Brock KB2 - McCullough or Rotax?

[BrianInVa]

Hi folks,
I’m bringing home my first gyro soon, an ‘80s Brock KB2. It’s pretty well complete except the original McCullough engine is long gone.

I have access to a freshly rebuilt McCullough 72 hp. Mounting it would be easy because the original mounting brackets for a McCullough are still intact.

However I’ve heard that McCulloughs are notoriously difficult to keep running well and parts are getting harder to find, so I’m seriously considering upgrading to a more modern 2 stroke, a Rotax 582. It would be much easier to have serviced and more dependable but I’m told it may require extensive modification to the gyro itself to make it fit and work. It would also be much more expensive.

Any thoughts on whether to simply mount another McCullough or upgrade to the Rotax?

 

[MikeBoyette]

Learning to fly a Bensen with a Mac is a good inexpensive way to hone your skills after dual training. Changing to a Rotax will alter the thrust like making the machine less stable and safe. Better to built new one already made for larger prop of Rotax.

 

[BrianInVa]

Learning to fly a Bensen with a Mac is a good inexpensive way to hone your skills after dual training. Changing to a Rotax will alter the thrust like making the machine less stable and safe. Better to built new one already made for larger prop of Rotax.

It’s tempting to go with the McCullough. It’s the simplest option as it already has the right engine mounts..

However the guy selling this Brock KB2 is throwing in a 503 since the McCullough is missing. And I just found someone on Marketplace who has a 582 they’re looking to swap for a 503. So I might end up with a 582 I could use for no more money than the initial purchase of this gyro, making that my cheapest option.

 

[ultracruiser41]

Hey Brian,
The Carolina BarnStormers are having their fall Wing Ding October 17-20 …..we’re not too far from ya…..come on down for a few days of friends…fun…food…and gyro flyin’
Anson County AirPort
Wadesboro, NC

 

[BrianInVa]

Hey Brian,
The Carolina BarnStormers are having their fall Wing Ding October 17-20 …..we’re not too far from ya…..come on down for a few days of friends…fun…food…and gyro flyin’
Anson County AirPort
Wadesboro, NC

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Thanks! Believe it or not, I sent you guys an inquiry/ message via your website last night.

I’m picking up this Brock KB2 in Jacksonville NC on Friday evening. If any of your gang is going to be around the Anson County Airport late this Saturday morning I’d be thrilled to swing by and say hi.

 

[Doug Riley]

The KB-2 airframe (=basically a 1969-model Bensen airframe) must be extensively modified to accommodate a Rotax. That's because the Rotax's geared reduction drive lowers the prop RPM to the mid-to-high 2000's. The McCulloch (yeah, it ends with "ch" not "gh") OTOH is a direct-drive engine with a prop RPM of 4000 or more. This difference means that you'll use about a 4-foot prop for the Mac, but a 5-foot one for the Rotax.

You'll have to install a longer mast to fit the 5-footer. But then your gyro will have a prop thrustline far above the aircraft's CG. This is a dangerously unstable arrangement that has killed many pilots. You can fix it by also raising the seat about a foot -- another huge frame mod. Of course, you must raise all the controls to a comfortable location for your new seat height, and figure out how to steer on the ground when you can't reach a direct-steering nosewheel.

 

[BrianInVa]

The KB-2 airframe (=basically a 1969-model Bensen airframe) must be extensively modified to accommodate a Rotax. That's because the Rotax's geared reduction drive lowers the prop RPM to the mid-to-high 2000's. The McCulloch (yeah, it ends with "ch" not "gh") OTOH is a direct-drive engine with a prop RPM of 4000 or more. This difference means that you'll use about a 4-foot prop for the Mac, but a 5-foot one for the Rotax.

You'll have to install a longer mast to fit the 5-footer. But then your gyro will have a prop thrustline far above the aircraft's CG. This is a dangerously unstable arrangement that has killed many pilots. You can fix it by also raising the seat about a foot -- another huge frame mod. Of course, you must raise all the controls to a comfortable location for your new seat height, and figure out how to steer on the ground when you can't reach a direct-steering nosewheel.

I was aware of the necessity of some of these modifications, but I was unaware of the loss of ground steering control.

Surely this Rotax swap has been executed safely before on a Bensen or Brock frame?

Would it be better to go with a full VW motor with direct drive, or is that not an option?

Any alternative recommendations? A modification that creates a step down of the frame below the engine/ prop so the motor can be lower in position to avoid lengthening the mast?

 

[ultracruiser41]

Thanks! Believe it or not, I sent you guys an inquiry/ message via your website last night.

I’m picking up this Brock KB2 in Jacksonville NC on Friday evening. If any of your gang is going to be around the Anson County Airport late this Saturday morning I’d be thrilled to swing by and say hi.

Hi Brian…..swing on by….. I believe Stephen Chadwell will be there and a few others. I’m there Saturday night and Sunday…. but we’re usually there, Saturdays and Sundays.

 

[BrianInVa]

A modification that creates a step down of the frame below the engine/ prop so the motor can be lower in position to avoid lengthening the mast?

I combed through the forum and found images of what I had in mind. If a dropped keel is added to accommodate the Rotax, gear box and prop, no further modification to the mast etc is necessary, correct?

 

[Doug Riley]

Yes, the "drop keel" a.k.a. cranked keel, mod is one way to do it. Note that these drop-keel craft generally have a vertical fin that spans the prop -- a so-called "tall tail." And, of course, you'll have to build a sturdy nose-gear leg with appropriate pivots and anti-shimmy friction damping. To steer, you have your choice of turning the nose gear leg, or installing main wheel brakes that can be activated one side at a time. Heck, you'll need the brakes anyway.

