Two Gas Engines

Today I finished assembling the main helicopter structure and performed the break-in procedure for the second gas engine. The two gas engines are located near the center of the helicopter, so that if one stops running, the other can bring it down for a controlled landing.

On Friday, if I can successfully rent a helium tank, I may try doing a test flight. (The design still uses four helium-filled weather balloons.)

I'm increasingly concerned about the vibration of the gas engines, so I'm going to continue working on building an electric version of the helicopter.

One issue is with having both gas and electric helicopters is that different cameras would be suitable for the different versions. The electric one needs a very light camera, whereas the gas one can lift a heavier (higher-quality) camera.

Gas Engine

Two weeks ago, I decided that I should try using a gas engine, because the batteries for the electric motor take too long to charge.

One week later, I had obtained an R/C airplane engine and all the accessories. After much trial and error, I was able to run the engine.

Today I built a new test rig that allows me to turn the engine from horizontal to vertical while it is running (this requires positioning the fuel tank, fuel lines, and carburetor on the axis of rotation). Using this rig, I determined that the engine can run and be started in a vertical orientation.

Photo of the rig. Detail. Accessories.

This contraption (including the 2.5 pound weight) can take off vertically. Of course, it's not a stable flying machine, so you have to hold on to it with one hand (which requires a certain amount of insanity).

The amount of lift and fuel consumption are both good.

The biggest problem is vibration. Every nut and bolt must be very tight; otherwise it will shake itself to pieces. (I bought some nut-locking glue, which I'll use on the final version.) Normally the engine is rigidly attached to a heavy airplane body, but for this helicopter, the engine should be the heaviest component, so there isn't much to damp the vibration.

Of course, this much vibration will be bad for recording video footage. I'll have to modify the helicopter body to isolate the camera as much as possible. Perhaps I can use a smaller engine to reduce the vibration.

The engine has other drawbacks: it is rather loud, it coats everything in fuel residue, it is a bit tricky to start, it requires many parts and accessories, and it requires draining the fuel tank and fuel lines after use.

Nonetheless, I think I'll continue to experiment with the engine.

One option would be to have two gas engines fixed in a vertical orientation and use two electric motors for forward motion and turning. Alternatively, I could place the engines on actuated pivots, as was the original design for the electric motors.

Next step: laser-cut an engine support structure.

Propeller Test

I've finished the first round of propeller testing. The test system uses software to send commands to a serial servo controller, which is attached to the speed controller, which is attached to the motor and battery. I measured lift using a digital postal scale.

Photo of the test system. Photo of propellers.

I measured each propeller at 70% and 90% of max power. I expect 70% will correspond to hovering speed, leaving a reasonable margin for climbing, turning, and battery depletion.

I tested each prop in order, then recharged the battery and retested each prop in the reverse order. This should reduce the battery charge bias.

Results:

Prop Type/Size	Lift at 70%	Lift at 90%
APC 10x4.7	740g	900g
APC 11x3.8	800g	995g
APC 11x4.7	940g	1130g
EF 11x4.5	950g	1130g
APC 13x6	1085g	1335g
MA 13x6	1150g	1345g

Each prop is denoted with its manufacturer and two prop size numbers. The first number gives the prop diameter (in inches). The second number relates to the forward speed of the propeller. For the helicopter, I'd like high static lift, not a high top speed, so I want a low second number. (Though these experiments suggest that the second number shouldn't be too low.)

The best props from this set are the 13x6s. These provide over 1kg of lift at 70% power. The motor, gearbox, prop, electronics, and battery weigh a total of about 350g, so flying with these props should be reasonable. (Some of this lift may be due to the ground effect, though I tried to reduce that by raising the props up off the table top.)

I'm going to order more propellers near the best in this batch (e.g. 13x5 and 12x4). According to the motor specs, I shouldn't go above 13" diameter, so as not to overheat the motor.

Given the charge time for the LiPo batteries, I think I may try a gas-powered engine. More on that later.

Initial Parts List

Here is a partial list of what I have so far:

• brushless motor with 4.4:1 gearbox
• 1500mAh 11.1V 20A LiPo battery
• 25A brushless speed control
• propeller adapter (need to drill out to 4mm)
• 3.5mm motor connectors
• 1.7 GHz wireless video transmitter
• 1.7 GHz wireless video receiver
• wireless serial devices
• 2'4" diameter (inflated) weather balloons

I have most of the mechanical and structural components. I also have some microcontrollers (I'll probably use a BASIC stamp for the first prototype).

I have an assortment of 10" to 13" low-speed props. I need to buy a scale so I can measure the lift provided by each.