Wildman Blackhawk 75

[ChrisAttebery]

I'm weak. I admit it. I caved on Saturday and bought the Blackhawk 75. I've wanted a 3" minimum diameter bird for a while. It lines up perfectly with the 75mm hardware I need for my Punisher 4.

Out of the box it looks like this.



But there's roughly 15" of empty airframe, so let's chop it down.



Hmmm, now the fins look kinda big. You can get a couple thousand more feet out of it with smaller fins. Now it looks like the Falcon. Awesome, but let's not go there... yet.



According to FinSim the stock G10 fins have a Divergence Velocity of 1443 ft/sec and a Flutter Velocity of 1963 ft/sec. That's a little concerning to me. An AT M685 will push it to 1900 ft/sec. The CTI M2245 will push it to 3200 ft/sec. It looks like CF tip to tip will be a requirement.

View attachment Blackhawk 75.ork

 

[kevinkal]

That looks like a great project Chris. Bring that airframe to lunch! Oh yes, do you like my new avatar photo? =] Thanks!

 

[ColumbiaNX01]

Use Hysol for the tacking on and fillets.

 

[flynfrog]

Use Hysol for the tacking on and fillets.

you do realize there tons of different hysols right?

Sweet project Chris

 

[ColumbiaNX01]

hysol E-120hp

 

[CzTeacherMan]

Subscribed...
I follow all your builds... This one intrigues me the most

 

[smapdiage9]

My Falcon/M2245 flight survived FG fins held on by Scotchweld DP420 alone with no additional reinforcement just fine.

 

[markkoelsch]

Chris, replace the fins with carbon fiber plate.

 

[ChrisAttebery]

As soon as it arrives I'll bring it to lunch.

Hey nice picture! ;^)

That looks like a great project Chris. Bring that airframe to lunch! Oh yes, do you like my new avatar photo? =] Thanks!

 

[ChrisAttebery]

Agreed. I've already looked at CF plate at ACP.

Chris, replace the fins with carbon fiber plate.

 

[ChrisAttebery]

I was planning to use Rocketpoxy. It has a tensile strength of 7600 lbs. It looks like Hysol hp 120 and Scotch weld dp420 both have a shear strength of around 4500 psi. I think it should be fine, but if anyone can show me data that says otherwise please let me know.

 

[ChrisAttebery]

I'm assuming that you put a nice big fillet on the fins (1/2"), right?

My Falcon/M2245 flight survived FG fins held on by Scotchweld DP420 alone with no additional reinforcement just fine.

 

[WILDMANRS]

I'm weak. I admit it. I caved on Saturday and bought the Blackhawk 75. I've wanted a 3" minimum diameter bird for a while. It lines up perfectly with the 75mm hardware I need for my Punisher 4.

Out of the box it looks like this.

View attachment 306366

But there's roughly 15" of empty airframe, so let's chop it down.

View attachment 306367

Hmmm, now the fins look kinda big. You can get a couple thousand more feet out of it with smaller fins. Now it looks like the Falcon. Awesome, but let's not go there... yet.

View attachment 306365

According to FinSim the stock G10 fins have a Divergence Velocity of 1443 ft/sec and a Flutter Velocity of 1963 ft/sec. That's a little concerning to me. An AT M685 will push it to 1900 ft/sec. The CTI M2245 will push it to 3200 ft/sec. It looks like CF tip to tip will be a requirement.

View attachment 306368

Hi Chris
glad to see you picked up one.
Sorry you don't like the BH fin design but your new design looks familiar.
https://www.wildmanrocketry.com/ProductDetail.aspx?product=7141
If you want I will change the fins for you .
BTW there is no need to go to carbon on the fins as you can see from the data.

 

[smapdiage9]

I'm assuming that you put a nice big fillet on the fins (1/2"), right?

Yep, went fast worked fine.

 

[markkoelsch]

Looking at the data from fin sim, I really think cf plate is the best choice. At 3000+ ft/sec I would worry about the survival of a tip to tip lamination- delamination along the leading edge comes to mind.

 

[ChrisAttebery]

Hi Tim,

Thanks for the reply. Please don't take it the wrong way. I like the lines of the Blackhawk. Unfortunately, I can't leave anything alone. I raced R/C boats, planes, and cars for about 20 years. I never had an engine that I didn't tinker with. My rockets are the same. I rarely build anything stock. I'll take you up on that offer. I'll send you an email.

I was really just wondering about the FinSim results. Either his G10 numbers are very pessimistic or the fins are too floppy. So, this morning I ran the fins from the Space Cowboy. According to Finsim the Divergence Velocity is 1600 fps. That's with .125" thick fins. The SC fins are machined to about .060" at the tips and the rocket survived a flight to over 1760 fps so I think I can safely say that FinSim is a bit pessimistic.

