September 2019

250km/h, Under 250 Grams

After having built my first plane using a mini-quad motor (an already heavily modified Parkzone Radian) early in 2018, I was amazed at their performance compared to traditionally designed RC plane motors. The mini quad motor that I was using was 2300kv, had a 2205 stator and weighed only 27g, with wires. I originally bought it for £2.40, on sale, when I was making my first mini quad 6 months earlier. In comparison, the motor that the Parkzone radian comes with weighs almost 110g, despite that, the mini quad motor provided significantly more thrust at a higher pitch speed (with its less efficient propeller), drew only 10% more power and only reached the same temperature as the stock motor (1/4 the mass, so 1/4 the heat energy).

This experience (and some calculations) prompted me to use mini quad motors on all three versions of the Long Range FPV Plane that I have built so far, and I plan to put them on the V4 as well. They have proved to be very efficient while having a having a ridiculously high output power for their weight.

Exactly what you want for something designed to fly fast.

Story Time…

I first wanted to build something tiny and fast when I saw the now infamous turbine powered, 1-meter wingspan hotliner in 2014. It was recorded on the day going over 250mph with a radar gun. Not convinced, my Grandad and I shoddily timed the plane between 2 trees near either end of the display line. We calculated that it was going closer to 300mph. To me, at the time, it seemed like something so small, and self propelled, going so fast was just unimaginable.

A few years prior (~2011) I had built one fast plane (probably ~ 150mph) from the fuselage of an old powered glider and wing from a pylon racer (pictured left). However, at the time I didn’t have enough experience building planes and the horizontal tail folded while attempting to pull out of a high-speed dive. Naturally, the plane was far away from anything it could have damaged at the time, but the severity of the crash made me reconsider building anything overly fast until I had the experience to make it safe. The plne had buried itself almost entirely in the ground.

My next attempt (picture and video below) to make something fly fast was in August 2017, spurred on by a large, spare motor that I had taken from my 1:10 scale RC car. I designed a delta wing plane with a 700mm wingspan and an all up weight of 800g and to be somewhat similar to a Multiplex Funjet 2. It was designed to have a thrust to weight ratio of aprox 1.75:1 with a pitch speed of 180mph, on a 4s Li-Po battery. Unfortunately, I had lots of issues with the motor/speed controller combination and only ever ran the plane (in the air) on a 3s Li-Po battery, and even then, it wasn’t running healthily. Optimistically, it might have hit 100mph, a very big disappointment. I was busy at the time with other projects (the RC Car) and school, and I lost interest.

My most recent endeavour in speed was with a used Parkzone Stryker F-27Q (picture and video below) that I picked up at a model airshow in 2018 for £30, great value. Straight away it hit nearly 90mph without any modifications. With a high discharge rate 3s Li-Po battery it reached 95mph. I decided I wanted to hit 120mph without making any changes to the power system. I cut off the ventral vertical fins, I removed the rudders and all related linkages and servos, I removed the landing wheel, I redesigned the elevons, and many other small weight and drag saving modifications. After all that, I did manage to reach a GPS recorded 125mph, however only one flight later I crashed the plane at full throttle, due to pilot error. The plane fixed up fine, but it was already becoming evident that without serious time and effort the plane was never going to be ‘fast fast’ and it just being ‘fast’ wasn’t fast enough. It still flies fine to this day, still in the same configuration.

Story Time Over.

I decided to start this build after I finished the Mini FPV Plane V4, it flew just well enough and with enough weight to spare that I knew I could fit a seriously powerful motor/ESC/battery into a plane of a similar size and have it not fall apart in the air. However, when I started researching exactly what motor, prop, and battery combination would be best, I found out about ‘speed croco’ and their FDD class for aircraft under 250g. ‘speed croco’ is a German record setting event where aircraft have their speed measured over a 200m sprint, and their 250g class that was only started two years ago has already hit 292kph. This was very disheartening. To see that many people had not only beat a challenge that I thought I had come up with, but by a fair margin.

There was a saving grace though, most of the competitors were veteran 25kg class competitors, where everything is made out of carbon fibre and thousands are spent just on wind tunnel work. Therefore, they were still running full 1-piece carbon fibre moulded aircraft (mostly scaled down versions of their big planes) in the 250g class. I wanted mine to be on a much smaller budget.

Then I found something else, an RCGroups member ‘MikeRX’ had already been copying the speed croco guys for over a year making his planes mostly from balsa wood and preformed carbon strips - much more attainable, and best of all, he hadn’t formally measured the speed of any of his aircraft, so no disheartening feeling for me! I almost regret finding MikeRX, seeing his amazing work and his insanely high-performance aircraft is great, but seeing the equipment and design ideas he was using began to limit my creativity, with my ideas slowly getting closer and closer to building a plane identical to one of his. Almost.

By the time I found MikeRX I had already settled on using an E-Max Eco motor and a 65a Tekko32 Metal ESC, but I hadn’t decided on what batteries or prop I wanted to use. Based on what MikeRX was using, I decided on 2x 3s 500mah 65c lipos in series to make one 6s 500mah pack, and I would decide the propeller choice based on current draw once the plane was built. Due to my relative inexperience with wood, I decided that I wanted to go with a built up foam wing, very similar to my Long Range FPV Plane series. The main spar would simply be the largest square tube I could fit between the two foam wing skins, and the trailing edge would be made from balsa stock, for added rigidity.

I also decided on a hard balsa tail, about the same size as that on the Mini FPV Plane V6 and using a flight controller with a GPS to send data back to the transmitter. I wanted to add a full FPV setup, but figured it could wait for the next version.

With the rough lay of the land figured out, I got started on the build.

First Flights

The first flight was very scary, it was immediately obvious that the plane was flying very unusually, and it was a real struggle to keep it in the air. At first, I thought it was out of trim, but after doing my best to get the trim centred it was still a real handful, it became apparent that the plane was tail heavy, causing it to have very little natural stability.

This was a particular problem because I was intentionally trying to build the plane nose-heavy so that I could add weight to the tail to tune it, however, with it being tail heavy, I only had the very short nose to put weight on. Instead I decided that I would leave the plane tail-heavy and would instead add a curve to the elevator input, giving me finer control around the centre of the elevator travel making, it easier to manually stabilise. This was an okay solution for first few test flights, but would need to be sorted in the second version.

Other than that, the first few flights went without a hitch, the plane proved to be very predictable and had a great glide slope. At first it seemed like it wasn’t flying very fast, but once I really opened it up and looked at the logs, it became apparent it had plenty of speed! On the first flight I hit 168kph flat and 193kph in a downline. Over the next few flights, changing propellers to find the best trade-off between battery voltage sag and current draw, I hit 200kph flat and 244kph in a downline.

Currently my limiting factor is the performance of my Li-Po batteries. The ones I’m using are over two years old and have had plenty of cycles with other planes in that time, as a replacement I have ordered 2x 3s 300mah 80c lipos and 2x 4s 500mah 80c lipos. The 3s 300mah lipos should be able to output more power than my current batteries even though they have a smaller capacity, partially due to their higher discharge rating, but mostly because they won’t have any wear.

The 4s 500mah lipos will put the plane over the 250g weight limit, but they will allow me to push the limits of the prop + motor + ESC combo more easily, which I will then be able to apply to the light setup.

I have also ordered a selection of APC propellers, mostly ones that are smaller than the propellers that I have been trying. This is because I believe at the moment that I am on the ‘backside’ of the motor power curve due to the load being too high. If I reduce the load on the motor significantly, the motor will be able to operate at a higher RPM where it is more efficient, so even though the power input will be lower, the output power may be equal or higher.