flight attitude), unclamp it, and slide it out sideways or straight up for testing or flight.

Tomorrow I'll add clamps so I can hold the Small-Pogo in place while I work on it at any angle. Saturday I should be able to try my test runs again. This time I know I won't need such elaborate, and annoying, tethers, though some will be required. I hope to do more throttle runs, rudder tests, and stability tests.

March 14, 1998

Completed the field test bench and made some engine runs. The bench works fine and the tilt table is great. Still having trouble with the fuel. By adding the check valve in the pressurization line I can get the engine to run better in the vertical and start ok. It's still sensitive to fuel level and blows a lot of fuel out the exhaust. I'm thinking about a custom tank that would sit just below the engine and outside the lower body tube between a couple of vanes. So far, none of the ideas I have come up with are very good and I don't want to redesign (and rebuild) a major portion of the vehicle.

March 15, 1998

Several more engine runs concluding with an almost free-flight. The field test bench worked very well. Running the engine with the check valve in the muffler pressure hose allowed the engine to run very well, though it pumped a lot of excess fuel and was still sensitive to fuel level. Since the the last run was the climax and gave the most information I'll talk about it. For this run the Small-Pogo was tethered only from above (i.e. hung from a cable over a tree limb).

The good news is that the thrust was plentiful. Anything above 1/2 throttle caused the vehicle to want to fly into the cords suspending it. The bad news (things to be worked out) are lack of rudder authority and rotation. The rotation problem has two facets. The first is simple rotation due to propellor wash. The second is due to unexpected gyroscopic nutation. The rudder authority and rotation due to prop wash should be fairly straightforward. The nutation is a head scratcher.

The nutation is like when you hold a bicycle wheel by the axle, spin the wheel to make it act like a gyroscope, then push up or down on one side causing the wheel to rotate 90 degrees from the direction of force. The Small-Pogo, when suspended from above and not constrained any other way, would spin about 30-40 times, almost come to rest, then spin again, all the same direction. It looked (and I'll have to review the video a few times) like as soon as the Small-Pogo would start to tip one direction the gyroscopic force would take over and spin the Small-Pogo. Additional rudders can be added to take care of most of the spin but I'm not sure it will fix the nutation. I believe that the actual forces are small because there is very little moment of inertia and the spinning was not too severe.

So, back to the drawing board. I've come up with a way to build a float bowl to take care of the fuel problem which I'll work on this week. The vanes on the upper body (ref Jan 17-31) should help some on the rotation. On the bottom of the lower body vanes not already in use I'll add adjustable rudders since I don't know how much rudder I'll need. I've also found a design for a delay circuit that will fire the model rocket engine about two seconds after it leaves the first stage.



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