Benson B8-M Improvements, Upgrades and repower


Super Supporter
Well This is the first Post On a long project.

I will be cleaning and repowering my Bensen B8-M I will give it some updated gauge and other equipment like fuel tank and so on.

But the Majority of the work will be concentrated on the proper installation of a Yamaha RX1 4 cylinder 1000cc I know some will say this will be
a bit too much power for the bensen and I will need to raise the throtle slowly so not to make the torque twist and roll but this will be gear to use the lower part or the rpm range
9000 rpm on take off and hoping for 7500 rpm on cruise i'm sure that it's possible to do with the weight of this machine.

I give my self a 3 year project for this but I'm mostly looking at 5 years and 5000$ investment.
But with this powertrain I will have a resent engine and not one from the 1940's

now give me pointers on that to look for and tell me if my setup makes sense like redrive gearing I will have about 120hp out of this 150hp engine.

installed weight of Continental A65 175lbs 65 hp
installed weight of Yamaha RX1 engine 165lbs 150hp but 120hp usable in my setup.


Super Supporter
For Sure I plan to install bigger tires and re-locate the hubs on top of the axle to lower the C-G on the wheels and make it more stable with bigger tires.
I will have support arms build from engine to axle to support engine better.
I will keep the engine at just about the same location but I will move it 1" to the right to compensate. I have a adjustment tab on my rear appendage to make it go strait I will try to eliminate this need in my new setup.
Probably will need to run a 3 or 4 blade prop on this since I can only use 54" long blades I don't want to change my Engine Centerline since it's like in the center of my chest at this setup
with what I have learned I need to keep it like this.

Jean Claude

Junior Member
The engine of the Bensen B8M is initially a Mac 72 Hp of 85 lbs, not a Continental 65 hp of 175 lbs
The thrust line of prop must be aimed at the GC of the aircraft in flight order (ie including pilot and rotor). The pilot's chest has nothing to do with it.
Last edited:


Gyroplane CFI
In my opinion if you want to have something completely different than what you have you should start over Frank.

There is simply not that much there to salvage.

In a happy world the thrust line goes through the center of gravity and a large horizontal stabilizer is mounted well back from the center of gravity.

In other words you need a taller mast and a longer keel.

So in our pile of trash we have an engine, engine mounts, propeller, empennage, keel, mast, rotor controls, seat and rudder controls.

Not much left of the original aircraft.

My advice is to learn to fly before designing and building a gyroplane.


Super Supporter
Well Thanks for your input .
My chest mention was just to tell you that I wanted to keep it at the same spot for thrust line.
I'm thinking of building a nice tail for it. but if you tell me it's not worth it then it's not.
Since it took me so long to get my medical my Instructor put me on a waiting list and it's a 2 year wait, I could jump on a plain fly 4 hours and take my course some where else but I won't this instructor is worth the wait.

Thanks for your comment I will reevaluate my setup and what I will do and keep you posted.

In 2015 I was told if I modify a Gyroplane not to reinvent the wheel use stuff that is proven many times by others I was sure this setup would be a good idea. but I guess it's not.
Last edited:


Super Supporter
I will now ask you guy's the most Important question.

If I build this project will you help me along the way I will need help and I will have many questions, so I ask will you help me?
If I get no help from you guy's then my Gyro is dead in the waters (Punt) So I need to know before I start.


Gyroplane CFI
I will help any way I can Frank and will not be upset if you don't follow my advice.


Super Supporter
I got skills I got tools and I can build just about anything. but I will need help with the small stuff that is big stuff for me.

Example. I have a tab on my rear Vertical stabilizer I know that is because it was flying sideways a bit and this was the best fix.
But I know that if you relocate engine 1" to the right this will not be needed since it will be cancelled out so I will need help for stuff like this.
since I want to do it right the first time and not have do do it 2 or more times.

I still have not made my mind but I'm leaning towards this build. I know I could have something different but I fell in love with this machine and from the start
I wanted to drop a yamaha in it.


Gyroplane CFI
It would be unusual to have a tab on your vertical stabilizer and I feel it would be a mistake to assume what it was for.

Sometimes the vertical stabilizer is mounted offset or the engine is offset.

Often the rudder has a trim tab to trim for a particular power setting and airspeed. This is to allow adjustment because there are very few hard and fast rules in building a gyroplane particularly with a one of a kind gyroplane.

In my opinion you are going to need to experiment to get her flying well.


Gyroplane CFI
That looks like a rudder trim tab to me Frank and generally speaking it is used to trim the rudder for a particular airspeed and power setting. Most production kits have one because it is difficult to predict how much trim it will need. Lots of thing work together to need a particular trim tab setting.
The rudder controls yaw.


Super Supporter
So After Taking all your info in account and doing some calculus I can tell you this project is still born.

Max Weight on take off is 631lbs Gyro is 385lbs so that leave me 246lbs for pilot and fuel and this is at max take off not a good thing to fly at max take off.
so about 50lbs to 60lbs for fuel so I would need to be 186lbs all dress up ready to fly and that would still be at max take off weight.

tell me calculation are wrong since there is no way I can go down to under 180lbs last time I was 180 I was 19 years young.


Dave Bacon
Hi Frank, I think you have more than enough power, you will need longer blades. To do that you will need to do a drop keel. It is very easy on a Bensen.That also allows you to put a bigger prop with more thrust.


Super Supporter
But like others told me easy to build from new then mod this one. plus new blades are expensive and if I add all cost it makes this build more expensive then buying a ready to run 2 place machine.

But thanks for the info.