Thanks,


Chris

Hi Chris
glad to see you picked up one.
Sorry you don't like the BH fin design but your new design looks familiar.
https://www.wildmanrocketry.com/ProductDetail.aspx?product=7141
If you want I will change the fins for you .
BTW there is no need to go to carbon on the fins as you can see from the data.

 

[ColumbiaNX01]

Hi Chris
glad to see you picked up one.
Sorry you don't like the BH fin design but your new design looks familiar.
https://www.wildmanrocketry.com/ProductDetail.aspx?product=7141
If you want I will change the fins for you .
BTW there is no need to go to carbon on the fins as you can see from the data.

Why is t2t not needed; b/c of the epoxy used for the fillets and/or the fin shape is just that good?

 

[ChrisAttebery]

I was looking at the specs for the Falcon and saw this:

"7000’ MSL flight condition, M2245IM motor, Barrowman 3-D lift slope prediction technique"

Using the Barrowman 3-D Lift Slope option under CN-alpha in Finsim almost TRIPLES the divergence and flutter velocities. Finsim shows that the stock fins are good to 4664 fps and my trimmed down fins in 3/16" G10 are good to 6574 fps. I might even get away with .125" G10 at 3661 fps.

 

[Mustang67]

Subscribed

 

[rfjustin]

Subscribed as well...

 

[ChrisAttebery]

The kit arrived this week. The nose cone looks like any other FWFG cone, but "HT" was written inside in gold Sharpie. :wink: The tubing looks good. It's smooth with a sanded finish. At 60" the tube is LONG. I should easily be able to cut 12-15" off of it and still have room for a 6XL motor. The switch band and coupler are both CF as well which concerns me. I don't think that the GPS will work inside a CF tube with aluminum lids. If I have time this weekend I'll throw my Telemetrum in there and run a test. My guess is that I'll be ordering a FG coupler and switch band soon. It's not a big deal if I do.

 

[ChrisAttebery]

I've been quietly working on this rocket for about a month. I made a couple decisions so I can move ahead now.

I'm going to try to keep it under M2 so that I don't have to worry about heat damage. That leaves me with the M685 for my largest motor.

I designed the fins specifically for the M685. There will be 4 smaller fins that are completely machined. They have a diamond cross section and are tapered from the root to the tip. I pulled the first pair of fins off the mill this morning.

 

[mpitfield]

I've been quietly working on this rocket for about a month. I made a couple decisions so I can move ahead now.

I'm going to try to keep it under M2 so that I don't have to worry about heat damage. That leaves me with the M685 for my largest motor.

I designed the fins specifically for the M685. There will be 4 smaller fins that are completely machined. They have a diamond cross section and are tapered from the root to the tip. I pulled the first pair of fins off the mill this morning.

View attachment 312299

Wow, as with all your work it certainly appears to be machined to a high quality...that fin looks like a real performer! Did you adjust the geometry at all from the stock design, beyond the obvious tapering, and diamond air foiling? The sweep angle looks aggressive.

 

[ChrisAttebery]

Thank you. Yes, they are highly modified. See the first post for the stock fin shape.

 

[ChrisAttebery]

I'm finally shaking off my winter lethargy. This project has been stalled for quite a while mainly because the fin guides are too big to cut in one shot on my machine. I decided to have Nat at UpccaleCNC cut the guides for me. I gave him the files on Sunday night and I received a shipping notice this morning. :clap:

Check out the quality of that cut.

 

[ChrisAttebery]

It's been a while since I've worked on this project, but I made significant progress this week. I have the altimeter bay design about 90% complete. I have a Telemetrum V2, an RRC2+, a Mobius Mini, three screw switches and three 850ma LiPo batteries in this design. The blue pieces will be machined from aluminum. The battery and altimeter trays will be machined from G10.

 

[rfjustin]

Will this be "all out the top" dual deploy via line cutter or chute release?

 

[ChrisAttebery]

Yes it will be all out of the top. I'm looking at keeping the main in the coupler with a bulkplate shear pinned into the bottom of the coupler.

 

[mpitfield]

Very clean design, I do not see where you connect the pyros in the bulkplate, how are you connecting them?

 

[ChrisAttebery]

Thanks Michael. I'll be drilling a pair of 1/8" holes for the ejection charges. I need to get everything else sorted out before I commit to their positions. I'll be using surgical tubing charges for this rocket. It's projected to hit 35k' on an M685.