Comanches in the 21st Century

A 300 MPH Comanche!

By

Charlie Horton

Happy me with my normally aspirated race winning Comanche 400 and its blunt nose bowl, but now I've found a way to make myself even happier ... and much faster .....

No doubt about it, a Comanche 400 is fast. Add a supercharger and you have people scratching their heads saying, "You've got to be kidding! You installed a supercharger on a Comanche 400?" Move over Bonanza, Mooney, and almost every other single engine aircraft, and most twins too, because the supercharged Comanche 400 is alive!

It all started about a year and a half ago when I was at a Corvette birthday bash in Bowling Green, KY. I saw a Vortech supercharger on a Corvette and the thought occurred to me that this supercharger might fit on my Comanche 400. After that Corvette show, I was on a mission to fit a supercharger on my Comanche. I studied it, made drawings, and got completely engrossed in the project. Just as I was ready to start, I met a couple of people at Sun-n-Fun who had just completed a supercharger installation on an experimental aircraft with a Continental engine. After talking a while, I joined forces with them. Six months later, I had a supercharger on my Lycoming IO-720.

Why a supercharger? A supercharger is belt driven and doesn't require a lot of extra plumbing. The inlet temperatures are 80 degrees above ambient temps so an intercooler is not necessary.

By contrast, a turbocharger is exhaust driven and uses hot exhaust gas to spin the impeller. With the turbocharger you have to chop a tuned exhaust to fit the exhaust plumbing, which reduces horsepower. Then you add a waste gate which produces more heat and more plumbing. And that adds to the intake plumbing including inter-cooling in some cases. Inlet temps on a turbocharger system can reach upwards of 290 degrees. That's detonation range. Then you have the heat problems to handle. To get rid of excess heat you sometimes see shark gill louvers on the side of cowlings. These louvers cause cooling drag over the wing root and fuselage resulting in slower true air speeds. Then you have exhaust back pressure problems causing CHT problems at altitude. So now you have to pull back some of the power to keep the temperatures under control. Even though turbocharged systems have the potential for more boost and power, they are usually limited by CHT's. It is necessary to reduce the power to keep the temps in the green. That does not do a lot of good when your primary goal in boosting is to go fast! Another problem is if the turbocharger fails, the flight is basically over.

Now compare the supercharger. If the supercharger belt fails, you continue to fly normally aspirated. There has never been an impeller failure on a Vortech supercharger. Not ever. And hundreds of thousands have been sold. If the impeller fails, you can still fly normally aspirated because the engine still breathes through the impeller although at a slightly reduced manifold pressure due to interference (about 1" less MP).

To control boost, we designed an inlet control butterfly that's operated from the cockpit by a vernier cable. With the butterfly closed, it normalizes at 30" MP. Since full boost is never utilized (full boost at sea level would be 60" MP), it only takes 12 horsepower to operate the supercharger. As another safety feature to protect the engine, there is an over-boost valve on the pressure inlet which is set to 34" MP. If I inadvertently over-boost the engine, the excess is dumped overboard. The concept is to have sea level pressure in the flight levels.

A 14" drive pulley bolts to the prop flange and drives the supercharger pulley, either a cruise pulley or a race pulley. At 2650 propeller RPM the ratio with the cruise pulley spins the supercharger at 41,000 RPM. The race pulley spins the supercharger at 54, 000 RPM. The maximum limit on the supercharger is 70,000 RPM. So you can see we are not working the supercharger very hard. In comparison, turbochargers spin up to 90,000 RPM.

The cosmetics of the nose changed a little as the nose bowl had to be extended to house the supercharger. We also added round cooling air inlets while we were redesigning the nose bowl. As a result, the prop had to be extended 4 inches. The Hartzell propeller was too heavy to extend that far because of the polar inertia moment, so I found a composite propeller in Germany that was certified for an IO-720. The composite prop is 27 lbs lighter than the Hartzell, has no AD's, and has a TBO of 1800 hrs. I was starting to like this composite prop idea.

