Wildman Sandhawk L2 DD build.

Finally got time on my hands.
Started the Av Bay since I had Rocketpoxy already mixed up.

Drilled some quarter inch holes in the fiberglass bulkheads.

Ordered 3 quarter twenty eyenuts from MAC.

Dremeled the fins down for anti flutter reasons. I’ll post files in a minute. Guess it’s now a SpeedHawk.

Here are the fin geometry changes I made to Wildman Sandhawk for safe operation on J-K 38mm motors. Now I’m calling the sucker Speed Hawk.

What did that do to the stability? I know it's a long rocket so probably not too much, but I'm curious. You seem especially concerned with fin flutter, have you had any rocket re-kit itself from it?

A fin's semi-span being less than the airframe diameter is a poor choice. Just sayin'...

More to rocket flight than a CP/CG relationship and what Finsim says...

viciouspeanut said:

You seem especially concerned with fin flutter, have you had any rocket re-kit itself from it?

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Never. And I don’t plan to have a kit rekit itself if I can help it.

As far as stability it decreases from 3.06 to 1.39 with same motor. The J350W would’ve rekitted the stock Sandhawk by finsim.

Well if you guys think I’m being extremely dangerous here, I can throw it in the trash and find a new kit. As far as I’m concerned the kit sims stable.

I've been told that long rockets need a larger stability margin. For your rocket that would be 2-3cal which the original kit had. Not saying that it won't launch properly, but it will have a higher chance of going unstable due to the relatively small stability margin and short fins. You are using OpenRocket right? OpenRocket tends to be a lot less generous than RockSim when it comes to stability so the actual stability margin might be much higher (.5cal or so) than OR says.

For example my scratch L2 build had around a 30 to 1 length/diameter ratio. With a J250W it had around 2 calipers (openrocket) on the pad and reached a peak stability of around 3.5 calipers during flight. It flew straight as an arrow. The next flight was on an I180W and it had 2.5 calipers of stability on the pad and 4 calipers in flight. This flight was heavily windcocked by the 5-10mph wind so there certainly is a limitation to the stability margin for long rockets.

I’ve launched L1 rockets with 1.4-1.7 stability past Mach. I view 2.0 or higher as overstable. They were yardstick length or less. And anything under 1 cal as unstable. This is my first L2 build. Call me ignorant here. I can always add some mass to the nose and find out.

I can promise a J350 would not make that kit blink. I have seen 1/16 Nike smoke shaped plywood fins launched on J285s just fine. A bit of flutter is not the same as catastrophic flutter. Quite a drastic change from 3.06 to 1.39, it should still be stable but beyond that it sounds like Eric has more knowledge than me regarding the span and diameter.

Andrew_ASC said:

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Hello Andrew, what's the name of the software?

Openrocket.

I also wanted to throw K1127s into it. That’s what really scared me flutter wise. That Mark guy before he past away told me not to stick a K1127 into the stock Sandhawk geometry.

Andrew_ASC said:

Openrocket.

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Thanks ^^.

So the L/D is 23:1. I figure if I add some noseweight and bump stability up close to 2.0 it will be fine. Honestly I have a RB05A scale rocket with a very short span since it’s a scale missile and it flies fine at 1.7 stability cal on H130’s.

Andrew_ASC said:

Well if you guys think I’m being extremely dangerous here, I can throw it in the trash and find a new kit. As far as I’m concerned the kit sims stable.

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Hi, Andrew !

Dangerous ? Nah, that's over on the "other thread" - LOL !

Seriously, the Sandhawk is a 4-fin rocket and should be stable in this configuration. If it were a 3-fin rocket, I might "hesitate", but I think you are fine. Just be sure to watch the CG location. I've had a few "stable" rockets become unstable when, presumably, acceleration forces caused the chute to slide rearward.

BTW - If you decide to "throw it in the trash", I volunteer MY trashcan - LOL !

Dave F.

Andrew_ASC said:

So the L/D is 23:1. I figure if I add some noseweight and bump stability up close to 2.0 it will be fine. Honestly I have a RB05A scale rocket with a very short span since it’s a scale missile and it flies fine at 1.7 stability cal on H130’s.

