Quantcast

High Powered Flis Praetor? Will the fins stay on?

The Rocketry Forum

Help Support The Rocketry Forum:

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
Back in 2005 I built a bt80 sized upscale of the FlisKits Praetor. TTW wall fins. The fins are carbon fiber with Kevlar honeycomb sandwich....edged in basswood.

The thing is still in one piece after all these years and still flies great. The biggest motor I've flown it on is an AT G79W. Rocksim says that's about 390 ft/sec max velocity. Get's a cool whistle on the way up.

SO…..I'd like to do an even bigger upscale using 4" LOC tube. That will put the total fin span at approx. 33" across. Each fin's span at about 14.5 inch.

If I can build it and select motors to keep the velocity the same range as my mid-power version do you think It will be OK? I think the key to it's longevity is that it is very light weight for it's size. I'd have to keep the 4" version light also.

Here are some pics of the mid powered version. Will this scale up OK? What are your thoughts?
07.02.05 MacPraetor Composite Fins a.jpg
07.04.05 MacPraetor 001a.jpg
07.06.05 MacPraetor 002a.jpg
07.09.05 MacPraetor 001a.jpg
07.11.05 MacPraetor 002a.jpg
07.24.05 MacPraetor 001a.jpg
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,487
Reaction score
1,106
You're doing glassed bassedge honeycomb? That's crazy stiff, should be able to take M.3 fairly well. If in doubt find a motor that gets off the rail in a hurry but only hits about half that top speed, then inspect your fillets and/or basswood edges for cracks.
 

David Schwantz

Well-Known Member
Joined
Jul 23, 2018
Messages
1,522
Reaction score
557
Location
MN
I cannot say about Mach1 rockets, but with very fast RC jets if you have a thin control surface it will tend to "hunt" if you have razor sharp LE. I would suggest rounding the LE and then taper back to TE. This should help to reduce the chance of flutter. Love the looks of it. Good luck.
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,774
Reaction score
718
Yeah I'm going to keep an eye on this thread. Are you going to post build pictures here?

Also, I am just going to echo what's already been said and that is with such a fin configuration your biggest issue is going to be flutter and I believe using a composite construction like you did on your first iteration is going to reduce/eliminate that. Also, I just want to compliment you on choice of fin material on that first build. Using a lightweight and strong material for those fins is great because you're not adding a huge amount of weight to the wrong end. That sucker looks great! :cool:

-Dave
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
In the planning stage on this one. The mid powered version shown in the pictures has a 29mm mount. The proposed 4” version at least 38mm....May build with 54mm.

If I build it there will definitely be pics.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
175EC209-EEF8-4F27-BF86-0984A3272FBF.jpeg
3AF6C2C5-2051-47B2-93BB-9BCC1A3A223C.jpeg


I’ve decided to give the 4” Praetor a go. Mocking up some parts. Still figuring out how I’m going to build this thing. Biggest question is fin construction. I’ve ordered some spread tow carbon fiber fabric plus vacuum bagging supplies. Maybe carbon over balsa core...or may try to make a mold and make two piece fins.

Guess I’ll decide when the materials get here.

I did go with 54mm mount. Got the LOC fin lock adapter kit like in their 4” Goblin. Will adapt 38mm or 29mm.
 

Wallace

Sponsor
TRF Sponsor
TRF Supporter
Joined
Feb 16, 2017
Messages
2,514
Reaction score
460
I'd say landing is where you'd be more likely to have problems due to the extra "upscale" mass..
 

Wallace

Sponsor
TRF Sponsor
TRF Supporter
Joined
Feb 16, 2017
Messages
2,514
Reaction score
460
Maybe kick the motor out the back on a tether to reduce landing weight?
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
D36C59EE-6BBF-4894-A899-825A3EE7D723.jpeg
F12D5469-DBEE-4197-AF7A-6863C9BAE109.jpeg
I’ve been gathering materials for some vacuum bagged composite fins. Spread tow CF fabric. Going to make a mold to lay the fin halves in and resin infuse.... then epoxy the halves together after cured. If it’s stiff enough, I’ll leave them hollow. Waiting for some warmer weather.
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,487
Reaction score
1,106
I’ve been gathering materials for some vacuum bagged composite fins. Spread tow CF fabric. Going to make a mold to lay the fin halves in and resin infuse.... then epoxy the halves together after cured. If it’s stiff enough, I’ll leave them hollow. Waiting for some warmer weather.
I just saw "F-1120" and thought "those madmen!"
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
C67DC657-7064-4D92-9176-B307D3821C32.jpeg
FB44E112-3EB7-4542-8C8A-C0AC0EB06B6E.jpeg
B15CF672-22E2-4FCC-A657-8D2797765E8D.jpeg
AE028257-4C9A-412F-B650-915AF160816C.jpeg

A little progress. I’ve been working on the fin pattern. This will be used to make a mold. Trying to get as smooth of a finish as I can. Nitrate dope since it dries fast even in the cold weather. Wet Sand and polish. Give this some time to cure / harden the I will wax and put on release agent to make a mold.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
B6FA81E7-00DF-43BA-8BFD-9ED3AEE75F27.jpeg
99245796-FCF4-406D-B9C9-25E938F7B1EA.jpeg
Mounting the fin pattern in some corrugated plastic. This will be the flanged area for vacuum bag sealing.

