PB Models Crescent Bullet

probably a bit late starting a thread on this one! I picked this kit up 1/2 price at one of the model show bring'n'buys last year and got bored the other night.
First off, the wood selection and cutting are 95% perfect, the only error so far is that the wing chord is 1/4" greater than the wing cut-out so the trailing edge needed a trim in the centre section. I also beefed up the 4mm ply bulkhead with another layer of 1/16" epoxied on as it just didn't look right.
Yes, in two very casual evenings and a wet Saturday this kit is half done - foam wings built and joined ready for a f/g bandage, fuselage sides doublered, edged and joined with the two main formers, closed loop exits and elevator snake installed ready to accept the front bulkhead, fin and tailplane parts assembled and lots of balsa shavings (and dust!) generated.
I'm going old-school with a single aileron servo and torque rods, but using Corona 339 MG digitals all round on 6.6v.
I did look at making it leccy for all of 5 minutes but an OS 40 FSR won the day.
Pictures may follow...

I'm greatly tempted to leave the rudder and elevator trailing edges squared off, it's one of those jobs I dislike intensely is tapering 1/4" balsa, anyone know of a downside to this on a fast model?

Can only rely on my long term memory on this one Rob, but had a Cresent Bullet donkeys years ago when I used to fly I/C. If I remember correctly I left the edges square, and it seemed to make no difference at all, as a mate had one and he DID sand them down, and the performance was identical . Both models were powered by an Irvine 40, if I remember correctly

Garry

nice model the crescent.
will interesting to see how you get on with it sir.

tony.

Conventional wisdom is that thick square cut trailing edges increase the drag although at small sizes there is some evidence to suggest it is not that significant. It has little impact on the effectiveness of a control surface.
Where drag is of great importance (full size gliders and airliners) very fine trailing edges are used and it would be fair to assume they do this for a reason.

I remember reading somewhere that square cut trailing edges help to prevent flutter.

OF is correct, fun fly models, and several aerobatic models have thick trailing edges, it was expained tome once, but the grey cells wont remember the reason

It's getting there in that it just needs the battery charging, but I have a minor question - does anuyone know the recommended CofG range, or at least the safe rearmost balance point? This kit came without anything more than a small scale drawing. Best I can make out the CofG appears to be at 3". By swapping the OS 40 FSR I had fitted for a heavier 45 SF with a JE super-quiet silencer I've got it to balance at 3 3/8" from the root LE, which I make 30% mean chord. AUW 4lb 8 oz which seems reasonable, be a shame to start adding lead.

It may only be an extra 3/8 of an inch, but is it worth it when you could risk crashing because the model is too twitchy?
I'd add the weight for the maiden & make it nose heavy. Each subsequent flight after that, take off a bit of weight & lose some of the "UP" trim..... Just do this in stages until is does become too sensitive for your liking, then put that little bit of weight back on if needed.
You'll probably find you don't need any weight after all.... but I'd much rather have a docile model for the maiden, then when all the other trimming is done & you're happy that the surface throws & any expo etc. is sorted,... finish off the COG to your liking.

Pol.

Ive got an original dressy bullet plan, I was only looking at it Wednesday, I will dig it out, and check for you, I've also got an original model, built, unflown, and a brand new OS 40FSR

I've had a reply on the other site that it should be 82mm back (recommended by Paul Bardoe himself), which I make 3 1/4" back so it's not so far off after all. I weighed it again with the bigger motor, silencer, prop and battery (1700 LiFe instead of 1300 LiPo) and it's up to 4lb 10 1/2oz so I've effectively added 3oz up front - the 4lb 8 oz was with the 40 fitted.
I've always wondered how people manage to state or measure a Cof G with such precision. Maybe I've just got stubby fingers? Use pencil points and then you have to ask at what exact angle of incidence should the model rest?
In all honesty I've only known two models where the CofG really was that critical, both of them flying wings. Real bicycle clip territory with the slightest degree of tail heaviness.

Always inverted, with just the tip of the finger, and a correct cg is very important, It's perhaps the most critical measurement on any aircraft, Bullet CG is 80 mm from leading edge at the root, inverted, slightly nose down, empty tank, or in a lekky version, with the batteries in flying position

alan cantwell wrote: Always inverted, with just the tip of the finger, and a correct cg is very important, It's perhaps the most critical measurement on any aircraft, Bullet CG is 80 mm from leading edge at the root, inverted, slightly nose down, empty tank, or in a lekky version, with the batteries in flying position

That's my point Alan, just the width of your fingertips gives 5mm or more latitude, then there's different peoples interpretation of 'slightly nose down'. Unless the centre of mass lines up with the wing upper surface this can add a few more mm variation into the mix.
Anyway, thanks for checking as that confirms I'm within 5mm now.

I always use a Great Planes balencer on all my models except the SR 71 i hung that from the ceiling the GP one is acurate to a mm