Stretching The Flight Time On A Compressed Air Plane

[Tom Stanton] has been experimenting with compressed air motors on model aircraft for a good few years, but keeping them aloft (and intact) for more than a few seconds has proven a tough nut to crack. His latest design represents a breakthrough — pulling off an impressive 1 minute and 26 seconds flight on 4 liters of compressed air.

The model incorporates an enhanced engine design featuring an expanding seal on the piston, a concept inspired by the old Air Hogs toy plane. For the airframe, he constructed lightweight wings using 3D printed ABS ribs on a carbon spar and reinforcing rods, all of which were wrapped in heat shrink film. Additionally, [Tom] incorporated a thin balsa former along the leading edge of the wing to help maintain its shape. The fuselage is also composed of a carbon fiber tube, and is outfitted with printed fittings to install the wings, V-tail, RC electronics, and soda/air bottles. A hollow nylon bolt holds the two bottles together end-to-end while allowing the motor to be screwed directly onto the front bottle. To conserve weight, each of the two V-tail control surfaces are actuated by single cables linked to servos, with piano wire torsion springs in the hinges to maintain tension

Despite successful flights, [Tom]’s trials were not without challenges. One crash threatened severe damage to his airframe, but thanks to a central 3D printed bracket that absorbed most of the impact, total destruction was avoided. Similarly, a printed shaft saved his expensive carbon fiber propeller from being damaged during multiple landings, an outcome that led [Tom] to devise a readily replaceable consumable connector.

A second video after the break offers a behind-the-scenes insights into this project including some fascinating technical details. For more on this project’s history, take a look at the initial diaphragm engines and his attempts to make them fly.

23 thoughts on “Stretching The Flight Time On A Compressed Air Plane

  1. Compressing your airplane may allow you to store more flight, but may degrade the accuracy. Personally, I don’t notice the clipping off the top so much, but I really miss the full, low landing gear.

  2. Didn’t/can’t watch the video, so maybe he covers this, but:

    4L of compressed air at 5 atm stores about 1 Wh of energy.
    That air alone adds 25 grams to the mass of the airplane (plus the ~100 grams of the bottles) .

    For comparison, a 1 Wh (~300 mAh) lithium polymer cell is just 6 grams.

    1. Some villainous troublemaker put dry ice in 2 liter soda bottle at a party I once attended.
      That bottle’s failure pressure was much higher than 5 atmospheres.

      I suggest using a road bike pump and valve. Stopping when the bottle starts creaking. One bottle or two separately plumbed. Glue idea is bad.

      To be fair to the engine, the whole plane is pig. 3d printed ribs! No strength taper, wing tip is as strong as root.
      Would have been better off using slow stick parts. Same size.

    1. IIRC there was at least one iteration where he played with the pressure decay curves. A full-on regulator would add weight so that’s pretty much a (cough) non-starter.

      It’d be an interesting “Drop the pressure bottle assembly” powered launch for a full-on glider though.

      ALSO – WTF is with “Email me ‘stuff'” in the comments now? Selfies next ,oh pursuers of dopamine addicts?

  3. Cooling the air coming from the pump would give him more air in the tank, as compressing it makes it hot. When the bottles cool down, some of the initial high pressure is lost. The pumping should be done slowly in stages, so the air already in the bottle can reach ambient temperature and more will fit in.

    Likewise, as the air expands out of the bottle, it cools down and loses pressure. Adding a long thin walled aluminum tube between the tank and the engine would act as a heat exchanger to warm the air, conserving pressure and air. Maintaining isothermal compression and expansion can easily double the efficiency of an air engine.

  4. It needs one or two thin carbon fiber rods angled down / back from the front to work as skids to not break the prop off. A couple of holes in the engine would be perfect for those.

  5. In a plastic jugs for drums forum years ago someone ran 2 liter bottles to 100psi (6.8atm) and stated in Europe that transportation regs set something like that as a safety factor for the bottles. They ring quite high and with a lengthy sustain at that pressure.

  6. am not going to watch two sponsored youtube videos, but..
    not sure what his self inflicted design goals are but looks like a fun project. Extending flight times is directly correlated to amount of air/gas/other “in the tank”
    1. CO2 cylinder like a BB gun- or propane, butane, N2O or any other gas with a favorable triple point diagram. Under compression those gasses condense and you can get way more moles of “fuel” in the tank. They are also somewhat self-regulating for pressure, as long as the temperatures stay reasonable with expansion of a gas as noted by others above.
    2. High pressure cylinder for aire and by high pressure, I mean something like a pneumatic air rifle- quick g***gle search shows they fill to astounding pressures, like 4000 PSI with a special pump that looks like bike pump. Of course there would be a weight penalty but the massive increase in power available, I’m guessing, would be favorable. Even a couple wraps of pre-preg carbon fiber would substantially increase the pressure the tank can hold.
    3. As mentioned above, solid CO2. Benefit of cheap soda bottle is if it ruptures, who cares. and the tank can be smaller.
    Finally, since he is hand launching anyway, give a look at “discus launch” gliders. You can get a ton of “free” altitude with a much better launch technique. Along those lines, also as mentioned, drawing from the massive amount of research for RC gliders, anything to reduce drag. Folding prop, dedicated airfoil design, wing-root area (notorious)

      1. [craig] says at the start of his comment that he doesn’t want to watch sponsored content. And yet he’s still trying to contribute. I for one found some of [craig’s] suggestions useful, and a much quicker way of sharing than watching ads on youtube.

        I’ll make my own contribution… in the 1990s before electric propulsion got really cheap and practical, CO2 motors for lightweight aircraft were quite popular. The Czech “Modela” (IIRC) motors ran from CO2 cartridges and had a good reputation.

  7. Fun!
    One correction though (might help). Reference 6:14 mark in first video. That CO2 cartridge doesn’t just contain 800psi CO2 gas at 0.7L. It contains 12? grams of CO2 liquid. So when expanded, it would be >6L vs the 1L you mention.
    Although still not as much energy per gram as the bottles, its close enough that the massive reduction in drag and consistent pressure delivery would more than make up for it. Actually, once you consider the bottle’s % of wasted energy at low pressure vs CO2 % wasted … the weight per gram might change.

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