Hot glued to the plastic board. I’ll use some plasticine clay to fill the gaps.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
8B82776E-A48F-49B8-AD32-A591271C00EA.jpeg
The weather is warming and I’ve been able to make some progress. The fin plug was waxed several times and then a coating of PFA release agent brushed on. I made a gel coat with west systems epoxy, graphite powder, and colloidal silica. A couple of heavy gel coat layers, then the 10 oz glass. I used Smooth on epoxy clay as a filler then more glass to finish.
247EFF11-D0DD-4D03-AA65-D6220FF4915B.jpeg

3ABF9648-5144-4862-8A9E-530E848DA8EC.jpeg

3AE621E7-2EE0-4334-9F10-52CC2172CF67.jpeg
 

Attachments

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
25D2D3CF-E4C7-42FA-9602-18DBD8E94F18.jpeg
03ECC3F1-1DB3-4D74-AE61-1C6ECB21022B.jpeg
C8A66472-C2A8-497F-9D15-B16DDC5DEFF8.jpeg
It takes time between each layer. Things need to set up but still be in a green tacky stage for the next layer. I was worried it wouldn’t separate, but it popped right off. Still needs some clean up. I’ll trim the edges etc., but here’s how the fin mold turned out. I’ll let it continue to cure for a few days before I vacuum bag with it.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
C2FB2B53-BB02-4EC5-AF59-69017BEE516D.jpeg


Trimmed it up with hacksaw. Edged with some vinyl relective tape that’s been hanging around in the shop. Can’t remember why I bought it, but put it to good use. Protects from sharp fiberglass edge.
 

Wallace

Sponsor
TRF Sponsor
TRF Supporter
Joined
Feb 16, 2017
Messages
2,514
Reaction score
460
For safety? Looks pretty cool, nice work.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
992D751A-8183-4600-A1E6-0586393CDBB0.jpeg
232022CD-4347-46E1-8434-85A589263B2D.jpeg
I’m going with 1 layer of spread tow on the outside with 2 layers of -60/0/+60 under that. For my first trial layup I don’t want to go too thick....too heavy. I’m calculating about 0.7mm thick. The plan is to mold each side. Trim and epoxy together then fill the inside with 4lb density polyurethane two part foam.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
421B7119-B485-4D66-BE4C-BD26E437CA1E.jpeg


My first ever resin infusion. Everything was going so well.... I was afraid my resin cup was going to run out so I mixed up another batch and poured it in. Still going good until I noticed a bunch of bubbles in the feed tube. Too much resin, it got hot and set off.

Closed off hose clamps on both sides. We’ll see what I get tomorrow. Note: don’t put too much resin in the pot.

What’s better on the pictures? Thumbnails or full size? Or do I need to lay-off all the pictures?

CA807F26-A382-49F3-B2B2-540D684677FD.jpeg


6106A819-8760-4B94-9BC1-3D8ABF8B3251.jpeg


15871EF0-C02C-4936-BF3A-B8D30D6EFACF.jpeg
 

Attachments

Wallace

Sponsor
TRF Sponsor
TRF Supporter
Joined
Feb 16, 2017
Messages
2,514
Reaction score
460
Personally I like full size pics, less left clicking is a good thing and they don't seem to slow even my crappy old laptop much if any. And more pics are always better. As far as epoxy batches, maybe pre-mix multiple smaller ones, or set the larger ones in a bowl/bucket of ice water? It's looking you have a really solid plan btw, keep up the good work.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
Success! Got home from work and popped it out of the mold. Came out pretty easy. Hardest part was removing the peel ply. Looks pretty good. Thickness at .032” / 0.8mm. As expected. No weird warps. Now to clean everything up and do it 7 more times.
5D4374F3-B2AA-4235-ACC0-4E6A145BCDF9.jpeg
A7899749-C38E-49C9-B2E4-B4126A6CB3D2.jpeg
CCF2889B-DEB1-47BD-8259-2ADF615E6B90.jpeg
2795EA60-0C18-4EC0-B6AB-714D9D5CB3D7.jpeg
A40BD521-93D3-4385-977D-05B088A43187.jpeg
C977FCE0-1C5C-42B0-8C3C-8A0F562B3E22.jpeg
 

Attachments

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
I forget if you mentioned what will go inside the fin halves?
My plan is to fill them with 2-part polyurethane foam...4 lb density. I'm not sure how much pressure the foam will make, so I'll probably make up a jig to hold the sides so they don't bulge out with the foam.
 

Wallace

Sponsor
TRF Sponsor
TRF Supporter
Joined
Feb 16, 2017
Messages
2,514
Reaction score
460
I had considered doing similar with the exception of "pouring" the foam cores first, then bagging the carbon to 'em. Really curious to see your results.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
05926B6C-1298-4BE2-B5E2-D3DC383510A4.jpeg
I’ve got three of them molded so far. I think I’ll trim a couple up and see how it works as a complete fin. I can check the final weight and see how stiff they turn out to be.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
2157B234-1846-4038-8D80-3D0BE73C8374.jpeg


Trimmed them up and dry fit.

611088CD-FA22-43EF-BB81-E37DDF3A1347.jpeg

42104B6F-216B-44F4-B271-05FEACCFDB71.jpeg
 

neil_w

Chuffed as ninepence
TRF Supporter
Joined
Jul 14, 2015
Messages
9,492
Reaction score
2,684
Location
Northern NJ
Very impressive work so far.

Were you planning to cover that lovely checkerboard pattern with paint? It looks really cool.
 

OKTurbo

Well-Known Member
TRF Supporter
Joined
Feb 20, 2009
Messages
1,523
Reaction score
342
Not sure on paint scheme yet, but the checkerboard pattern will definitely be showing. I’ll have to shape / sand the leading edge, trailing edge, and tip...so those will have paint. I’ll use 2K clear for UV protection on the exposed carbon fiber parts.
 
Top