acronymn {border-bottom: 1px dotted}
logo for

leftimage for

Advantages of the Automotive Rotary Engine in Small Aircraft

There are several reasons I chose to use a rotary engine in my plane: its power to weight ratio is better than any other engine, it is new technology, and it is safer than a horizontally opposed engine.  Besides, I was interested in the challenge of getting the automotive engine to work as an aircraft engine.

About the Engine

I selected the Mazda Renesis, a 225 horsepower rotary used in the RX-8.  It is an improved version of Mazda's 13B engine that has been used in the RX-7 for years.

Power to Weight

Installed completely, including radiator and coolant, the Renesis engine will weigh about 50lbs less than an IO-360 and will give me 225HP verses 200HP.  This table has the details.

Weight Comparison Table (lbs)

Engine 301.1 194.5
Reduction Gear Box   42.0
Inlet duct, cleaner, etc. 8.5 8.0
Coolant heat exchanger   7.0
Coolant   14.0
Exhaust system 17.0 8.0
Oil heat exchanger and lines 11.6 8.0
Engine oil 11.5 12.0
Baffles and ductwork 4.2  
Engine mount 13.5 22.0
TOTAL 367.4 315.5


The lower weight of this engine lets me move the battery to the engine compartment instead of behind the rear baggage compartment.  The shorter battery to starter cable means lower resistance, allowing the starter to turn the engine better.

Air Conditioning?

I still think I will have to add weight to the front of the plane to get into CG.  My plan is to install the engine, saving weight where I can without sacrificing safety.  I will then put the plane on the scales and see how much extra weight I need up front. I can use that extra weight to add backup safety or, if I have enough extra weight, I can add air conditioning.  Wouldn’t that be cool (all puns intended) to have refrigerated air in a small airplane?

Technology difference

Face it, the horizontally opposed engine is 1920’s technology.  There have been improvements, but it is still the old “suck, squish, bang, blow” of a four stroke  where parts are moving in one direction, stopping, and then moving in the other direction 2500 times every minute.  There are loads of moving parts to fail and wear.  The rotary engine has three moving parts that all continue to move in the same direction the entire time it is running.  There is no stopping and reversing direction.


It is this stopping and reversing direction that causes wear, fatigue and failure in the parts of the horizontally opposed engine.  That is why we see so many busted rods and valves, broken cranks and cracked cases. This kind of catastrophic failure causes the engine to either windmill, resulting in no thrust, or to stop completely, producing drag.  The most catastrophic failure of a rotary engine would result from overheating.  Because the housing is aluminum and the rotor is steal, the housing expands faster than the rotor.  This means the rotor still spins, you just get blowby, causing you to loose power.  You will still have some power, just not the 225HP anymore, but the engine is still producing thrust.

The challenge

I am building my own airplane because I love the challenge.  I take the same approach with the engine installation. It is another puzzle to solve.  I am not buying a firewall forward kit, I am doing it all myself.  Call me crazy, but sometimes I wonder which I like more, working on the airplane or flying it.  Assembling, building, and installing my own rotary engine kit is exactly the right kind of challenge for me.

Return from Advantages of the Rotary Engine to Engine Decisions

Return from Advantages of the Rotary Engine to Kit Plane Advice Home

Kit Plane Advice Privacy Policy
© Copyright  All rights reserved.