Brock KB2 - McCullough or Rotax?

[BrianInVa]

For better or for worse, I’ve modified the Butterfly frame a bit.

Barry (Carolina Barnstormers) recommended a wider main landing gear for this Ultralight model. Apparently the original main landing gear was a bit too narrow, a compromise Butterfly may have made to help reach its part 103 weight compliance, but making it tippy in ground maneuvers.

I used the main landing gear from my old Brock KB2 frame as well as the part of the Brock keel that connected to it. So with the wider / taller tires I got for it, it should be well planted now. This also raised the entire gyro frame 2” higher overall.

Since I had the keel apart for this already and I had the extra height from the main landing gear modification, I decided to drop the keel 2” directly below my 64” Powerfin B prop. This gives a bit more clearance for the prop between the frame and the rotor.

The original Butterfly tail frame and rudder assembly stepped back up from keel level about 4” as in this image

I never did like this part of this model’s design, so I only brought the frame at the tail back up to the level of the keel fore of the prop. (This can still easily be raised back up another 2” in the cheek plates if that’s more prudent?):


Both the keel drop for the prop and dropping the tail gives just a little more clearance to the rotor for that large prop and that tall tail.

I realize I’ve been informed repeatedly that with proper rotor management and training that just ain’t an issue but psychologically I feel like I’ve done “something” to address it.

 

[BrianInVa]

Any idea how strong or rigid the arm needs to be connecting the top of the tall tail to the mast? What type of tubing or angle or other material is best?

I have the front console parts off the Brock gyro frame I was considering using to connect the tall tail to the mast on the Butterfly:



They’re actually fairly strong and would fit to the mast easily.

 

[Aerofoam]

On the Dominator, the top tubes are about 5/8" Dia. x .052 wall AL tubing. if you are going tall tail, they are usually right behind the prop, to the lower pivot would actually be at, or slightly ahead of the cheek plates on the keel.
You will need new, shorter cheek plates flush with the upper surface of the aft keel tube.
The aft section of your keel behind the pivot is really only important for the rear wheel location and elevation above the ground, so you could shorten it quite a bit and I think the rear wheel, may need to be lowered on cheek plates so you can't rock back too far....
These are all geometry issues you can play with by taking off the aft keel section and putting it back on with clamps, checking how far it can rock back and if the blades could strike the ground etc.....

 

[BrianInVa]

On the Dominator, the top tubes are about 5/8" Dia. x .052 wall AL tubing. if you are going tall tail, they are usually right behind the prop, to the lower pivot would actually be at, or slightly ahead of the cheek plates on the keel.
You will need new, shorter cheek plates flush with the upper surface of the aft keel tube.
The aft section of your keel behind the pivot is really only important for the rear wheel location and elevation above the ground, so you could shorten it quite a bit and I think the rear wheel, may need to be lowered on cheek plates so you can't rock back too far....
These are all geometry issues you can play with by taking off the aft keel section and putting it back on with clamps, checking how far it can rock back and if the blades could strike the ground etc.....

Ok, so you’re saying the frame under my rudder currently extends too far aft, too far away from the prop? And my tall tail should actually be closer to my prop?

I had asked one of the guys at the Barnstormers specifically about this, whether I should shorten the keel/ frame behind the prop so the tall tail is closer to the prop as seems to be the case on the Dominator etc., and he replied,

The longer the keel… the better …so it’ll be fine with the longer keel

 

[BrianInVa]

You will need new, shorter cheek plates flush with the upper surface of the aft keel tube.

These were the original cheek plates off the Butterfly Ultralight tail frame assembly. I didn’t cut them down yet in case I needed to go back to their original configuration.

 

[Aerofoam]

Ok, so you’re saying the frame under my rudder currently extends too far aft, too far away from the prop? And my tall tail should actually be closer to my prop?

I had asked one of the guys at the Barnstormers specifically about this, whether I should shorten the keel/ frame behind the prop so the tall tail is closer to the prop as seems to be the case on the Dominator etc., and he replied,

There is truth in that statement, but it needs to be balanced with, the farther aft the rudder, the shorter it needs to be to not get hit by the rotor.
You could leave the extra length, actually, from the looks of it, you could just replace the whole bottom tube and skip the cheek plates.
It really looks like it can rock back too far, but I am comparing it to my AC and Dominator,
so anyone who knows this aircraft will no more than me!
I would leave the original cheek plates alone, if you need shorter ones, just make them.
The LE of my Domi rudder is only about 5" from the prop hub, so with that in mind, I would be thinking about a new longer straight bottom tube
that is about a foot shorter over all than your 2 piece bottom tube section.
My Domi only has about 5" of air space under the front tire when rocked fully back on the tail wheel.

 

[BrianInVa]

You could leave the extra length, actually, from the looks of it, you could just replace the whole bottom tube and skip the cheek plates.