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Andrew,

Not saying you're dangerous. Not saying you should throw it away.

I have had a few Saabs too, not my favorite design. Had all the Wildman Saabs in fact! One has to work pretty hard to get that rocket to fly consistently predictable and straight...so probably not the best example to use.

Your stock Sandhawk will be stable and will fly well with the J350W, without modification, without shredding fins all over the place. I'm fairly certain Mr. Wildman would back me on this one. He tends not to sell stuff that shreds on modest motor choices.

What several high/fast flyers have discovered over time is that in order for fins to be out in "clean air" and sufficiently do their job, usually a semi-span of 1.4-1.5 times the diameter is necessary...or at least a really good idea. NOT SAYING LESS SPAN WON'T WORK...but the rocket may tend to wander off the pad, struggle at increased angle of attack, coning may become an issue, etc. Have seen (and experienced) this enough to know that in the real world, using less span may be better for flutter mitigation, but is NOT better for rocket guidance. I was humbled this past summer when one of my rockets experienced this....and I even knew better!

All I'm saying is chopping off the span to the extent that you've done is probably not the path I'd have taken. Decrease tip span, sure! Increase fin thickness, sure! Decrease span to less-than-airframe-diameter....asking for trouble IMO. If it were me, I'd call up the Wildman and check on the price of a new set of fins. If you're concerned about flutter, increase the thickness of the fins to say... .125" (Tim should have that). Then use your dremel to open up the slots a tad.

Not trying to be a downer, trying to help out here and there!

Andrew_ASC said:

So the L/D is 23:1. I figure if I add some noseweight and bump stability up close to 2.0 it will be fine. Honestly I have a RB05A scale rocket with a very short span since it’s a scale missile and it flies fine at 1.7 stability cal on H130’s.

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Look at the models of the Patriot and Hellfire missile . . .

https://www.erockets.biz/public-mis...-1-4-patriot-38mm-available-by-special-order/

https://store.heavenlyhobbies.com/06-023-0054.html

Dave F.

Ez2cDave said:

Look at the models of the Patriot and Hellfire missile . . .

https://www.erockets.biz/public-mis...-1-4-patriot-38mm-available-by-special-order/

https://store.heavenlyhobbies.com/06-023-0054.html

Dave F.

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Dave, you've presented a pair of guided missiles. These are designed to be semi-stable, so that they turn when their fins move. Not to mention their warheads = noseweight.

Also note: most Patriot "flying models" out there have fins larger than true scale.

What most of us fly in hobby rocketry are essentially small sounding rockets...IE: non-guided without the huge mass of a warhead up top. Thus, the irony of chopping a Sandhawk's span drastically...the Sandhawk is a sounding rocket designed to fly in the same manner as we fly hobby rockets....the fins are the size they are for a reason.

Again, several of us have flown quite a few rockets fairly fast...M2+. Several of us have found through practice that having a span equal to, or smaller than airframe diameter, can be problematic. It may not be in Andrew's case, but trying to pass on practical knowledge without reinventing wheels.

And part of the hobby is learning. So there’s a practical lesson and limit to how much fin span we can trim. I might just push it on a J500G to learn.

ECayemberg said:

Dave, you've presented a pair of guided missiles. These are designed to be semi-stable, so that they turn when their fins move. Not to mention their warheads = noseweight.

Also note: most Patriot "flying models" out there have fins larger than true scale.

What most of us fly in hobby rocketry are essentially small sounding rockets...IE: non-guided without the huge mass of a warhead up top. Thus, the irony of chopping a Sandhawk's span drastically...the Sandhawk is a sounding rocket designed to fly in the same manner as we fly hobby rockets....the fins are the size they are for a reason.

Again, several of us have flown quite a few rockets fairly fast...M2+. Several of us have found through practice that having a span equal to, or smaller than airframe diameter, can be problematic. It may not be in Andrew's case, but trying to pass on practical knowledge without reinventing wheels.

