My first build.

Discussions on low powered model rockets using E powered motors and below.

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Digit
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Re: My first build.

Postby Digit » Sun Mar 13, 2016 9:16 pm

Dimensions and weights were all done with vernier callipers, rule and scales as it was being built and then again once completed for overall weight with a real motor fitted and my heavy finish (putty in airframe spirals). CG was adjusted nose ward from simulation after checking actual by suspending from a loop of dental floss. A thought that only just occurred to me was I only had my recovery wadding just below the parachute instead of right down to the motor mount, it's a relatively long airframe could that have contributed?

Here's footage of one of my C launches.
[youtube]http://youtu.be/e1WFUIqnpyI[/youtube]
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Re: My first build.

Postby Digit » Sun Mar 13, 2016 9:22 pm

vance2loud wrote:I would guess that the balsa soaked up a lot of dope when you were sealing it.

Also looking at the stability graphs that SpaceManMat posted it might help if you can find a longer rod to allow for a higher speed as you leave the pad.


Yea I was actually thinking that when I brushed it on, it just vanished. Each fin got two coats on each face, so there's a good chance of three different weighted fins. I don't think dope was the correct stuff to use out of this learning experience.

Also how do I read these graphs?
cheers.
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Re: My first build.

Postby martymonsta » Sun Mar 13, 2016 9:30 pm

Hey Digit, Nice looking rockets and good job with your 1st day flying with QRS, usually weather cocking is caused by the rocket being overly stable, which is what Matt is trying to diagnose. You will likely have seen that 1 Cal of stability is about the minimum that is recommended for stable flight. This means that the center of gravity (where the rocket balances with a motor installed) in 1 body tube dia. forward of the center of pressure (there are ways to work this out manually, however a simulator like OR is probably the easiest and some kits will provide the CP for you).

So less then 1 cal of stability and the rocket might become unstable under thrust and this is very bad. A stable rocket will naturally want to correct its orientation so that it maintains an angle of attack of 0 deg. and will thus weather cock a little. The more cals of stability a Rocket has and/or the stronger the wind the more the rocket wants to turn into the wind.

With a lot of beginner kits they are designed over stable, this allows a good margin for error for the motor mount to be over built and are designed to be stable on the biggest motor that will fit in the mount, because that is always going to happen. This makes them over stable when you fly them on smaller motors. One of my LPR kits has a horrible case of weather cocking if I fly it on an A motor. That said the data that you have provided doesn't look like it is over stable, the graphs that Matt posted show the stabilty over the fight as propelent burns off and the center of gravity shifts forward starting at just under 1 Cal and leveling off at about 1.6 Cal. That should provide you with a good stable fight.

Looking at your video it did have a bit of a wiggle, but that could be fin alignment.

Once again good flying
Last edited by martymonsta on Sun Mar 13, 2016 9:39 pm, edited 1 time in total.
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Re: My first build.

Postby vance2loud » Sun Mar 13, 2016 9:35 pm

The graphs posted show the CoG throughout the flight (Dropping as the motor burns) in green, the CoP (Changing relative to drag at different speeds throughout the flight) in Blue (I think) and the resulting stability margin in red.
It shows that you didn't have much stability until around 0.6 seconds into the flight, which could possibly be after leaving the rod.
If this is the case then the weathercocking occurred prior to reaching a stable flight situation at 0.6-0.7 seconds.
In the simulation software there is the option to adjust the launch rail length and angle, If you adjust that correctly you should also be able to see what the velocity is as you leave the rod.
I will leave it to the more experienced here to suggest minimum speed as I personally am unsure as well.
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Re: My first build.

Postby OverTheTop » Mon Mar 14, 2016 9:32 am

Congratulations on your first build and flights. Well done!

My personal preference for rail speed is around 14m/s (50km/h). Below that I give a bit more consideration to the launch conditions (need calmer winds). Certainly no slower than 11m/s (40km/h). Check with your RSO or anyone else helping with your future certifications what their thoughts are too.
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Re: My first build.

