Rotor identification?

[BrianInVa]

Last night I brought home a 24’ rotor for my Butterfly Ultralight rebuild.

I traded a 22’ Bensen rotor and parts off my Brock KB2 even up for it so nothing out of pocket.

The seller was selling it as, and apparently was told that, it was a Dragon Blade rotor. After an initial comparison to online images, and looking for the brass weight at the root for the Dragon Wing serial number it became obvious these are not Dragon Rotor blades. This was confirmed by Mike Boyette.

They have a hard porous core visible on the side of the root that I’m assuming is some type of carbon composite. The skin is a continuous layer of aluminum with no rivets. They’re 7 3/4” wide.

What looks to be a serial number is stamped near the root on both blades, “RR 1084.” There are pencilled numbers on the same area of “1414 1/2” and “1415.” I’m assuming these have to do with weight / balance.

The most common opinion I’ve received so far is that these are RotorDyne rotor blades, with at least one individual asserting they look nothing like his 1980s RotorDyne rotor.

I have not found any similar appearing root area cores after an extensive internet image search except a similarity to TAG rotor images.

Any assistance in their identification would be most appreciated.

 

[BrianInVa]

Hub bar:

 

[BrianInVa]

The side airfoil profile most closely resembles the RotorDyne profile in this image, especially the blunt leading edge. My rotor is only 7 3/4” wide but this schematic says “RotorDyne Large.”

 

[GyroCFI]

Straps almost make me think they're Rotor Hawk blades, but IIRC they had more bolts attach on to the hub bar...

 

[Doug Riley]

The blunt leading edge is a Rotordyne thing. Once they had their extrusion die made to the shape they'd drawn, they were sorta stuck with it, for better or worse. Custom extrusion dies are expensive.

When you build a sheet-metal airfoil with a wraparound skin, you have to "preform" the leading-edge radius by folding and squishing the strip of sheet metal. The final L.E. radius will be determined by how hard you squish that tin.

A standard leading-edge radius for a wing is around 1.5-2% of the chord. Perhaps by chence, early Rotordyne blades had a leading-edge radius that was finer than the spar extrusion's L.E. radius -- IOW, the skin was folded tighter and this caused a gap between the front edge of the spar and the inside surface of the (prebent) skin.

These early editions reportedly performed better than later editions because the gapped version had a more normal L.E. radius. As Rotordyne got better at matching the skin prebend to the spar, the actual skin leading edge radius better matched the spar -- i.e. it was too bunt. Oh well.

 

[BrianInVa]

The blunt leading edge is a Rotordyne thing. Once they had their extrusion die made to the shape they'd drawn, they were sorta stuck with it, for better or worse. Custom extrusion dies are expensive.

When you build a sheet-metal airfoil with a wraparound skin, you have to "preform" the leading-edge radius by folding and squishing the strip of sheet metal. The final L.E. radius will be determined by how hard you squish that tin.

A standard leading-edge radius for a wing is around 1.5-2% of the chord. Perhaps by chence, early Rotordyne blades had a leading-edge radius that was finer than the spar extrusion's L.E. radius -- IOW, the skin was folded tighter and this caused a gap between the front edge of the spar and the inside surface of the (prebent) skin.

These early editions reportedly performed better than later editions because the gapped version had a more normal L.E. radius. As Rotordyne got better at matching the skin prebend to the spar, the actual skin leading edge radius better matched the spar -- i.e. it was too bunt. Oh well

The only markings on these rotor blades is an “RR 1084” stamped near the root of each blade. Is that a serial number?

This is a comment I received elsewhere:
They are early Rotordyne. The straps and cuffs are the tip off. Later Rotordynes, the Joe Souza era after Tracy, he discontinued the cuffs and the molded end caps. The blades are easy to hand start and provide nice lift.

Given their age, are they still safe to fly?

 

[Doug Riley]

Brian, welcome to the magical world of Old Secondhand Homebuilt Aircraft Stuff.