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Here is an unguided "Loki-Dart" - type rocket, definitely NOT a "model", the Princeton "Space Shot".

https://static1.squarespace.com/sta...b07/1525994353280/SpaceShot_IW_Poster+(1).pdf

NOTE - I'm not going to "trash" Andrew's thread, any further, by getting into a "urinating contest" with you, Eric . . . I respect the man and I trust that you do, too !

Dave F.

Dave F take it easy.

Eric C is a very accomplished flyer. He really knows his stuff and I am sure he is trying to help Andrew out. Eric would give you the shirt off his back. Not that I would want a Hawian shirt. He's just a great all around guy.

I think I’ll be okay with the fin modifications, the only real way to know is fly it. That’s fine if Dave and Eric get into a pissy contest I don’t care. The kit in stock form was fluttering with a I1299N-P and the volume accepted up to a 38-1200 casing. I got it on sale for $80 and destroyed it’s scale appearance for functional utility.

I respect Dave for pointing out designs that have worked. I respect Eric for telling me behaviors that may happen when finspan is less than tube diameter. I’ve flown probably three rockets with finspan equal to diameter or less while under Mach 2 no problems. The reality is there is no easy way to predict Eric’s noted behaviors.

Ez2cDave said:

Here is an unguided "Loki-Dart" - type rocket, definitely NOT a "model", the Princeton "Space Shot".

https://static1.squarespace.com/static/594607e1be65940bfedfa4e7/t/5af4d366352f53a69d581b07/1525994353280/SpaceShot_IW_Poster+(1).pdf

NOTE - I'm not going to "trash" Andrew's thread, any further, by getting into a "urinating contest" with you, Eric . . . I respect the man and I trust that you do, too !

Dave F.



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Sounds good, Dave.

I put a lot of smilies in my posts above in an attempt to show that I'm trying to help in a light-hearted fashion. Most that know me do not know me as an overbearing know it all type...that's not what I'm after here. I am, however trying to convey the many experiences of some friends and I that have been learned through real life trials and errors over several trips to places like Argonia, Black Rock, Ash Grove (Mach Madness), Princeton (old waiver days), and beyond. I have also seen many computer optimized fin designs not work in the real world...I have more to say on that but don't want to be seen as argumentative.

Not urinating, just clarifying.... Loki's and Super Loki's are spin stabilized out of the tower and travel at a super high velocity that we typically don't achieve in hobby rocketry. I've seen a few launch...they're sweet! I'm not certain if the Princeton attempt is spin stabilized...if they're attempting to mimic a Loki without spinning, and haven't done so successfully to date...we shall see I guess.

I'll be quiet now. I was honestly trying to help. Sorry.

An Alternate idea I had was to get carbon fiber Sandhawk 3/32” fins milled as those would take m1.27 in theory. That wasn’t exactly cost effective. Who knows dremeling the fin span might be the most idiotic thing I’ve ever done: it did solve the flutter problem without a lot of work. I’m in no hurry and can mull this decision over.

ECayemberg said:

Sounds good, Dave.

I put a lot of smilies in my posts above in an attempt to show that I'm trying to help in a light-hearted fashion. Most that know me do not know me as an overbearing know it all type...that's not what I'm after here. I am, however trying to convey the many experiences of some friends and I that have been learned through real life trials and errors over several trips to places like Argonia, Black Rock, Ash Grove (Mach Madness), Princeton (old waiver days), and beyond. I have also seen many computer optimized fin designs not work in the real world...I have more to say on that but don't want to be seen as argumentative.

Not urinating, just clarifying.... Loki's and Super Loki's are spin stabilized out of the tower and travel at a super high velocity that we typically don't achieve in hobby rocketry. I've seen a few launch...they're sweet! I'm not certain if the Princeton attempt is spin stabilized...if they're attempting to mimic a Loki without spinning, and haven't done so successfully to date...we shall see I guess.

I'll be quiet now. I was honestly trying to help. Sorry.

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Have you witnessed any crash from finspan less than tube diameter?