Postby SpaceManMat » Mon Mar 14, 2016 9:55 am

Ok, with sim details confirmed...
1. Your model is a bit on the heavy side so longer rod and larger motor will help
2. Your stability is very marginal for about .2 sec after launch. What this means is that there is not enough aero forces being generated to keep the rocket straight. BTW the graph in OR only starts once the rocket leaves the rail/rod. So the very start of the red line is the critical part.
3. To correct the issue the normal option is to add weight to the nose. This moves the CG forward thus increasing stability. However you need to be careful with this as it will increase your already high liftoff weight. Possibly you want to look at reducing weight instead or as well, particularly from the aft end of the rocket. Other ways of fixing stability are larger fins and also if you can extend the body you may be able to move the CG further forward, but you probably don't want to rebuild your rocket too much. To get more speed you could go to an 18mm D motor. A D21 will make it scream off the pad, but this may require addition nose cone weight and also difficulty in recovering it.
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Re: My first build.

Postby Viking » Mon Mar 14, 2016 1:21 pm

Congrats Digit, sounds like you had a fun day. The bug has bitten?

OverTheTop wrote:...
My personal preference for rail speed is around 14m/s (50km/h). Below that I give a bit more consideration to the launch conditions (need calmer winds). Certainly no slower than 11m/s (40km/h).

I sim'ed my Big Daddy clone(29mm) on an F12 for its maiden, OpenRocket said ~12m/s off the rod which I knew was going to be a little dicey but the WARS rail is much longer than the rod length in OR so I figured all would be fine there.
Come launch time there was a light breeze, really nothing to be concerned about, so it surprised me when she weathercocked so badly! The subsequent flight on a G53 couldn't possibly have been any straighter.
In hindsight, the BD has huge fin area in relation to her overall length, at 1 cal the diff between CG/Cp is ~76mm over a 470mm short bird, so ~16% of overall length...
15m/s min for me from now on ;)

SpaceManMat wrote:... BTW the graph in OR only starts once the rocket leaves the rail/rod. So the very start of the red line is the critical part.

Thanks for bringing this up, I wasn't aware of this. I hadn't even considered it, just assumed the plot started from ignition :oops:
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Re: My first build.

Postby martymonsta » Mon Mar 14, 2016 2:02 pm

Just to clarify some weather cocking is a perfectly normal in a stable rocket flight, I would expect to see a rocket turn into the breeze strait off the pad. To illustrate assume that a rocket is stable throughout its flight, has a off the rail speed of 15m/s and the weather has a moderate breeze of 20km/h or 5.5m/s. The rocket will naturally want to correct so that it maintains an AoA of 0 deg, so using basic trig it would try to correct to an angle of 20deg off vertical, that said something like the MFR on a B6 would have a Max velocity of 50'ish m/s which would only be 6.28deg off vertical and that angle will start increasing as it looses vertical velocity.

Also longer launch rods might provide higher off the rod speeds however if you get rod whip it kind of defeats the purpose.
Many years ago the great British explorer George Mallory, who was to die on Mount Everest, was asked why did he want to climb it. He said, "Because it is there."
Well, space is there, and we're going to climb it

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Digit
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Re: My first build.

Postby Digit » Tue Mar 15, 2016 6:11 am

Thanks all for the help and advice. I know have a better understanding of that part of open rocket, and some guidelines for motor selection and future builds. Can I ask what a better sealing solution for balsa might be?
Cheers
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Re: My first build.

Postby SpaceManMat » Tue Mar 15, 2016 9:25 am

I normally use a wood grain filler. Other use a spray filler.
QRS: 124
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Re: My first build.

Postby Lister » Tue Mar 15, 2016 11:09 am

Just paint a coat of pva on balsa or use the spray putty and paint that on and it will seal it... or if you want more strength too, get some epoxy and laminate paper onto the fins.

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Re: My first build.

Postby SpaceManMat » Thu Mar 17, 2016 6:48 pm

Forget what I said about D motors. Add 6 - 10 grams to the nose cone, it will fly nicely to 1000 - 1100 feet. Should not be an issue recovering from that altitude. Do check your CG to make sure. Note that you will not be able to fly with C motor once modified.

BTW if you measure your CG without the motor, make sure you also check your sim with no motor installed. Also if you use the CG override for your entire model, this value is always the without the motor CG (motor weight and CG is added on top of this by OR).

Take a look at the modified sim.
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QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 10,849 feet
Largest Motor: CTI 1115J530 IM
Current Project: X Wing


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