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Author Topic: From VIT to not VIT  (Read 366 times)
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tctele
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« on: November 01, 2018, 05:20:48 AM »

Hopefully the headings not too confusing!

Without going into exact measurements, moment arms, wing and tail areas etc etc. Here's my predicament:

I want to build another 1 or 2 R/L coupes with a single function electronic timer, I have them so why not use them, fed up of mechanical timers letting me down. Now I have one VP hub left and I know this will work with the stab area as is. But I don't have another VP hub and I don't intend to buy one either. So, is there a formula for calculating how much bigger the stab needs to be? All things relevant I want to keep everything nearly the same, I have multiple jigs etc for reproducing parts so they all are interchangeable. The CG woulkd probably go further back than the 70% due to a slight increase in weight granted. Or, is it a case of use a fixed pitch prop, I have them on my other models, and just use a bunch of different sized stabs until I don't have to use the VIT function in the burst? Like I said just wondering if the is a % formula to save me building a bunch of stabs, not that they take to long anyways.

Thanks in advance, Tony
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flydean1
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« Reply #1 on: November 01, 2018, 07:35:53 AM »

Someone else will have to come up with the stab formula.  It has to do with Tail Volume.  As for flying without gadgets?  The late Jean Wantzenriether (I have surely misspelled his name) developed the TOP/PGI trim technique.  This gets rid of the gadgets for very little loss in performance.

Don't know if there are any threads here on the Forum, but Free Flight quarterly has multiple articles by Jean.  They are quite informative.
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tctele
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« Reply #2 on: November 01, 2018, 04:35:06 PM »

I have his FFQ edition. Tried PGI didn't get on with it for some reason. I think I probably answered my own question by saying build a bunch of stabs, not ideal but probably very practical and real time results. A ball park formula would be nice so I'm not to far off would be good though
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flydean1
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« Reply #3 on: November 01, 2018, 08:43:58 PM »

Ratz or Sundance should move this as it deals with Coupe which are rubber powered rather than engine powered.

I did use TOP/PGI on a couple P30's.
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tctele
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« Reply #4 on: November 02, 2018, 02:54:02 PM »

Didn't realise I'd posted into "Power" got an error on my first post so didn't take notice, my fault
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Yak 52
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« Reply #5 on: November 03, 2018, 05:35:15 AM »

Hi Tony,

The Tail Volume formula could definitely be a useful tool to save you a few iterations. If you can work out the tail volume of similar designs without VIT (from other successful plans?) it would give you a fairly accurate starting point.

Also if you can get a copy of Hepcat's 'NPandTrim' spreadsheet, it gives an estimate of tail incidence for a given lift coefficient. You can then size the tail to give a minimum trim change across the speed range - ie tail size for a locked down model.

It depends if the time spent on the theoretical approach or empirical building of a few iterations is less hassle for you personally I suppose.

Jon
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tctele
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« Reply #6 on: November 03, 2018, 11:33:24 AM »

Thanks Jon I'm sure I have the formula somewhere has anyone got John's trim spreadsheet please. Tony
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