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[Jerry Hill]

I'm guessing this is the right place to put this, from and r & d point of view it seems to be unique. It's also a bit pointless but the 'why do we climb mountains' thing kicks in here.

The idea is simple enough. I like airboats, and many years ago I resigned to one day making a boat where contra rotating propellers would remove the troublesome torque issues they have. Using model aero engines these airboats don't like turning right, the torque effect on such skimmy bouyant designs is pronounced. Now its been years since I thought of this, and a long time since I started to make it happen, but I get to it when time allows between more regular projects.

I like trialling things before comitting, especially if there are unknowns in there, so the first thing to do was put together a basic boat to check out the principle. The result of that is here: https://youtu.be/Q6BiGlR36Uc

Those are two tiny engines, a .20 and a .25 I happened to have spare. After many trials a couple of things came to light, not least that I was getting more cumulative thrust from engines arranged like this than from the engines capabilities running alone added together. A happy accident I looked into and developed. In essence the front engine, the smaller one, is pre accelerating the air into the second, which is able to run a much heavier pitch prop than usual.  After much debate and alternate engine trials I've settled on a .32 at the front, and a .40 at the rear. And this design of craft for them:



My tests showed two main issues. Vibration from two engines on a common mount was quite destructive, so I made a setup with isolated mounts. No existing rubber mounts suited space restrictions so I made my own from polyurethane.  After this is was getting a bit silly on the weight front on the mount, which is where the fuel tank on rc airboats is usually kept. On my trial boat though I had the tanks on the deck as can see in the video, but inevitably there were issues, so as the tanks could ceratinly not be up on the mount, I turned to Perry fuel pumps. I tried a few variations on plumbing on these and settled on a system that works really well. The  pumps deliver a surplus of fuel to a point just by the carb inlets, the carbs draw off what they need, and any excess is teed off back to the tank. That rig is shown here where the fuel tank distance is representative of the final boat: https://youtu.be/4a-mfEhPY3k

Here's where I am, top image it's next to a 10cc hydro





I'll get back to it now

[derekwarner]

Jerry.....this example is interesting, ...[E1 = supercharge, E2 = driven]

"In essence the front engine, the smaller one, is pre accelerating the air into the second, which is able to run a much heavier pitch prop than usual.  After much debate and alternate engine trials I've settled on a .32 at the front, and a .40 at the rear''

This is not dissimilar to the concept of two pumps in hydraulic circuits used by the  American FMC corporation in their military applications term of a servo - supercharge tandem pump system ....however with air, you can force more volume in a given space than you can achieve with fluids]

The only difference is that the servo pump is a positive displacement gear pump E1], and the primary pump is a piston pump [E2]

The logic here is that whilst a piston pump is technically positive displacement, the fluid output is literally a series of pulses [number of pistons*RPM]

So based upon this premis, I would have assumed, by placing the higher displacement engine [E1(0.4)] as the servo force, the final difference in thrust [E1*(0.4)-E2*(0.32) = Ex], however the total thrust is [E1+E2-Ex]

So [Ex] is the thrust that causes steering offset.......in your trials, are both engine speed throttles controlled from a common servo?

I don't believe you could simply +/- these to attain an answer....still interesting... Derek

[Jerry Hill]

Theres a few things to compare it to out there, 'pressure recovery' marine systems touches on it too.  Cessna has the only real world example, we've all seen contra rotating props right next to each other, as per Fairy Gannet or even some Mustangs, but this is not that, only the Cessna 337 has this spaced out arrangement.

It's important that I add that this is not an efficient arrangement, any thrust advantage is easily lost in engine weight, but it's not about that. It's just about the the fun of doing it 

The engine mount incorporates dedicated servos for each engine, waterproof mini Hitec's, they are driven by a Graupner MZ18 Transmitter set up so that the throttle lever controls both engines together, and a seperate turn knob adjusts the balance between them, from WOT FWD/IDLE AFT to IDLE FWD/WOT AFT depending on the main throttle positon. I've programmed in variable rate though so that the fwd engine comes up slightly ahead of the aft, because the aft engine is dependent on cooling from the fwd one, so I don't want it revving higher than the front one can cool, as it were. I also have a tacho sensor on the fwd engine and temperature sensor on the aft fed back through the transmitter's telemetry system.

[Jerry Hill]

Here's the servo position, same on other side:





And alongside the test boat for comparison:

[TheLongBuild]

This should be fun 

[Martin (Admin)]

Lovely engineering!  

[Jerry Hill]

Well we'll see. There's no hiding from the difference between minimal sufficiency and over engineering such as this, but as the enjoyment for me is the act of making things, rather than assembling purchased parts (which I also do when it suits) even if it simply doesn't work I've already had my fun. The looming issue with this idea is weight, but, airboats can run happily even when carrying huge weight, take a look at this boat we made for my lad, 18lbs drven by 10cc: https://youtu.be/LtAxVY2elgM

If it runs like this, then I'm there: https://youtu.be/B8BTwHYusJ8

[Tim_M]

Very interesting subject. I'll be watching this. Nice workmanship too!