Written by Tony Chaveiro Friday, 21 November 2008 00:00
A few months ago I became aware of a new device that CSM Ltd. was testing to improve engine mixture control. The purpose of the unit was to maintain a more or less constant engine head temperature through the entire flight so that the ‘usual’ engine leaning towards the end of the tank wouldn’t happen. This sounded like a great idea and I was eager to try one of those units right away.
As it happens, I’ve only seen this unit work on O.S. engines so I wasn’t quite sure the Carbsmart would provide an improvement on a YS since they are known to be quite constant during flight (due to the regulated pressure feed) and past experiences had shown me that this particular engine was quite insensitive to outside temperature variations. I patiently waited until the production version became available and made my order as soon as possible. Since I’m a skeptical type of person, I decided to conduct a series of real life tests to confirm or disregard CSM’s (and sponsored pilots) claims about the unit. The result of these tests (and corresponding raw data) can be found further down in this document.
Since I haven’t seen any factual data about the CSM Carbsmart, I was interested in logging its performance during flight and see if all the hype about it was really true or just another marketing move like I’ve seen many times in this hobby. I decided to use my Align T-Rex 600N which is equipped to date as follows:
* YS 50 ST (latest generation)
* Hatori #522 tuned muffler
* 3 x Futaba 9351 servos on cyclic
* 2 x Futaba 3155 nano type servos on throttle and mixture control
* Logictech 6100GT combo as gyro and tail servo
* QuickUK fan and tail clamp (explanation about this later on)
* Arizona Regulator and 2600mAH Relion Li-on Eagletree Systems USB Micro logger with rpm and temperature sensor
* CSM Carbsmart and Revlock 20 (modified with CSM USB Interface)
* Radix main blades, SuperStubz as paddles and K&B Dream Design tail blades.
My intended goal was to mount both sensors (Carbsmart and Eagletree) on the engine so I could record the temperature during the entire flight and check if the engine control unit was performing the task. I also started to record data before installing the device so I could have comparative charts to use later on.
When my unit finally showed up, I was surprised at the size and weight of it. CSM quotes 5gr as the unit’s weight but apparently this must include all wires and corresponding sensor as I weighted the unit on my precision scale and the actual weight is rather 2.5gr. My first problem appeared as I was deciding where to mount the sensor on my YS. The Carbsmart official manual only provides a brief explanation and diagram of the installation on a Hyper-type head and since the YS has a standard cooling head, the installation suggestion wasn’t adequate. The biggest problem here is the top head mounting vs. wall mounting position as each location has its benefits and drawbacks. The crankcase wall mount is excellent for providing the most accurate temperature reading as it is away from the fan airflow and at 90 degrees from the exhaust port. Without external ambient influences, this type of measurement will be the most accurate of the engine internal temperature. The drawback is that the inner cylinder liner (made from steel) will increase the lag time between temperature changes in the combustion chamber and temperatures one will actually record on the cylinder wall. On the other hand, the top head mount position will do exactly the opposite. Since the head is made of bare aluminum, the heat transfer is quite fast so even small temperature changes in the combustion process will almost immediately be visible to a sensor placed there. The biggest drawback of this method is however the fact that the top of the head of our engines is under constant cooling that is provided by the fan itself which could lead to some temperature deviations.
After several emails to CSM’s tech support (seems like there must be a black hole on the Internet between my email server and CSM since some of the emails never reached CSM’s tech support), Wendy Mill provided me a new version of the manual which also shows a diagram for standard type heads and their suggested location was precisely the top of the head.
After I got this new information I decided to mount the Carbsmart sensor between the fins on top of the engine head (to reduce the unit’s lag time to a minimum) and the Eagletree sensor would be placed on the cylinder wall (to get the most accurate temperature reading of the engine internals).
CSM recommends the use of heat transfer epoxy or RTV so I started to look for an adequate epoxy to cope with the job. I was finally convinced by a fellow modeler to go with the Arctic Silver Alumina Thermal Adhesive which seemed to be the best at the time. This epoxy is extremely heat conductive as it is packed with aluminum particles yet it doesn’t conduct electrical current making it the perfect choice for this application.
After I finally had everything I needed, I proceeded to install the Eagletree sensor at the designated spot (see picture 1). The thermal epoxy worked wonderfully and was completely cured after 24 hours.
In the meanwhile, I decided to check the operation of the Carbsmart on the bench before proceeding with the installation but to my complete surprise, the sensor OK led was lighting up intermittently instead of showing a steady light. After some tests I concluded the sensor was actually defective. I promptly contacted Andy from Real raptors and before I completely explained what was happening, Andy had already sent out a replacement sensor for me. That’s what I call great customer service.
