See what happened when I took one of the fastest ready-to-fly RC planes in the world and attempted to make it faster, using multiple rockets spread across the wings.
I wanted to find out exactly how much faster I could make this already very fast RC plane go through first increasing it’s performance on electric power and then adding rocket boosters.
For this project we would start with one of the fastest planes you can get off the shelf, the E-Flite Electrostreak.
This plane is highly streamlined with a very small frontal area for cutting through the air, and this means it can apparently reach speeds of around 100mph in straight and level flight.
After a quick assembly, we took it out for a test flight to find our baseline speed we would then improve upon.
Baseline Test Flight
94mph - a good start, but I thought we could get this thing to go a bit faster.
The cameras were creating a lot of drag, and we could see this through testing a CAD model in a virtual wind tunnel, using a software called Airshaper so the first problem was figuring out how to optimise the camera setup for higher speeds.
The best idea would be to reduce the number of cameras to an operational minimum with this single camera embedded as much as possible into the wing which would lower the frontal area.
Now we had made the plane more efficient, we could next work on adding more power!
The plane had a very powerful motor, but the stock battery only provided it with 14v.
I knew we could probably squeeze a bit more out of this motor without burning it out, especially seeing as though it would only ever be on full throttle for a few seconds during our speed runs, so I decided to test out a 6s battery with a nominal voltage of 22v to increase the RPM of the motor dramatically.
Now to test the motor to see if it would turn faster, firstly with the original battery as a benchmark.
With the 4s battery it reached a rotational speed of 14,000rpm
And with the 6s battery it was at 22,000rpm.
With this change it was time for another test flight to see if we'd increased our speed
Power Increase Test Flight
The plane had gone a bit faster, but not by as much as I’d thought, probably because of the weather conditions. For the next flight, we’d wait for the perfect day.
Now, it was time for Phase 2 where we would take the plane to the next level with some rocket boosters.
The first problem was to figure out which rocket engines to use.
I decided we would use these off the shelf rocket engines for this project, as in the UK you can’t make your own.
Previously my rocket plane designs had only ever used a single rocket booster, but this time I wanted to experiment with multiple rockets.
One option when using multiple rockets is to mount them symmetrically on the wings
Next, though, we needed to build some reloadable rocket pods to hold the rocket engines under the wings.
Some model rocket tubes were first cut to size and then fitted with some 3d printed aerodynamic noses.
After a bit of paint to make them moisture resistant, they were pretty much ready for mounting, but first we needed to make the plane a bit more visible in the sky to make it stand out at higher speeds.
For mid air rocket ignition I would use some RC switches that would send power from an extra battery to the rocket engine igniters.
After wiring the new electronics into the wing of the plane, our new speed plane was now finally ready for it’s big test.
Carefully, I installed the first rocket engines, the two C size engines that would produce a D size impulse of around 3kg of thrust over 2 seconds of burn time.
The igniters were next, and these had to be very precisely fitted so that the igniter was in contact with the fuel.
The conditions were great, and the plane was ready, so now it was all over to me not to mess this up.
In position over the field, I throttled the plane to around 50% and fired the rockets
Rocket Test Flight 1
Success! Both rockets had fired and plane was still in one piece.
It seemed like we had achieved the first Project objective to prove that multi engine rocket plane flight was possible in RC form.
Now we could size up and see if we could increase our previous speed record with this airplane with the four c size motors which would double our impulse, with 6kg of thrust over 2 seconds, equivalent of a single E size rocket motor.
Time for the second test flight!
Rocket Test Flight 2
We had a problem! The rocket engines had ripped off the airplane!
The wings had twisted violently after ignition and the control surfaces almost ripped clean off!
They were hanging on by a thread and the plane was still just about flying - but now I had a big job on my hand to get the plane down.
Assessing the footage back at the workshop it was clear to see what had gone so wrong.
The ignition of the rocket motors had been successful, but the rocket pods had detached due to a catastrophic aerodynamic flutter.
Critically, this foam RC plane just hadn’t been rigid enough.
The force of the ignition had twisted the wing and resulted in an extreme aerodynamic flutter which ripped the pods clean off.
So, we had increased the speed of the stock plane and found multi-rocket ignition to work reliably - we just probably needed a stronger plane!
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