View Full Version : Rotax on a Bensen.
07-25-2004, 09:10 AM
Does some one know the mersurment to install a 503 or 532 rotax on a Bensen?. I am not goin to do it now I just what to have it handy if some day I change my mac 90.
07-25-2004, 05:06 PM
I did it to a KB2 to a KB3 I made the mast 72" and used a 60" prop
07-26-2004, 02:44 AM
Brent, did it work out real well?
07-26-2004, 03:18 AM
yes it did but that was not the only mod I did. I put a tall tail and Mt. bike froke nose wheel linked so you push right to go right not the old push right to go left, that KB2 had. Added a seat tank and you get this.
07-26-2004, 03:26 AM
Wow, That looks great, Brent. What engine did you have on it?
07-26-2004, 03:37 AM
rotax 532 and I realy liked this gyro very light and no pre rotoator it had 24' rotordyne blades
07-29-2004, 07:23 AM
The Rotax redrive engines all require a longer prop than a direct-drive VW or Mac. If nothing else is done to a KB-2 except to swap engines, you'll end up with a prop thrustline that's 6" above the CG, with the very weak stock horizontal stab. That's not good. At minimum, add a good-sized horizontal stabilizer located in the propwash. A tall tail like Brent's will fill the bill. So will a Ron Herron T-tail. Thses H-stabs should ideally have at least a couple degrees of negative incidence (leading edge lower than training edge).
Even better, raise the seat enough so that the prop thrustline intersects the CG. Then do some flight tests to see if you've achieved positive static pitch stability.
07-29-2004, 07:54 AM
Doug: but I have to raise the engine mount, if not the prop will hit the keel! How much will that be?
07-29-2004, 07:56 AM
Other thing I am use to Mac 90 engine. Will I feel alote of change of power? We are talking about changing a 90 for a 50, 60 or 70 HP.
07-29-2004, 08:12 AM
Joe: The usual prop for a Mac is about 48" in diameter. The usual prop for a Rotax with 2.58:1 or 3:1 gearbox is at least 60" in diameter. So you'll have to raise the location of the center of the prop by about 6" above the standard KB-2 location.
The best position to mount the engine is with the sparkplugs UP and the gearbox DOWN. Many people put the gearbox UP, but that puts the engine lower and keeps the CG down. In any case, DON'T put the sparkplugs down (as in the early Air Commands).
You want the CG to be as high as the possible. This minimizes the "thrustline above CG" problem.
You still should install an effective horizontal stabilizer in the prop slipstream, however.
07-29-2004, 10:08 AM
Will I feel allot of power difference? :confused:
07-29-2004, 01:42 PM
Joe, perhaps some other folks will answer that. I haven't made the comparison myself. The thrust figures suggest that there isn't much difference. The Mac may have more horses (or not!), but the Rotax has a reduction drive. The re-drive gives you better prop efficiency -- more thrust per horsepower.
07-29-2004, 04:07 PM
Thank you Doug for you good info.
07-29-2004, 04:53 PM
the performance of a MC72 with a super mack runin 100 LL was dyno tested to 68 HP. a 90 HP I can strapolate it will be in the 72 HP range. Now the rotax swing a bigger prop so you will be moving a more air.
In a rcae withan ari comand with a 582 and a 72hp MC the aircoamnd was doing 60 and the MC was trailing behind at 70% power. This are facts.
07-29-2004, 05:45 PM
Thank you Charles!
07-29-2004, 06:00 PM
A couple of months ago I had a chance to fly an Air Command CLT with a 582. It wasn't a very good flight test as there were a couple of quirks we were investigating, but (and it hurts to say this :) ) the 582 seemed to have as much thrust if not more than my Mac. It had a 3-bladed Warp Drive on it, no pod other than a little round one for the instruments, and 25' Skywheels. It also had the regular short Air Command tail, and would yaw all over the place with power changes.
There was excess friction in the control system, so I was paying more attention to keeping it pointed in the right direction than anything else. But when I put da hammer down, it really climbed!
I hope to fly it again soon, and may be able to do some back-to-back comparisons.
Here is a not-so-good picture of the machine with Rod Reed (l), and Bob Simmons, the owner(r). As you can see, it's a fairly minimal machine.
07-30-2004, 08:25 AM
Mike thank you for that good info. When you do your back to back comparison please post it on the topic to know the result. We never had a comparition of a old school engine with the new ones allot of people will like that topic. Mike that will be a good topic.
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