|Apr 23, 2012, 06:47 AM|
Free Flight Junior 60 C of G Position
I plan to build a Junior 60 to participate in the British National Chamionships' Bowden Contest.
I already have a wing from a defunct radio model. This event is open to any free flight model which features a cabin or cockpit; it doesn't have to be a vintage model.The object of the Bowden Contest is for the model to rise off ground and to land 45 seconds later. If your flight lasts for more or less than 45 seconds you incur penalty points. The most consistent flyer wins.
I have a plan of the radio version of the Junior 60 which shows the balance point at about 30% of the wing chord.
Should I use this same balance point for free flight?
|Apr 28, 2012, 03:50 PM|
I've tried it!
I've flown a J60 free flight, years ago, and three RC ones since.
The question isn't whether yours flew hands off or not (all these old J60/Tomboy/Madcap/Bandit type models are super stable).
The only point you have to settle is the trim change when the engine stops.
The issue is confused in normal RC flight by using an RC throttle that you can adjust as required.
I always fly mine using an ED Racer or Hunter, or various team race diesels, so I only have full power or no power! (like in the Bowden!).
Flown RC with an unthrottled 2.5 diesel and a 30% CG, the Junior 60 needs slight down elevator under power, which is removed when the engine stops.
Flying in the Bowden, you obviously won't be able to remove down elevator when the engine stops! So, unless you want a load of unsightly downthrust, you need to move the CG back to a position at which the glide is OK with the slight down elevator needed for power. Typically, on a FF J60, this means a CG at around 40% and about 2 or 3 mm packing under the stab. leading edge, in relation to the incidence setting shown on the plan.
This effect could be lessened by reducing the power, i.e. if you only used say a Comp Special, or if you reduced rpm on your 2.5 diesel then the power trim and glide trim "come closer together".
The bottom line is that you'll need a CG of around 40% to fly FF, and the more power you use, the further back the CG will have to be... Which is why "fairly moderate" power is to be recommended!
|Apr 29, 2012, 08:48 AM|
This what I was thinking too. A forward CG (high pitch stability) is going to give you a single speed model - trim for the glide and it will be power stalling or trim for the motor run and it will dive in the glide.
Moving the CG back (weaker pitch stability) and reducing power makes sense.
Do people still put props on backwards to reduce power?
|Apr 30, 2012, 02:45 AM|
I was planning to use a PAW 19!
I suppose I'll have to adjust it so that it produces far less power than its potential.
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