For my Bugatti 100p project I wanted to use a contra rotating motor as this is a prominent scale feature of the aircraft. As commercially available options were cost prohibitive and would not provide the power required for the project, it was necessary to tackle the creation of a custom solution. I’ll go over, step by step, how I went about my very first foray into mechanical engineering for my custom contra motor.
The idea is rather simple. Two in-line propellers turning in opposite directions. There are two main advantages to this:
1: The props spinning in opposite directions cancels out any torque that would otherwise be sent back into the airplane.
2: Efficiency. The accelerated air from the fore prop allows the aft prop to perform the same work with less energy and in so doing accelerates the air further. The efficiency is measured by an increase in thrust and decrease in energy use compared to two engines mounted separately, for example on the wings. Further streamline efficiency is gained by reducing the frontal prop disk area and thereby reducing drag in the process.
Contra rotating is not to be confused with counter rotating props. Counter rotation implies that the props are indeed spinning in opposite directions (eg. P-38 lightning) but are mounted separately rather than in-line.
Traditional full-scale contra setups utilize a gear box of one form or another. A planetary gear system was typically used so as to use just one large engine to turn both propellers. The full-scale Bugatti used a gear box that utilized two engines, one mounted behind the other with drive shafts going on each side of the pilot to the gear box at the front.
In the modeling community there have been many contra setups fashioned using gear boxes. For F3A competition aircraft there is a commercially available planetary system that is quite impressive both in performance and price. Other small systems (and large) have used gearing of one kind or another to achieve the same effect.
The advent of the brushless electric motor has drastically improved electrical efficiency as well as ability to drive fantastic speeds. The National Model Pylon Racing Association (NMPRA) has published rules surrounding motors and props that are intended for speed. Given the size that I planned for the Bugatti, the F-1 class of motors was a perfect fit.
To use brushless motors in this way is not new. There have been many folks who have done this in the past so I read a lot of information on it and decided the best course was to follow their lead. A custom outer shaft would need to be fabricated to adapt to the use of two identical F-1 motors, the NTM PropDrive EF-1 1300Kv motor from HobbyKing. I’ve used many of the NTM motors over the years and have never had issue with them.
The idea is that the two motors are bolted together by their X-mounts. The aft motor receives a new shaft of the original diameter but a bit longer. This shaft goes through the fore motor and drives the front propeller. The fore motor has modifications made to the bell that houses the magnets to accept a custom shaft that allows the shaft of the aft motor to travel through and spin freely on ball bearings. This is the part I had to design.
Others have used bolt-on custom mounts that I did not much care for. They added mass (lowers efficiency and increases gyroscopic effects) and required very precise alignment for drilling and tapping of the bolts from the adapter to the bell. I decided to try drilling out and tapping the bell and have the custom outer shaft threaded so that it could be screwed into the bell. A nut could be used from inside the bell to lock the depth of the outer shaft. This required precise machining of the outer shaft of course but further precision of the drilling and tapping of the bell. If the bell was slightly off-center the bell would wobble and vibrate and be useless.
To ensure the bell was modified on dead center the drilling and tapping of the bell had to be done on a lathe. The bell was sent to a recent FTFF2016 friend who spoke up when I was looking for a machinist. His friend does this sort of thing as a job. The inner lip of the bell that provided spacing was first removed.
Then it was determined that the thinness of the wall of the hole drilled in the bell would potentially crack while the hole was being tapped. A reinforcement piece of aluminum was put in place and left there for safety concerns.
After being tapped to M12x1.25, the bell was sent back to me.
The Outer Shaft
The outer shaft started life as a single piece of steel rod. It was threaded to M12x1.25 and drilled out with an inner diameter of 6mm to allow the inner shaft (5mm outer diameter) to easily pass through. Each end was then milled out by 2.5mm deep and 8mm diameter to accept the bearings that would support the bell over the inner shaft.