The simple answer to your question is that nobody knows. Back in the day, Rotordynes sometimes experienced delamination, but I don't know of any catastrophic delams. What happened instead was that people would notice the hollow "thud" that delam from the upper spar surface would make when you tapped it with a nickel. I believe some people also found gradual separation of trailing edges.

Gluing metal is not like gluing wood. It relies totally on a chemical bond. Obviously the chemical composition of the glue is part of the story, but not all. The temperature at which the glue set is important. Most important, though, is the cleaning and stabilization process that should occur before the parts are glued up. Aluminum forms an oxide essentially instantly when you strip it of older oxide. This oxide coating is not the ideal substance for gluing. Good prep involves a chemical alteration of the naked aluminum. Dragon Wings, IIR, used a caustic bath -- Ernie also employed heat-setting glue, and baked each blade in a purpose-made electric "hot box." Mike Boyette can correct me if I have this wrong.

Rotordyne's full assembly process involved some, but possibly not all, of these steps. The aluminum can gradually re-oxidize under the glue if that chemical stabilization wasn't done, or wasn't done well enough.

But after 20-30-40 years or more, you just don't know for sure.

 

[MikeBoyette]

The blunt leading edge is a Rotordyne thing. Once they had their extrusion die made to the shape they'd drawn, they were sorta stuck with it, for better or worse. Custom extrusion dies are expensive.

When you build a sheet-metal airfoil with a wraparound skin, you have to "preform" the leading-edge radius by folding and squishing the strip of sheet metal. The final L.E. radius will be determined by how hard you squish that tin.

A standard leading-edge radius for a wing is around 1.5-2% of the chord. Perhaps by chence, early Rotordyne blades had a leading-edge radius that was finer than the spar extrusion's L.E. radius -- IOW, the skin was folded tighter and this caused a gap between the front edge of the spar and the inside surface of the (prebent) skin.

These early editions reportedly performed better than later editions because the gapped version had a more normal L.E. radius. As Rotordyne got better at matching the skin prebend to the spar, the actual skin leading edge radius better matched the spar -- i.e. it was too bunt. Oh well.

This is how the Mosquito helicopter blades are made in house now. Not a good way to do it IMHO. You never get a tight wrap at the leading edge and have a gap where it isn’t bonding as well.

 

[Sv.grainne]

My opinion - but - I would not try to use the old blades. Bite the bullet and get a set of Razor Blades. Don't become a statistic!

 

[MikeBoyette]

Good thing about the Rotordyne design is the fact that the spar is captured. If it happens to delaminate it can’t really seperate from them. From my experience these blades, if inspected by an experienced gyro person familiar with Rotordynes and deems them safe, will be great to learn on. The will work great with an electric prerotator. If they are set to the correct pitch they hand start easily, thus the 60-100 RPM’s will be adequate for takeoff.

 

[BrianInVa]

Good thing about the Rotordyne design is the fact that the spar is captured. If it happens to delaminate it can’t really seperate from them. From my experience these blades, if inspected by an experienced gyro person familiar with Rotordynes and deems them safe, will be great to learn on. The will work great with an electric prerotator. If they are set to the correct pitch they hand start easily, thus the 60-100 RPM’s will be adequate for takeoff.

This is great to know!

This Butterfly Ultralight originally came with a prerotator with brushless motor that ran on Lipo batteries and was capable of 220 rrpm.


The frame I picked had everything stripped from that assembly except the bendix and mount for the brushless motor. Fortunately I found a former Butterfly Ultralight owner, a retired machinist in the aviation industry, who helped develop that prerotator and he still has all the documentation and photos.

He said one fully charged pair of 5s 6000ah Lipos was good for at least five prerotations, and it was rheostat controlled for soft start up and gradual rrpm increase.