When I put the order in with the Germans, I told them I wanted to go fast at 25,000' and that I would be making over 400 HP at that altitude. The Germans, being very precise, built me a prop that works only at 25,000' and 2300 RPM and no where else! I lost 30 mph at the lower altitudes and 1000 fpm rate of climb. I explained to them that I had to reach 25,000' to try the prop out and needed the prop to work at 2600 RPM, not 2300 RPM. They said "oops". When I ordered the prop, I assumed it would work in the normal flight profiles, but wanted it to go extra fast at 25,000'! I assumed wrong! Not wanting to make the same mistake again, I ordered new blades that, "have a good rate of climb, good efficiency and performance at the low and high altitudes and work at 2500-2700 RPM." That should cover it all! They are building me new blades right now to fill that order. The company is being very gracious; they are building the blades and mounting them on my hub free of charge. An honorable company like that should be recognized: It's MT-Propeller.

Also, on my ferry flight home, I tried to reach the mid 20's but started to get spark scatter from the magnetos at 19,000'. I have eight of those plug wire holes in the same size distributor block as a six cylinder, so the holes are real close. When you lose the resistance of thick air, the spark can easily jump to the next wire. So I stopped at FL190 and worked the problem out by reducing power.

So you ask, "How does it all work?" I have a serious go-fast machine now. Consider that the prop is not efficient at FL190, I reduced the power to 2400 RPM for spark scatter The supercharger is exponentially more efficient at 2600-2700 RPM. Remember, it is belt driven and needs high RPM to produce boost., and I only have the "cruise" pulley mounted on the supercharger for flight testing, not the race pulley. The cruise pulley reduces available boost by 25%. With all that said, my TAS was 275 mph at 19,000'. On my ferry flight from Las Vegas, NV, where the supercharger was installed, to my home base at Diamondhead, MS, I was able to find some tailwinds and had an average ground speed of 310 mph!

As this is being written, the magnetos are being pressurized and should be on soon. I will take the plane up to FL250 and see what it will do, considering the prop doesn't work above 2300 RPM. Projected true airspeeds are over 300 mph with indicated airspeeds well within the structural limits of the aircraft. Next I'll put the race pulley on and see what difference that makes. Ultimately I'm looking for 315-320 mph TAS! The new blades will be ready for installation at the end of December, then I can flight test them and compare the numbers to the "slow" blades.

Once I get the problems solved I plan to run NAA speed record runs at altitude. There are some speed records in my class, C1c, that have been in the record books for 20-30 years. The closed circuit record is held by a Bellanca Super Viking at 305 mph (I didn't know a Super Viking could go that fast. Do you think ATC radar wasn't working very well 30 years ago?). Also, I plan to attempt the coast to coast speed record run. It's held by a Mooney at about 240 mph. Remember, the clock keeps ticking while refueling, so time is critical. The projected course is from San Diego, CA to Jacksonville, FL. If anyone on that course line would like to help with a re-fueling stop, please contact me. Initial plans are for one re-fueling stop somewhere in the mountains. If we land early at a high altitude airport, we will not have the climb penalty from sea level. It would help if we knew someone to personally handle the arrangements for the refueling stop so there will be no misunderstandings that result in delays. We will be coming in with our hair on fire looking for about 100 gallons of fuel and a quick turn-around. I'll firm up the plans and fuel stops as we get closer and keep you all up to date.

Next question you may ask is, "What does the FAA think about all this?" I have been working with the Wichita FAA since the beginning of the project and they have been wonderful. There are rules and regulations to follow but they are willing to help with anything to guide this project to completion. They have been more than patient with my delays; safety has always been the top priority. I know you all have heard some bad things about dealing with the FAA but these guys in Wichita really know how to make a flying project happen. So far, I couldn't be happier. Also, Bob Weber from Webco Aircraft has been a tremendous help. We are lucky as Comanche owners to have Bob's knowledge and parts inventory available. And lastly, a big thank you to Hans Neubert. Hans is the structural DER on the supercharger project. Hans owns a twin Comanche, and for those who haven't heard of him, he is probably one of the smartest men on the planet when it comes to airplanes (I'll let Hans explain "polar inertia moment"). The supercharger project could not have been completed without the help of these experts.

If you have any questions you can contact me at: com400@mindspring.com

Here are some pictures of my Comanche 400, as well as some details of the installation:

New aerodynamic look with nose bowl extension

Belt driven supercharger with new baffling.

Belt tensioner. Cruise pulley on right, race pulley is smaller.

Note the NACA air inlet duct on the lower panel

The finished product