Best Engine

[ElJay]

Ok gang....I'm just about ready to start research on the best engine/prop for the bensen I'm rebuilding. Pros/Cons on Rotax and/or Hirth. Only interested in air cooled, don't want the hassel of water cooling...Best size/hp-performance-for my weight @ 235 lbs?? Will probably buy new unless a really good thing comes around, but will have to be VERY low hours....Thanks in advance

LJ

[Brent_Brown]

I hope you plan to change the mast height, so you can use that type motor.

[ElJay]

No, I didn't know that. Why? Please share your knowledge with me. I really don't know what I'm looking for and cant afford to buy wrong....The only reason I said those two names as they are the only ones I have seen/heard of, since I can't get a mccollough anymore.....thanks again for your help...
LJ

[Brent_Brown]

I bensen that had a Mac on it will have a 58" mast the rotax or other redrive motors will need a taller mast so you can use a prop 60+". I think you can still get new Mccollough.

[dabkb2]

Has Anyone Tryed A 912 On A Kb3?

[GyroDoug]

Dave,

The only reason I can think of to be flying a KB3 rather than one of the newer and more modern designs is because money is tight and you are trying to get in the air more cheaply. If that is a major factor in your decision making process then putting a Rotax 912 on your Gyro is not something you would want to consider because it is much more expensive than any of the other options out there.

Gyro Doug

[PW_Plack]

LJ,

I see you have few posts, so this may be new info for you.

When the high-RPM engines with reduction gearing came into use on gyros, their advantage was that they could turn a larger prop at slower RPM through the gearbox, producing more thrust for a given HP level.

This required a taller mast to provide prop clearance between the rotor and the keel.

This, in turn, led to machines designed with the propeller thrustline well above the gyro's center of gravity, which resulted in pitch instability, because the prop thrust was always trying to rotate the airframe nose-down. (This is the genesis of the "centerline thrust" debate you'll read much about in the forum archives.)

Since the original Bensen design anticipated a prop of 56 inches diameter or less turning at 3200 - 4000 RPM, you'll have some redesign to do if you want to adapt an engine turning a much larger prop at 2500 RPM.

An early choice for Bensen fliers seeking an inexpensive four-stroke upgrade was the air-cooled VW auto engine. They were considerably heavier than the Mac, and problematic if not properly converted, but they all make their max HP at about 3200-4000 RPM, and so are appropriate for props of the proper size to work with the original Bensen frame layout.

There are at least three reputable choices for VW-based aircraft power these days, and all have years of experience addressing the crankshaft and bearing issues of the early days:

Revmaster (http://www.revmasteraviation.com/products/r2100_engine/index.htm)

Great Plains (http://www.greatplainsas.com/)

AeroVee (http://www.sonexaircraft.com/engines/engines.html)

There are also small, air-cooled aircraft engines available, but most make max HP at lower RPM which, again, requires a larger prop to take advantage.

Good luck!

[dabkb2]

I really like my KB3 with a 582 rotax, I was just thinking about the reliability of the 4 stroke.

Does the 912 weght more than a VW?

Can I put a 60in prop on a 912?

[GyroDoug]

Dave,

The Rotax 912 is a lot lighter than a VW engine and a much preferable engine. The catch is it costs $15,000.00. It will easily swing a bigger prop than your airframe will be able to handle. You could modify and change your KB3 but then it really wouldn't be a KB3 anymore it would me a modified one of a kind Gyro and if you don't have the engineering expertise to make those changes properly you may end up killing yourself. That's why I said if you want that kind of engine and you can afford that kind of money you should just get a newer design that has already been professionally designed for that power plant and will be safe to fly.

Gyro Doug

[Friendly]

Jack,
I started my gyro adventure with a Bensen with Mac Motor. I had it a very short while and added CDI which was about $500 . Then more and more instruments etc.
If I were building a Bensen to accept the aircooled (redrive) engines, I would drop the Keel 6in and add a Rotax 503. I think you would have a very good machine for your weight. If you add a set of 22 or 23 foot DW rotors you would be very happy with it as a 1st gyro (in my opinion). It would be about the same performance as a ultrawhite. It is more work to add the tail, but if you are building a Bensen and decide not to go with the Mac. Consider the tall tail from Ernie at RFD. It will be about $300 more than what you would be spending on a CDI unit and you will probably want to add a HS to any gyro you have so the price will break even.
The 503 is probably the most reliable aircooled engine in use today. I am not a rotax advocate, I just had to join those that have use that engine because it has proven to be so reliable. I would also consider the MZ202, from Compact Radial. I would not buy a Hirth myself.
Hope this helps and vist the Drop keel KB 3 thread to see what others have done.
I will include a picture of a Bensen frame that is converted to the dropped keel for a Rotax 503, or any 60 in prop.

[Timchick]

There's a Sport Copter Lightning (503) listed in the new PRA mag for around $10,000. Thet route is better than spending half that for a new 503 and then having to modify your Bensen.

[Oskar]

Many years ago I threw away the Mac on a Benson and put on a 503. Had to extend the mast and make new engine mounts, but that was about it. I mounted the engine with the gearbox pointing down, and raised the seat a bit to get the CG higher.

It flew really well, but at 5500ft and 165lbs performance was marginal on hot days. The 503 was at full throttle most of the time, but never missed a beat. My latest gyro has an MZ202, and from the limited experience so far I prefer it to the 503.

[Brent_Brown]

http://www.sabretrikes.com/Propeller.htm

This web site has very good photos and details of the MZ 202. if I use it again I will get this pipe they have.

[Friendly]

Many years ago I threw away the Mac on a Benson and put on a 503. Had to extend the mast and make new engine mounts, but that was about it. I mounted the engine with the gearbox pointing down, and raised the seat a bit to get the CG higher.

It flew really well, but at 5500ft and 165lbs performance was marginal on hot days. The 503 was at full throttle most of the time, but never missed a beat. My latest gyro has an MZ202, and from the limited experience so far I prefer it to the 503.

Oskar,
do you have some pictures of your gyro??

[Oskar]

Friendly,
There are some on
http://www.starbeegyros.com/gallery.asp
Look for the RDA registration.