Construction of a fin

Discussions on high/extreme altitude and mach busting rockets.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby PK » Thu Sep 29, 2011 7:23 pm

Passaretti wrote:
PK wrote:Hmm, too technical for my mortal brain.

Come on PK don't tap out now!

Sorry Mike, I was using a Australian linguistic technique known as 'sarcasm'.
Translated to American, that sentence reads: "Your mindless babble defies analysis and, at best, hints at a deep lack of understanding of the fundamentals of the problem <pre-recorded audience laughter and applause>"

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby cryoscum » Thu Sep 29, 2011 7:39 pm

hehehe
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby hybridhighflyer » Thu Sep 29, 2011 7:40 pm

I thought talking technical was good with rocket scientists....

Heat issue is just using the wrong epoxy. I must say 2.2 Mach is impressive but there is always a way to beat it. I still believe the airfoil will not happen if you designed the fins with the folds I advised as well as the methods.

With all my projects going on at the moment I would love to build a rocket to have a drag race with you nic to demonstrate my fin design but first we don't have clearance to 10k and second I don't think I will have the time. I will see if I can find time to make you 3 fins for you to try yourself.

At the moment I just hope the airport will be able to send me to Perth Saturday. If not I will go home. I was caught in the Melbourne massive storm in the CBD in Brisbane we just call that a storm but at 40 levels up the rain still has snow.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Andrew Burns » Thu Sep 29, 2011 7:55 pm

As I understand it (which isn't very well) what Paul said is on the mark, gust of wind/disturbance cause an angle of attack on the fin which results in lift being generated.

Because the lift force of a flat plate (and with any kind of aerofoil you'd find on a rocket) averages out to occur on the 1/4 chord point the lift force causes the fin to bend and twist in a positive direction and angle (which causes a further increase in angle of attack). The twist on the fin increases until the fin is strong enough to resist it (stiffer fin twists less before resisting the torque which gives them a higher resonant frequency). The disturbance force eventually subsides or the fin unloads as the rocket yaws to face into the wind slightly and the torque on the fin drops off a bit causing the fin to spring back to its original shape. As the fin spring backs it overshoots to the opposite direction slightly, causing a negative angle of attack and the whole cycle repeats in reverse. Depending on the stiffness of the dynamic system (increases with the square of velocity) and the structural system stiffness and damping ratios the feedback between dynamic/aerodynamic and structural systems might be stable (good) or unstable (bad) causing increasing twisting and bending angles until the fin fails.

If you cared enough you could simulate this system however there are some empirical models around for predicting fin flutter envelopes. It can be a bit tricky to estimate the stiffness of composite structures however as unlike metals the orientation of the fibres has a huge impact on the stiffness of the structure (as does the type of resin used and how it was cured).

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby hybridhighflyer » Thu Sep 29, 2011 8:05 pm

As I mentioned before mike like a wing as the wind passes it creates preassure forces that act on the wing or in this case fin. This causes vibrations by the fluctuations in acceleration shaking the wing/fin. Think of it this way when you pull a ball on a string and fluctuate the speed you are pulling it jerks the fin. As the wind passes the fin it creates a back draft and varies the preassure behind the wing creating vibrations. Higher speed higher variances. Take this with heat and fiction it adds to the situation.

I hope this is the answer you are after.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Passaretti » Thu Sep 29, 2011 8:06 pm

Andrew,

So in theory, in a perfect world, in laminar flow (planar to the fin surface) this feedback should not occur. Would you tend to agree?
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby RobAust » Thu Sep 29, 2011 8:10 pm

Hi Andrew,

I'm with you and believe PK summed up it nicely.

One image which is widely used to partially illustrates this phonomenom can be found below;
FAA-8083-3A_Fig_15-9.png
FAA-8083-3A_Fig_15-9.png (29.31 KiB) Viewed 3422 times

From my review, I believe the issue is the transition through Mach and then sustained Mach flight is where the fin stresses become enourmous.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby hybridhighflyer » Thu Sep 29, 2011 8:10 pm

I disagree. You cant apply that thinking to the real world. That makes a lot of assumptions there. That was to mike.
Last edited by hybridhighflyer on Thu Sep 29, 2011 8:11 pm, edited 1 time in total.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby cryoscum » Thu Sep 29, 2011 8:11 pm

cryoscum wrote:Really need to come up with some sort of electronics that can do the ramping...