After three days the sensor finally arrived and I repeated the operation checkup and this time the light stayed perfectly solid. I prepared the thermal epoxy and installed the sensor carefully. Ten minutes later, while the glue was still curing I decided to repeat the operation test and to my deep sorrow, the sensor was dead again! This time, as I knew it was my fault, I extracted the sensor from the epoxy bed and removed the heat shrink to check out the sensor itself. As I suspected, the sensor is nothing more than a SMD Thermistor soldered between two wires. Way too flimsy to be handled normally. I contacted Wendy at CSM and asked for the sensor spec so I could find an alternative while I was waiting for another couple of sensors from Real Raptors. The thermistor is a Murata specific type so I tried to buy a handful of them but minimum order quantity was around 100 units so I decided to wait for my new sensors.
Finally, after another weekend the sensors arrived and this time I proceeded with extreme care with the installation. I even removed a little material between the top fins so that the sensor would drop right in without applying any type of pressure. This time things went smoothly and the sensor was fine even after the installation (see pictures 2 & 3).
The last thing left to do was installing the actual mixture control unit and the futaba 3155 nano type servo (see pictures 4 & 5). At this time I started to worry a little about the weight of the entire machine as it was already hitting the 3500gr mark. I ended up removing the standard 9254 throttle servo (40.1gr) and replace it with another 3155 servo (13.4gr). The only thing I had to add was a custom reduction plate made out of carbon (2gr). The final weight difference was about 8.8gr less than before installing the system (40.1gr – 26.8gr – 2gr – 2.5gr). Even though I was happy with the weight save, I noticed that the bird started to be slightly tail heavy (because of the removed weight in front). After some trial and errors I ended up replacing the stock horizontal fin and boom support with a QuickUK boom clamp that Budd kindly provided me last month. His clamp is exactly 5.5gr lighter than the stock clamp and horizontal fin it replaces and since it’s located at the middle of the boom, the effect is far superior to the 8.8gr I removed from the front of the helicopter. In addition, the lack of the purely aesthetical horizontal tail fin, the performance in fast backwards and backwards inverted flight is greatly improved. I would have never thought a little clamp could make my day!
After everything was installed and tested, I proceeded to set up the unit as explained with the supplied manual. It’s pretty easy to follow and besides the fact that one should not forget to remove the ratchet that is installed on the mixture needle, there is nothing complicated in the setup. The only thing I would have liked to see is a USB connection to a PC interface as I have for my Revlock and other CSM devices like the 720 Gyro. It would be nice to be able to fine tune the unit for each individual needs. I think that 10C steps are rather wide and a perfect setup would probably need something closer spaced. I set the preset target temperature at 100C just to be on the safe side.
When I arrived at my local field, I double checked everything to make sure I didn’t forgot anything and started up the engine. The servo drives the mixture control needle right at its leanest setting during startup so I was rather worried if it wouldn’t bee too lean at idle (the idle needle is only a percentage of the main needle) but I was wrong as the engine idled perfectly for over a minute while I proceeded to the flight line.
After some initial hovering around and seeing the needle move slightly in flight I proceeded to something more demanding to the engine. I first executed a couple of full climb outs followed by some vertical pitch pumping maneuvers and I immediately noticed that the engine smoke was changing slightly as the mixture was being richened by the Carbsmart. After this, I started to perform a series of back flips, knife edge and tail down tic tocs, immediately followed by some pie dishes and then wide open circuits. I could hear, feel and see the engine mixture being adjusted for the particular demand of each maneuver. I must confess that I had a wide smile all over my face as I was seeing my engine perform better than anytime before. It was really enjoyable to see my helicopter execute swiftly and without the slightest hick everything I could throw at it. Unfortunately, my flight time was reduced to about 7 minutes while my normal flight time used to be around 9:30 minutes. After landing, I immediately downloaded the Eagletree data for inspection and noticed on the rpm charts some bogging that was probably due to the rich settings at which the engine was running. I decided to increase the target temperature to 110C and also increase the gain up to 75%. The second flight was even more astonishing than the first and I truly thought for some seconds I was the new Curtis Youngblood in town. I particularly appreciated the sight of the engine getting automatically richer as I was performing knife edge tic tocs faster and faster. I would have never thought the unit would react in less than 3 sets to the increased load. After this flight (which lasted 8:30 minutes – good enough for me!) I downloaded and checked the logs again and this time bogging was almost non existent and the average engine temperature was only 1% off the preset target temperature. Really amazing I must confess.
