MAC 4" Black Fly Build Thread

Barkley said:

Mike, what's your refund policy?

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Would be interesting to know your preference for conformal rail guides over rail buttons. Have you used them before?

Jozef said:

Would be interesting to know your preference for conformal rail guides over rail buttons. Have you used them before?

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I've used them on 2 other rockets and won't go back to lugs. You really need to score the surface before bonding, but it's lower drag and easy to mount. The previous pic was a spoof (I just put them down and took the pic) so I'll take a pic of the scoring before mounting.

Barkley said:

I've used them on 2 other rockets and won't go back to lugs. You really need to score the surface before bonding, but it's lower drag and easy to mount. The previous pic was a spoof (I just put them down and took the pic) so I'll take a pic of the scoring before mounting.

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OK, you said reduced drag compared to lugs, but my question was relative to rail buttons. Delrin rail buttons have less surface contact are than either lugs or conformal rail guides.

Jozef said:

OK, you said reduced drag compared to lugs, but my question was relative to rail buttons. Delrin rail buttons have less surface contact are than either lugs or conformal rail guides.

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To my eyes there's less frontal area with rail guides than with buttons, and I have an assumption that the air would have a cleaner entrance/exit with two skinny appendages vs one much larger one. Absolutely nothing scientific about it, other than the marketing hype matching what I think makes sense.

Barkley said:

To my eyes there's less frontal area with rail guides than with buttons, and I have an assumption that the air would have a cleaner entrance/exit with two skinny appendages vs one much larger one. Absolutely nothing scientific about it, other than the marketing hype matching what I think makes sense.

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Makes for an interesting debate... drag during flight vs drag on the rail. Impossible to model. It"s good that options are available to meet builder preferences.

One benefit I’d see is that rail drag is during a short time of very high thrust.

DavidMcCann said:

One benefit I’d see is that rail drag is during a short time of very high thrust.

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Maybe. No way to measure it. I cannot imagine that either system makes more than an insignificant difference in flight performance or altitude. The only identifiable difference boils down to drilling and tapping holes vs sanding prep and epoxy....and appearance. Not sure why they need to be so long... unless it is for bonding concerns. They appear large enough to be cut in half.

As a fan of conformals myself I'd recommend JB Weld over other epoxies. Yes scoring but also fine sanding... I've previously posted saying that Devcon etc were fine, but I've had a character forming experience at the pad since then.. Lovely rocket and lovely build by the way.

If the conformal rail guide was say maybe .25% (arbitrary number) of the length that it is would it still not do it's job and would it also cause less drag? Why does it have to be the length that it is, is there a technical reason? Being made from aluminum it would be easy to cut them down to another size and re-angle the cut edge.

The linear/conformal rail guides will potentially keep the rocket "guided" by the rail longer, since a rocket can rotate on just one button, but not one guide. You could even launch on just one linear guide if you were careful.

For this bird it doesn't really matter, this is a 5 fin rocket after all... I found it amusing that this discussion got started in this thread of all places.

Jozef said:

Maybe. No way to measure it. I cannot imagine that either system makes more than an insignificant difference in flight performance or altitude. The only identifiable difference boils down to drilling and tapping holes vs sanding prep and epoxy....and appearance. Not sure why they need to be so long... unless it is for bonding concerns. They appear large enough to be cut in half.

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id wager there’s an aerodynamic drag difference between buttons and guides. There’s ways to measure it, but which are practical and available to us?

mpitfield said:

If the conformal rail guide was say maybe .25% (arbitrary number) of the length that it is would it still not do it's job and would it also cause less drag? Why does it have to be the length that it is, is there a technical reason? Being made from aluminum it would be easy to cut them down to another size and re-angle the cut edge.

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I like it the length that it is from an adhesion perspective. As has been mentioned, there's the possibility of just one guide keeping things straight, which would be a significant torque force.

And yes - great to have options on the build. And Tim51, thanks!

DavidMcCann said:

id wager there’s an aerodynamic drag difference between buttons and guides. There’s ways to measure it, but which are practical and available to us?

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Agree, David ...and you would win that wager. There has to be a difference. Might be interesting to chat one evening during URRF5 over a beer

Jozef said:

Agree, David ...and you would win that wager. There has to be a difference. Might be interesting to chat one evening during URRF5 over a beer

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Or... buy a kit and build it with it with buttons. I'll be at URRF. We could put the same motor in both and drag race. Pitfield too to increase the sample size.

Barkley said:

Or... buy a kit and build it with it with buttons. I'll be at URRF. We could put the same motor in both and drag race. Pitfield too to increase the sample size.

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If I get my 4" built in time then I am in. I will do a fly-away rail guide, you do the conformal and Dave can do buttons. I say we use the K-960-LW as the motor

mpitfield said:

If I get my 4" built in time then I am in. I will do a fly-away rail guide, you do the conformal and Dave can do buttons. I say we use the K-960-LW as the motor

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Deal... now mine is getting close... so get yours done...

Internal fillets and some internal support. Compared to full scratch, I'm loving this kit.

Barkley said:

Deal... now mine is getting close... so get yours done...

Internal fillets and some internal support. Compared to full scratch, I'm loving this kit.

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I can see that you're enjoying this nice spring day as well, lucky I didn't take off my winter tires.

I love the spring.....




There is something relaxing about building a kit. You have a plan, things all go according to plan, and no power tools need to be bought or animals sacrificed.

DavidMcCann said:

id wager there’s an aerodynamic drag difference between buttons and guides. There’s ways to measure it, but which are practical and available to us?

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Jozef said:

Agree, David ...and you would win that wager. There has to be a difference. Might be interesting to chat one evening during URRF5 over a beer

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Build a couple of identical GD29's: on one put three sets of A, on the other put three sets of B.

Fly M.5, M1, M1.5. If there's any appreciable difference, it should absolutely pop out in the speed / drag numbers.

Remember to leave tape flags that are easy for gloved hands to grab. I use a plastic spoon to spread the West with 405, but I'll leave it rough so that I can put on a surface coat with the fairing compound so that it's easy to sand - and it should make it pretty.

Internals fillets are done, externals are almost done. Still need to sort out my HED setup, but I have the plan.

I heard a rumour about a swarm drag race at URRF. Mine will be ready...

DavidMcCann said:

id wager there’s an aerodynamic drag difference between buttons and guides. There’s ways to measure it, but which are practical and available to us?

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Just an FYI, Mr. Tin Van Milligan has already done the research and presented findings at Narcon 2017. Check out Peak of Flight Newsletter - Drag On Differing Rocket Shapes [#438] (Link) or full report on Launch Lug Drag (Link).

A bit of epoxy on those nuts to keep them stuck and my HED setup is complete. Below, epoxied lead in the nose to keep the up side up.



URRF should be fun...

Tobor said:

Just an FYI, Mr. Tin Van Milligan has already done the research and presented findings at Narcon 2017. Check out Peak of Flight Newsletter - Drag On Differing Rocket Shapes [#438] (Link) or full report on Launch Lug Drag (Link).

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TVM's reports usually coincide with what has a better profit margin, but I'll have to look at what he did report.

edit: read it. my take away was the sim software is unable to deal with the size, scaling up is questionable and unsupported, and even wind tunnels would have an issue giving good data. Boo.