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Sorry, I guess it does give the worst case, I remembering modeling a model with drastically different side views just to convince myself of that..
Titan IIIE
- Thread starter Charles_McG
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Charles_McG
Ciderwright
I played around with the calcs.
Barrowman puts the Cp a half inch forward, compared to 'Rocksim'. Cardboard cutout puts it 3.5" aft - BUT doesn't take the boosters into consideration.
Barrowman puts the Cp a half inch forward, compared to 'Rocksim'. Cardboard cutout puts it 3.5" aft - BUT doesn't take the boosters into consideration.
Charles_McG
Ciderwright
Please excuse me if I seem paranoid, but I've got a lot of money in some of the parts for this beauty.
When I started, I built a BT50/20 (1x13mm motor) downscale to see how she'd fly. Made a number of flights in the neighborhood park. Tried different numbers of fins and adding weight to the tail to simulate the extra motors. Kept track of the Cg - and where I had good flights and where I had sky writing.
Then I built the big pretty one and loaded it it up motors - and to get the CG up to where the mini had good flights took what seemed like a lot of nose weight to me. Which made me wonder what was up.
So I got the trial Rocksim ('cause I'm not sure how to fake the pods with OpenRock) and simmed out the big one. Okay - Cp much further back than I had guessed, and the noseweight needed looks like an amount to counter my choice of thick, heavy fins, compared to the scale model. Reasonable.
But what about those test flights that suggest that the Cg should be much further forward?
So this morning I got out the calipers and simmed the Titan IIImini. There are a couple of important difference: I extended the core body tube in the big version. The little version had core fins with a shorter span. The Mini version has a pretty long launch lug on a standoff, which I simmed as an extra fin. A little tweaking brought the weight and Cg to my measured values.
And low and behold, it makes sense. The Cp is much further forward on the mini than on the big model. Adding the ballast like I did experimentally taks the stability to just under .5.
So now I have more confidence in the simming. Now I just need to get some flight time in. Eat Cheese or Fly is coming up this weekend, but my rocketeer boy is off at AirVenture with his Aviation Club - so I might have to wait until sometime in August.
When I started, I built a BT50/20 (1x13mm motor) downscale to see how she'd fly. Made a number of flights in the neighborhood park. Tried different numbers of fins and adding weight to the tail to simulate the extra motors. Kept track of the Cg - and where I had good flights and where I had sky writing.
Then I built the big pretty one and loaded it it up motors - and to get the CG up to where the mini had good flights took what seemed like a lot of nose weight to me. Which made me wonder what was up.
So I got the trial Rocksim ('cause I'm not sure how to fake the pods with OpenRock) and simmed out the big one. Okay - Cp much further back than I had guessed, and the noseweight needed looks like an amount to counter my choice of thick, heavy fins, compared to the scale model. Reasonable.
But what about those test flights that suggest that the Cg should be much further forward?
So this morning I got out the calipers and simmed the Titan IIImini. There are a couple of important difference: I extended the core body tube in the big version. The little version had core fins with a shorter span. The Mini version has a pretty long launch lug on a standoff, which I simmed as an extra fin. A little tweaking brought the weight and Cg to my measured values.
And low and behold, it makes sense. The Cp is much further forward on the mini than on the big model. Adding the ballast like I did experimentally taks the stability to just under .5.
So now I have more confidence in the simming. Now I just need to get some flight time in. Eat Cheese or Fly is coming up this weekend, but my rocketeer boy is off at AirVenture with his Aviation Club - so I might have to wait until sometime in August.
Charles_McG
Ciderwright
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