Need some help with a swing test

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Screaminhelo

Screaminhelo

Shade Tree Rocket Surgeon
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My Fatter-By is complete and the rocket balances about 1.5" fwd of the CP indicated by the cut out. I would like to confirm this with a swing test but the CG and CP are aft of the leading edge of the fin which prevents me from looping a string around the airframe. Here is a picture so you can see what I am dealing with.

Fatter-By.jpg

I have considered using a bridle that I can adjust until it balances. Has anyone tried something like this?

I am tempted to go with the results that I already have but I would prefer to verify this if I can since this is my first venture into scratch building.
 
If you have two launch lugs separated by a long distance, you can put a wood dowel or guide wire between the two and then slide a string wrapped around the dowel to find the c.g. This method only finds the c.g. and requires two launch lugs. (Perhaps, something like this could be used as a bridle, if it was strong enough.) Conceivably, you could tape another launch lug on. Usually, the c.g. is easier to find than the c.p., but surprisingly I have found times when all the glue, paint, finishing, etc. has moved the c.g. (BTW the overall weight may also be different) more than a simulation code predicts.
 
The launch lugs are plenty strong enough but the top one is just barely past CG and it may make it difficult to get the bridle to balance, that will be and option that I file away for future builds though. I can simply make a couple of loops around the fore an aft airframes and center the swing line at the CG.

That variation that you bring up is exactly why I want to swing this thing. I have it balanced for a total motor weight of 75g right now and I want to see if I have enough margin to keep it stable with heavier loads. If it remains stable with an F35 (105g total motor weight), I am confident that finishing will not have not have an adverse effect with the motors that I intend to fly. In the end, my ultimate goal is to reduce nose weight and minimize overall weight. It seems that swing testing will be the best of both worlds here as it should be simple to adjust to different test weights and provide concrete, dynamic data.
 
Screaminhelo said:
The launch lugs are plenty strong enough but the top one is just barely past CG and it may make it difficult to get the bridle to balance, that will be and option that I file away for future builds though. I can simply make a couple of loops around the fore an aft airframes and center the swing line at the CG.

That variation that you bring up is exactly why I want to swing this thing. I have it balanced for a total motor weight of 75g right now and I want to see if I have enough margin to keep it stable with heavier loads. If it remains stable with an F35 (105g total motor weight), I am confident that finishing will not have not have an adverse effect with the motors that I intend to fly. In the end, my ultimate goal is to reduce nose weight and minimize overall weight. It seems that swing testing will be the best of both worlds here as it should be simple to adjust to different test weights and provide concrete, dynamic data.
Click to expand...

Bill:
you'll want to have the model in full up flight ready condition Less the igniter and plug. You'll also want to have the model balanced with about a 7-10deg. Nose down condition. With that in mind using the dowel between the launch lugs should give you plenty of string adjustment just make sure you can keep the dowel and string secured with tape during the swing test.
 
Micromeister said:
Bill:
you'll want to have the model in full up flight ready condition Less the igniter and plug. You'll also want to have the model balanced with about a 7-10deg. Nose down condition. With that in mind using the dowel between the launch lugs should give you plenty of string adjustment just make sure you can keep the dowel and string secured with tape during the swing test.
Click to expand...

That is the plan. I have a 3.5" dummy motor that I made where I can adjust the weight for an initial test of various motors. The heaviest motor I am likely to use is and E11 reload. I plan to use that as my base line. Since the lugs are in the fin root, the dowel method may run into some interference issues with the big fins but it is worth a try since it will be more precise than a string bridle.

In the end, I'm learning a great deal here and having a good time in the process. Thanks for the advice gentlemen!
 

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