So with his assistance I’ve already ordered a little more powerful 36v brushless motor, which will reach the same 220rrpm but a bit more quickly, a set of 8400ah Lipos and all the other hardware and software to make it work again. He even offered to help assemble all the hardware and electronics if I run into trouble.

 

[Doug Riley]

I'd reserve judgment about reaching 220 RRPM. The electric-motor prerotators I've seen and heard about reach 100-150. They typically use automotive starter motors, which are pretty powerful. As Mike says, that 100-150 is plenty to get you going, with careful part-throttle coaxing of the blades past their flapping speed.

I hand-start the Rotordynes on my Gyrobee and that works fine. I do have to stand on the seat to reach them, but I do NOT do so with the engine running!

 

[BrianInVa]

I'd reserve judgment about reaching 220 RRPM. The electric-motor prerotators I've seen and heard about reach 100-150. They typically use automotive starter motors, which are pretty powerful. As Mike says, that 100-150 is plenty to get you going, with careful part-throttle coaxing of the blades past their flapping speed.

I hand-start the Rotordynes on my Gyrobee and that works fine. I do have to stand on the seat to reach them, but I do NOT do so with the engine running!

Ok thanks, as always. I was just going by what this individual is saying. Media at the time also claimed 220 rrpm for the Butterfly Ultralight prerotator. Time will tell.

 

[MikeBoyette]

Best bet is to learn in the beginning how to hand start them. Then if your prerotator ever malfunctions you still have the skill to fly.

 

[Doug Riley]

People who are marketing aircraft (and other products) have been known to "improve" on the truth. E.g. everything, from a little 447 Air Command on up, supposedly can climb at AT LEAST 1,000 fpm. Right.

Hand-starting your blades is a handy skill, IMHO. It was even possible to hand-start the 28-foot DWs on my tandem Dominator, with lots of arm work and plenty of taxiing at a slow walk until the blades "caught."

 

[BrianInVa]

I started working on the rotor blades with aluminum polish and microfiber cloths last night. It’s a slow process that requires a lot of elbow grease but definitely rewarding. They’re cleaning up nicely. And they are actually in good condition overall. I’ll get them checked by someone qualified to do so.

 

[BrianInVa]

Hand-starting your blades is a handy skill, IMHO. It was even possible to hand-start the 28-foot DWs on my tandem Dominator, with lots of arm work and plenty of taxiing at a slow walk until the blades "caught."

Is this included in current gyro CFI training? It should be but I doubt it can be attempted in the two seat gyros they use for instruction. Gyro glider training to learn rotor management would be helpful tool I would think.

 

[Doug Riley]

I don't know of any CFI's who currently include hand-starting rotors in their syllabi. I didn't include it myself, since messing up a hand-start with Dragon Wings can result in a stalled blade smacking the tall tail. A training environment needs to be more forgiving than that. Besides, it's hard enough to get students to stick around for the full syllabus without adding "optional" skills training.

You can hand-start the blades on even a tall 2-place machine as long as you can stand on your seat and reach the blades -- with the engine NOT running, please. Warm up the engine first if you like, then shut it off to "pat up" the blades. Get an assist from any breeze you have.

Gyrogliding was very helpful to me in my training. I put in some 50 hours of gliding in my unpowered Bensen, and even flew it towed at Oshkosh. As a result, even my earliest powered liftoffs felt comfortable and familiar.

 

[Aerofoam]

Where did that bendix originate? Is it custom?, the motor looks like a standard Turnigy brushless from a decade or more ago.
You can't use a Rheostat, you will need a brushless motor speed controller with something like a servo tester to provide an actual knob to control it.
I am curious if that bendix was custom made, it looks fairly light compared to the "Cut Off" automotive versions.
I would have used a cog belt rather than the drive chain, but it is nice and compact.
I don't think you need as much battery as you are getting.

 

[BarryK]

That electric prerotator system was from Larry Neal…..and yes…..it could hit over 200 rrpm.