Temperature and RPM data
In the following charts you may appreciate the engine performance and differences between a flight with no regulation on the fuel mixture needle and the use of a Carbsmart.
Please notice how the entire temperature curve has more of a soft wave form than on the unregulated flight and also notice the average temperature being very close to the defined target temperature. On the Carbsmart temperature chart, the highest temperature deviation is under 5C at anytime. There is a low spot almost at the end of both flights which is due to the fact that I have the habit of returning into a hover to check my fuel level and then go up for a final autorotation. On the Carbsmart chart, even at the leanest servo setting, the engine cooled down considerably mostly due do the use of the QuickUk fan. The recorded average temperature for the flight was 110.9C which is almost unbelievable as the target temperature was set to 110C. That’s less than 1% deviation of the preset temperature!
As you can see, the temperature curve is much more compact and has fewer variations during the duration of the flight. Finally you can also see that at the end of the flight, the usual temperature increase you experience on a non regulated system isn’t there anymore.
The unit definitely helps tuning an engine as it will keep temperatures under control thus allowing the pilot to extract more consistent power through the entire flight. The unit will potentially be able to save the engine in case of a fuel blockage or a leaking o-ring in the carburetor as it will detect the heat increase and react accordingly. I wish I had this unit last year when my OS Hyper o-rings started to leak and my engine died in flight as the head temperature reached 194C. Those that can’t tune an engine to perfection will appreciate the use of a Carbsmart too. You can even use it at the lowest setting to break-in an engine. I will never have another helicopter without one of those units installed. The price is very good for what you actually get.
As I already mentioned, a more sturdy temperature sensor would be nice and a clear warning in the manual like “handle with great care” should be in order. Let’s be realistic. The murata thermistor employed costs less than 20 euro cents per unit and even if you add labor, plugs and wires, the production cost of it is quite low. I’m sure that adding a drop of thermal epoxy on the sensor itself wouldn’t increase the total cost significantly but provide an almost bulletproof sensor that would be still very sensitive and accurate to temperature readings. Another negative point is the increase in electronics you end up having on your helicopter. We are reaching a point where a simple RC model has more electronic devices on board than some military UAVs. Crash costs will increase if you happen to destroy the unit or associated servo not to mention potential failures of any of the components.
Suggestions to the Manufacturer
I would like to take the opportunity and suggest some features for future developments of this product in case someone at CSM reads this article.
1. Sturdier temperature sensor or at least a clear warning in the manual.
2. PC interface port for those geeks that wish to fine tune the unit even further. The unit could display two temperature lights indicating that a custom temperature was selected in this case. Other features in the pc interface would be nice too.
3. A better fail safe. The unit will enter failsafe if the sensor breaks or shorts out but not if the sensor happens to fall off the engine. This can happen if someone just uses regular silicone as it may become brittle over time. In this case I would suggest something simple as an internal routine that would activate if the there is a sudden drop of temperature (like 30C) in less than 2 seconds, indicating that the sensor probably fell off. If this is coupled with the unit only starting to operate from 50C up, one could almost take out any chance of running the engine too lean due to a problem with the sensor.
4. Expanded features (maybe through the pc interface) that would allow the user to configure the failsafe position. In many cases a full rich setting in case of failure would be probably more beneficial than the currently employed center throw. This is specially true for those that didn’t tuned the engine properly beforehand in the first place but may also help to detect the failure if the engine becomes overly rich and sluggish.
5. Future integration with the Revlock Governor would be very nice as it would save further weight and make installation even easier.
Well, all in all I’m very satisfied with the Carbsmart’s performance. I did have some initial installation problems which made me think of giving up more than once but I’m glad I persisted and ended up with a wonderful setup. I’m going to stick with the unit in the future and I hope it will continue to perform as it does now. I would personally recommend it to anyone that isn’t afraid of adding more electronics to his machine and get the best possible performance one can achieve with the aid of modern electronics. Even if a pilot rather relies on manual tuning, the Carbsmart can add a valuable asset as a failsafe device in case of engine troubles which can become quite expensive.
Finally, I would like to thank Andy at Real raptors for his swift response and excellent care he had towards me, Wendy at CSM for providing me an updated manual, technical specifications of their thermistor and some good advice, Budd at QuickUk for helping me shaving off some excess weight and re-establishing correct balance through the use of their tail clamp, Derek Chan at RunRyder for recommending me the Arctic Silver Alumina Adhesive which is probably the best stuff I ever used and also to Martin at ArcHeli Cobra for pointing me in the right direction regarding the sensor installation.
( 10 Votes )