A huge thank you to my friend Kaid at Sprocket Specialists for doing this work. He was fast and precise. He also made me TWO of them.
At first the bearings did not fit in the custom outer shaft. Using my digital calipers, I measured the diameter of the bearings at 7.976mm. The part measured at 7.874mm. That 0.102mm difference was enough to prevent the bearings from slipping in. The bearings were sent to Kaid along with the custom parts and he further machined them out to a press-fit. He mailed them back and we discovered how much weather can effect steel. The bearings were free-spinning when they left ID but had become compressed and would not easily spin now that they had arrived in MD. In order to be removed from the part the bearings had to be destroyed and new ones ordered (at $10 each). At this point I did not want to send the part back again but I did notice some abrasions in the inner lip of the seat for the bearings. I polished the seat out with the dremel and rubbing compound and the new bearings slipped right in.
With all the parts finally in-hand I started the assembly. First, the rear bell got its new shaft and the two X-mounts were bolted together with the new shaft in place.
The custom outer shaft screwed into the machined bell like it was an original part. After setting the correct height of the outer shaft I screwed on the locking nut from the inside.
The nut would not clear the stator windings so I used a hack saw to cut 1/3 of the nut off. It was just enough (2 complete thread turns) to lock the shaft to the bell. You can see the size difference in the image below of the modified and original nut sizes.
A brass washer was used as a bushing between the bearing in the stator and the bearing in the outer shaft.
Another brass washer was used at the front end of the outer shaft as it would meet the compression collet prop adapter for the front prop.
After that a simple compression collet was used on the inner shaft for the front propeller and the assembly was complete.
I wanted to do a simple test of the assembly to make sure things were running true so I kept the battery on its storage charge and did some "light" testing. In this video I also talk in-depth about how the assembly works and you can see how the props spin independently from each other.
After this I moved on to the spinners. The rear spinner started life as a 3-inch Dubro spinner and I cut the top off and balanced it. I had to further modify the mounting plate to make it thinner to fit onto the shaft along with the prop.
A 2.75-inch spinner was a perfect fit for the stranition to the front prop so I balanced this one and it fit like a glove.
To really test this setup I was fortunate to have my friend Carl over and with a fully-charged pack we ramped it up.
We noticed that the force from the propellers was just too much for the collet prop adapter for the front prop. We added some nothches in the adapter to facilitate a tightening of the nut but as you can see in this video it was still not neough. I then used some 220 grit sand paper to rough up the inner shaft tip to improve the grip on the collet adapter. Remember, this not only holds the front prop in place but also the rear prop and motor bell assembly.
I also noticed that there was a bit of rubbing between the bearing of the outer shaft and the front prop adapter. I used my dremel to remove some of the brass around the outside of the washer and it seems to be spinning more freely.
Further scoring of the rear bell and back plate was made to ensure better tightness. This was also done to the back of each propeller. Additional grip was needed to get the nuts as secure as possible. Safety first, second, and third.
With the motor seemingly complete and the major mechanical bugs worked out I cut a ply ring for the firewall and used some blind or "T" nuts to mount the motor.
I had to use the dremel to remove a bit of the metal on the inner lip for better finish and clearance of the motor bell.
Next it was time to mount the plate in the nose of the fuselage.
Its a perfect fit and keeps the look of the airplane in its scale form.
I want to first thank my wife and children for putting up with me during this abitious project so far. I also want to, again, thank Kaid and Martin for their help with the machine work. None of this would have been possible without them.
I'd be remiss if I did not thank and acknowledge Hobby King for sponsoring the Bugatti build. Many thanks for the support and parts. They have been very helpful through the process.
Lastly, I want to thank this wonderful community. Thank YOU for YOUR support in the build thread. Suggestions and questions are always welcome and it has been wonderful to have the community behind me for such an ambitious project. If you would like to see me continue these complex builds, consider a like on my videos and subscribe to my YouTube channel. The more community support I gather there will help garner support for future builds.