If you gyro has a prerotator powered by the engine, its drive train likely will need to be reworked to span the added distance up to the rotor, as compared to the Mac.

 

[BrianInVa]

If you gyro has prerotator powered by the engine, its drive train likely will need to be reworked to span the added distance up to the rotor, as compared to the Mac.

Good point regarding the prerotator, I hadn’t thought of that either.

 

[BrianInVa]

Just out of curiosity, is it possible to build a prop that would work with the 582 on the original unmodified KB2 frame, without going to a larger diameter?

Would/ could a graphite prop with the original diameter but larger cord be found, or made, to make up for the shorter diameter, preserve the thrust of a taller prop, and thereby avoid modifying the airframe?

I realize the larger chord in many props would make it heavy and the gear box might not handle, it but I’m curious if the larger chord could be made with carbon prop instead.

 

[Brian P]

Just out of curiosity, is it possible to build a prop that would work with the 582 on the original unmodified KB2 frame, without going to a larger diameter?

Would/ could a graphite prop with the original diameter but larger cord be found, or made, to make up with the difference and avoid modifying the airframe?

I realize the larger chord in many props would make it heavy and the gear box might not handle, it but I’m curious if the larger chord could be made with carbon prop instead.

In the UK we've been flying Bensens with 52 inch props and rotax engines for decades. Won't set the world on fire but fly ok.

 

[Doug Riley]

Possible? Yes. Efficient? Sorry, it's more inefficient than usual.

The efficiency of a prop (how much of the energy delivered by the output shaft ends up doing useful work) depends on several factors. The disturbed air entering the prop disk already leads to a lower efficiency (and more noise) in our pusher props than in a tractor-prop arrangement. We're stuck with that loss to some extent, but it's painful to make things worse, because...

Another of those efficiency factors is disk loading. It's measured in pounds of thrust per square foot of prop disk area (area=prop radius x prop radius x 3.14). The smaller the diameter (hence radius) of the prop disk for a given thrust range, the less thrust you actually get. (That's why helicopter rotors are huge and direct-lift jets are obscenely wasteful.) So you'll lose some efficiency with a shorter prop on the redrive Rotax.

A nice perk of the drop-keel setup with a tall tail is torque cancellation. A redrive multiplies an engine's torque. The gyro tries to roll in reaction to this torque. The rotor normally prevents this, but, in low-G flight situations the frame is fee to roll. The tall tail cancels most of the roll torque, while the older Bensen-style "shorty" tail doesn't.

 

[BrianInVa]

I just had a detailed conversation with Dale at Warp props. He said if I used a c or e gear box with 2.62 to 1 drive ratio and we went with a 4 blade prop we’d need to go 60” (minimum) to 62” (better) diameter to achieve the same thrust of a full size Rotax prop on a 582.

They still have some 5 blade plates. If we went 5 blade we could get away with 56” (minimum) to 58” (better) diameter prop.

Warp recommends a minimum 1 1/2” clearance from keel.

So I need to find out the largest diameter prop that the KB2 could use in its original configuration to see how much I’d need to modify the frame, if any for a 4 or 5 blade Warp carbon prop.

Anyone know what diameter prop the KB2 originally used with the McCullough?

 

[BrianInVa]

If you gyro has a prerotator powered by the engine, its drive train likely will need to be reworked to span the added distance up to the rotor, as compared to the Mac.

It looks like this one used cheek plates to shift the rotor rearward. That looks like it could raise the mast a bit but would that also help the original prerotator reach the rotor?

 

[Butch]

It’s tempting to go with the McCullough. It’s the simplest option as it already has the right engine mounts..

However the guy selling this Brock KB2 is throwing in a 503 since the McCullough is missing. And I just found someone on Marketplace who has a 582 they’re looking to swap for a 503. So I might end up with a 582 I could use for no more money than the initial purchase of this gyro, making that my cheapest option.

Put on the Mac

 

[Doug Riley]

The gyro in the photos (with the green frame) is a Gyrobee derivative. That design doesn't use a drop keel, and instead employs a taller mast that is not angled back. Bensen and most similar gyros mount the mast with a roughly 9-degree aft rake. The 'Bee mast OTOH is straight up.

The "backward" cheek plates simply put the rotor head where it would be anyway, if the mast were raked. The fore-aft location of the rotor head is dictated by the weight-and balance condition of the aircraft, not by convenience in fitting a prerotator shaft.

Whether you'll need a different prerotator drive train (probably just the flex shaft, if that's the type of prerotator you'll use) will only be clear once you've (1) done your other frame work and then (b) performed a "hang test" to check weight and balance.

The KB-2 is a 1969 Bensen B-8M frame. They typically used a 48"-50" prop. At 4,000 RPM, you don't want one any longer than that on a direct-drive Mac or VW. The tip speed becomes problematic (not to mention loud).

 

[BrianInVa]

Hi Brian…..swing on by….. I believe Stephen Chadwell will be there and a few others. I’m there Saturday night and Sunday…. but we’re usually there, Saturdays and Sundays.

I picked up that Brock KB2 in Jacksonville NC this evening and I’m staying over. I’m pretty beat tonight so I won’t be at the Carolina Barnstormers hanger till sometime late morning or early afternoon. Maybe someone there can help me make heads or tails of the mountains of gyro related stuff filling my trunk and rear seat now.

(I realize this is a lot to ask, but what are the chances of getting a short ride along in one of the gyros tomorrow? Tonight when I picked up this Brock was the first time I’ve ever seen one in person. I’ve never flown in one.)

 

[BrianInVa]

Put on the Mac

Why are Macs so controversial. Seems folks either love them or hate them.