COMANCHES IN THE 21ST CENTURY
A 300 MPH COMANCHE!
- SUPERCHARGER UPDATE -
by Charlie Horton
I know I have the power! Since A 300 MPH Comanche appeared as an article last December, there have been some changes to some of the original designs that have slowed the completion date of the supercharger project. The changes were made to optimize and extract the most performance from the system.
The supercharger project has grown into two versions; one for cruise speeds for going to visit grandma, and one for airspeed records. The version for cruise speeds will keep supercharger RPMs lower, thus not producing enough compression heat to affect the inlet temperatures much. Speed with this version is estimated to be in the 275 mph range. The second version has metamorphosed into a truly testosterone machine. It is this version that has caused the delays. It is this version I will run for the NAA speed records. Projected estimated speeds are, at best, a guess because power and speed will be controlled by CHTs. I've got the power, the question is, can I use it? It is this version I will share with you in this article.
In the last article, I had a propeller that was not matched for the plane and power. In the beginning of the year I flew to DeLand, Florida where MT-Propeller installed a new set of blades on my hub. The blades we removed were type certificated (TC'd) for the TC'd hub. The blades we installed were not TC'd for the hub, but before I left DeLand, we had TC'd blades on a TC'd hub again.
While I was in DeLand I had the chance to work with the FAA FSDO office in Orlando on the propeller issues. Again, my relationship with the FAA throughout this project has been exceptional. Follow the rules and you have a better chance of carving a path to achieve your goals. However, due to rules, sometimes the answer may be "no." You just have to accept that or find an alternate means of compliance. It has really been a fun time for me working on this project and working with the FAA. It's probably because I don't do this for a living.
On the flight back from DeLand I took the opportunity to collect some flight data. It was this data that prompted me to change the design of a few components. With the supercharger turning up more RPM's, the original air inlet boost control plenum was not sufficient for the air demand. In other words, the supercharger was never satisfied; the manifold pressure (MP) would never peak. I had to completely redesign the air inlet box and, while doing so, engineered in a ram air door. If I can recover an inch of MP by ramming air into the supercharger, that will give me approximately two inches on the discharge end of the supercharger. Free power! I also added a filtered air source at the base of the air inlet box as an alternate air source. To reduce the wake behind the ram air door I added an air foil.
New design air inlet box – ram door on top, alternate air source on the bottom
The ram air/boost control door is operated from the cockpit with a vernier cable. The new inlet box took a couple of months to complete since I had only limited time to work on it. Believe it or not, I do have a "real" job where I own and operate a business in New Orleans, five days a week.
Baffling was the biggest challenge; yet the most important. Without proper cooling efficiency, controlling CHTs is a losing battle. If you recall, the nose bowl was extended 6 inches to accommodate the supercharger. That created a large cavity between the nose bowl and engine which Hans Neubert informed me was a huge source of cooling drag.
When Hans talks, I listen! So, I built a carbon fiber air inlet on the co-pilot's side and baffled it in, then baffled in an air "path" over the supercharger. Since the supercharger is belt driven it's impossible to stop all the air from passing around the belt into the nose bowl cavity but I baffled it tight enough that most of the air will be stopped.
Carbon fiber air inlet bridges the gap from the opening to the engine
The last change was the addition of an intercooler. The race pulley spins the supercharger in the 50,000 RPM range. More compression means more inlet heat. So now I needed a way to get back the performance lost from inlet heat. This is not your everyday, plain-Jane air-to-air intercooler. This is a state-of-the-art air-to-water intercooler. Yep! You heard right! Air-to-water!
As you may know, there is a limited amount of space under the cowl of a Comanche 400. So I had to find a product that would fit; then I could engineer the plumbing around it. I found a block charge cooler which measured 11" X 4". I built the inlet and discharge manifolds in such a way that the intercooler fits between the supercharger and the engine.
Intercooler before installation
Intercooler installed between the supercharger and engine
I welded the over-boost valve onto the air-charge cooler along with some pressure ports and temperature probe ports. The whole unit weighs 7 pounds. The air-to-water intercooler is much more efficient than an air-to-air intercooler, and has a lot less plumbing. The pressure drop across the core is .25 psi. Anyone who knows about intercooling knows that on an air-to-air intercooler the pressure drop is at least 3 psi. Since one psi equals two inches of manifold pressure, that relates to a lot of MP drop on an air-to-air cooler! I don't have the luxury of having twin blowers like dual turbochargers, so I had to find a way to keep the performance at peak with my single blower (supercharger). Graphs show that the air-to-water intercooler will give me inlet temperatures of 20 degrees Fahrenheit at FL200 and an additional 40 horsepower.
Completed installation of the supercharger and baffle
The intercooler system works as follows: There is a 15 gpm in-line water pump (weight 3 pounds) that circulates the water from the expansion tank to the intercooler. From the intercooler, the water goes to the secondary heat exchangers and back into the expansion tank. The total water/glycol mix in the system is about two gallons. There is an over-pressure relief valve, temperature probe, and low water level probe in the expansion tank. The water temperature stabilizes at a specific temperature and continues to circulate. That temperature is calculated to be 20 degrees at FL200, and zero degrees at FL250. As far as power management, it looks like for every 14-15 degrees the intercooler cools the inlet temperatures from ambient, the power should be pulled back 1" MP to maintain equivalent power. For weight and balance, since the propeller is 30 pounds lighter, the overall weight difference is estimated to be 15-20 pounds more for the whole project. Putting that into perspective, the original STC for twin turbos on the Comanche 400 allowed 56 pounds more on the nose.
The last "miracle" that has to happen is the mounting of the two secondary heat exchangers. There are two 5.5" x 14" x 1.75" heat exchangers that the water/glycol passes through. Their function is similar to a radiator. This is the final stage of the project. Everything else is complete. I am hoping to have the heat exchangers mounted and functional by the end of May.
You may ask, what does all this mean in terms of speed? Isn't it funny how we measure the success of a modification by how fast it goes? The race pulley will give me an additional 5.6" MP in the flight levels. The intercooler will give me 40 more horsepower.
The propeller? Only a flight test with data will give me that answer. The propeller is calculated to be 88 percent efficient in the flight levels. My critical altitude is projected to be FL200 to FL210 (critical altitude is the highest altitude the engine can make sea level power). If the critical altitude is FL210, I'll continue the climb to FL240 where the engine will be at approximately 75 percent power (giving dense air power equivalency). So how fast will that be? Hopefully, 300+ mph! Time and CHTs will tell.
After we break a few airspeed records I'll put on the "grandma" pulley and go visit … well, grandma! Real fast!
It's been a fun project, regardless, but the real fun is still to come! My next update to you should be after the flight tests. Meanwhile, enjoy flying and have fun!