Now that I look at it again, I think you’re right. The whole keel from mains back would be simpler. There was really no need to step back up from the 2” dropped keel height other than from mimicking the original design, especially since the new mains will raise the entire frame 2 + inches versus the original. It was the height of that rear wheel on the original tail design that bothered me anyway. It just looked like it would let it rock back way too much on taking off.

the farther aft the rudder, the shorter it needs to be to not get hit by the rotor.

This has been in the back of my head all along. So I’ll eliminate those cheek plates and that rise In the keel aft of the keel drop and I’ll shorten the overall length of the tall a bit. Just not sure how much.

 

[Aerofoam]

This has been in the back of my head all along. So I’ll eliminate those cheek plates and that rise In the keel aft of the keel drop and I’ll shorten the overall length of the tall a bit. Just not sure how much.

There could be merit in leaving the keep a bit long while the rudder is still very close to the prop.
I would think the ground stability, when racked back, would be increased with the tail wheel farther aft.
But I am guessing and would ask people who have actually tried varying the spacing of the tail wheel....
On the Domi, the tail wheel is slightly aft of the rudder's TE. So the keel protrudes about 2" farther than the rudder, which can also protect the rudder from hanger rash....

 

[BrianInVa]

There could be merit in leaving the keep a bit long while the rudder is still very close to the prop.
I would think the ground stability, when racked back, would be increased with the tail wheel farther aft.
But I am guessing and would ask people who have actually tried varying the spacing of the tail wheel....
On the Domi, the tail wheel is slightly aft of the rudder's TE. So the keel protrudes about 2" farther than the rudder, which can also protect the rudder from hanger rash....

Since this is a flying tall tail and I’m going to raise the hs half way up into the middle of the prop wash, I’ve been told I need to add an anti servo tab. It’s also been pointed out on another thread that the farther from the axis of the rudder it is the more effective the anti servo tab is, if I understand this correctly:


So the rear of the keel needs to be a certain distance from the rudder axis just to support the proper distance to the anti servo tab pin.

 

[Aerofoam]

Since this is a flying tall tail and I’m going to raise the hs half way up into the middle of the prop wash, I’ve been told I need to add an anti servo tab. It’s also been pointed out on another thread that the farther from the axis of the rudder it is the more effective the anti servo tab is, if I understand this correctly:
View attachment 1162695

So the rear of the keel needs to be a certain distance from the rudder axis just to support the proper distance to the anti servo tab pin.

It only needs to be a little longer than the tail to protect the linkage and the servo tab. The tail does not have a long chord, the linkage is only about 8" long and the servo tab protrudes about 3" aft of the rudder, so if the keel is long enough to pass under the tab, it is long enough.
Any longer would just be putting the wheel farther aft...

 

[Kpjcae]

A little help is this a KB2 or a Parsons with a dominator tail? Thanks Brain

 

[Aerofoam]

Wow that is short!!!!!

 

[BrianInVa]

A little help is this a KB2 or a Parsons with a dominator tail? Thanks Brain
View attachment 1162724

Nice and simple!

The frame itself is very similar to my Brock KB2 frame but this one has been modified for a liquid cooled Rotax? Maybe a KB3? The control itself is similar to my KB2 but more refined.

I’m so new to all this that I have to defer to the folks here who are much more knowledgeable about Bensens / Brocks etc than I am.

 

[MikeBoyette]

Looks to be a custom machine. The builder appears to have gotten inspiration from a few sources. It looks like a KB3 with a RFD Tall Tail and a snow bird instrument panel. The axel is also straight instead of bowed. The prerotator seems to be a version of Dad’s hydraulic driving a single place RFD head. I would be very surprised if it isn’t using Dragon Wings. Dang why not just build a Dominator?

 

[scottessex]

The tail wheel needs to be raised, no ability to rack back.....

 

[MikeBoyette]

The tail wheel needs to be raised, no ability to rack back.....

I was thinking the same thing. Take off distance is greatly reduced the more rock back you can get without tagging the blades.

 

[Kpjcae]

The tail wheel needs to be raised, no ability to rack back.....

Thanks for the heads up. I have it that way so it’s stable while I work on it. When ready to fly it bolted up 90 degrees. Do you see anything else I need to work on? I need all the help, I can get, thanks

 

[Roundwing]

How do you steer it? I do not see a swivel on the nose wheel.
If there is a pivot point straight up from the nosewheel axle, I think it will have a tendency to shimmy.
With it that short coupled it will also be very sensitive steering.

Rick

 

[Kpjcae]

How do you steer it? I do not see a swivel on the nose wheel.
If there is a pivot point straight up from the nosewheel axle, I think it will have a tendency to shimmy.
With it that short coupled it will also be very sensitive steering.

Rick

The front wheel is on a single pivot and in a keeper. Do you think I should put linkage to rudder to nosewheel, so I can steer the nosewheel, or will that make it too sensitive? ( or leave it free?)
Or just putting return to center springs on it?
Not trying to reinvent the wheel but I have just enough let’s try it in me to do both and see.
Thanks

 

[scottessex]

It should have either a soft linked nose steering or differential brakes on the mains.