Actually, I need to harass PK over this. Seems like he gives away free electronics if you show him photos! Friendly folks, these from WA!
:D
Last edited by cryoscum on Thu Sep 29, 2011 8:13 pm, edited 1 time in total.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby hybridhighflyer » Thu Sep 29, 2011 8:13 pm

cryoscum wrote:
cryoscum wrote:Really need to come up with some sort of electronics that can do the ramping...


Actually, I need to harass PK over this. Seems like he gives away free electronics if you show him photos! Friendly folks, these from WA!
:D


It's ok you offered my redundant set remember. ;)

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Passaretti » Thu Sep 29, 2011 8:34 pm

RobAust wrote:One image which is widely used to partially illustrates this phonomenom can be found below;


Rob can you point to where you found this image?

Thank you,
Mike
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Andrew Burns » Thu Sep 29, 2011 8:40 pm

Passaretti wrote:Andrew,

So in theory, in a perfect world, in laminar flow (planar to the fin surface) this feedback should not occur. Would you tend to agree?


No I don't think it would occur in laminar flow with no angle of attack to the fin however no rocket will ever have laminar flow with no angle of attack over its fins, all rockets (that I know of) go fast enough that flow over their fins will be entirely turbulent and unless you're flying in a wind tunnel there will be angles of attack.

Rob: Your images show what is basically a special case of fin flutter when flying at the transonic region. When the fin is at an angle of attack in the transonic region the airflow over the top side speeds up which can cause a local supersonic region. This weak supersonic region doesn't last however as a normal (or 'strong') shock forms to bring the flow back down to free stream velocity. This normal shock results in increased drag and huge pressure gradients over the surface of the fin which drastically increase the loads on the fin and can behave in unpredictable ways. I've seen animations of aerofoils like the one you showed where the supersonic pocket and normal shock were rapidly oscillating forwards and backwards along the chord of the wing in a positive feedback loop with angle of attack (classic flutter). Clearly it's best to move through the transonic region as fast as possible as you don't get these problems when the flow is fully sub or supersonic.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby kopius » Thu Sep 29, 2011 8:46 pm

Andrew Burns wrote:As I understand it (which isn't very well) what Paul said is on the mark, gust of wind/disturbance cause an angle of attack on the fin which results in lift being generated.


Now just throwing this out there (prob coz I was actually present at the time), the wind on the day was less than desirable. I am not discounting the fact that the fins may have let go on their own accord and I am all for the redesign which Nic and I have been discussing, however some form of wind shear is very possible, especially with the weight and size of the rocket.

Another flight earlier that day saw a coupler break on the way up which without going into great detail and taking this off topic is believed to be from a similar event of wind hitting the airframe at an angle/speed that caused severe turbulence. Unfortunately this rocket had about 8 or 9 sensors in the NC to test temps and the likes similar to Chris's project so we will have to wait for the rebuild to see data.

There are many valid points in this thread, however there is also many variables which may have played a role in the end result. I would not describe the fins as delaminated, rather torn off.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Passaretti » Thu Sep 29, 2011 8:58 pm

Andrew Burns wrote:
Passaretti wrote:Andrew,

So in theory, in a perfect world, in laminar flow (planar to the fin surface) this feedback should not occur. Would you tend to agree?


No I don't think it would occur in laminar flow with no angle of attack to the fin however no rocket will ever have laminar flow with no angle of attack over its fins, all rockets (that I know of) go fast enough that flow over their fins will be entirely turbulent and unless you're flying in a wind tunnel there will be angles of attack.


Thank you. That little nugget is what I needed to hear to begin to soak in this phenomena. I agree completely that laminar flow can only be achieved in a lab setting. The point that I was trying to make is that it is simply more than "a strong flow" that starts this process. What I am taking away from these conversations is that the driver is the angle of attack & turbulent flow about the fin/wing.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Passaretti » Thu Sep 29, 2011 9:28 pm

kopius wrote:There are many valid points in this thread, however there is also many variables which may have played a role in the end result. I would not describe the fins as delaminated, rather torn off.


Thanks Blake. Could you post some more pictures of the fins/mounts or what's left of them?
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