Sig Chipmunk headed for HEAVY...

I've got my Chipmunk pretty much all framed up and ready to cover. I put one coat of PC and silkspan on the fuselage and tail. I put it on the sacle last night: ouch! 38oz with engine and spinner...

What weights have others achieved with this kit?

For the most part I used the kit supplied wood and hardware. I did repalce some really hard stuff and all the "lite ply".

The engine is a PAW .35. heavy, but a lot of power. Constructon priorties were:
1) build it straight
2) assemble it "square"
3) KISS (no major mods: I could't resist a few...)
4) keep it light (plan is minimal finish, but at least one step up from just clear dope and silkspan)

The only mods were: beef up the landing gear mounts, add wing tip weight box, adjustable leadouts, "square" wingtips rather than the built up round ones, and I shortened the nose (moved the firewall aft 1.5" but because the PAW is a longer engine the prop moved aft only about .75". After the discussions here on this forum I stuck with the flat stab/elevator to make it easier to align. I tried to get a 0-0-0 thrust line, wing, tail incedence alignment. After all was said and done I think I may have a very small amout of positive incedence in the stab. Maybe about 1/8 deg. I figured "better positive than negative" and I'm not convince I can measure it that accurately...

Ron,
I have seen these fly really well in the mid 40 oz. range.
Just try and keep the final finish pretty light.
Bill <><

>Bill <><

Bill, is that a Jesus fish next to your name?

Yes.
Bill <><

Nice! It took me several days to figure it out. Lack of sleep does that to you...

>>Bill <><
>
>Bill, is that a Jesus fish next to your name?
>
>Steven Yampolsky

Wow, Steven, first time in this forum I see someone that not a certain crazy Brazilian mention the name of Jesus!

I also wondered about Bill's "fish", but never asked.

Why the fish? because in Greek fish is "ICTHUS", and that was an anagram for: (the declinations are not right, I'm a lousy greek student) Iesous Cristus Theos Uion Soter, (Jesus Christ God's Son Savior)

I esous
C ristus
TH eos
U ion
S oter

Len is a better scholar, he probably knows how to write it correctly.

>Ron,
>I have seen these fly really well in the mid 40 oz. range.
>Just try and keep the final finish pretty light.
>Bill <><
>
>
>Disclaimer: I am neither a NATS Champion, engine reworker,
>engine producer, column writer, Hobby Shop owner, *rockeet*
>scientist, nor otherwise. Just an old Advanced flier.

The plan is dyed silkspan with some solid color trim... trouble is: that shows all my poor joinery work

We'll see what it looks like (and what it weighs) when I get that far along. I suspect that "... mid 40 oz. range..." will be hard to meet.

Probably closer to 50 oz.

Rick Clark won a number of contests in Advanced class flying his ~50oz SIG Chipmunk. It is powered by either an OS 35-S or OS 40FP(not sure which one). He's been flying this ship for a number of seasons now with great success.

You're saying with an engine, and the engine is a PAW 35?

You're bragging, man. And a tip of the hat to you for getting the wt so low.

Mine is 44oz with a 40FP, doesn't seem to be too heavy. Light and crooked or flimsy is worse than heavy. You'll probably be alright.

You might want to start trying to figure balance. Ballast to correct imbalance will probably get you into "heavy" range quicker than anything else you'll face at this point. It may not be too late to move the engine around if you need to (or are able to, is this going to be classic legal?).

Mine is 50 oz. with an LA .46. Ain't the greatest,but still does a decent pattern. Have seen 'em heavier than this and still perform with enough ponies to pull 'em.

>Light and crooked or flimsy is worse than heavy.

...yeah, that was my thought, too

>You might want to start trying to figure balance. Ballast
>to correct imbalance will probably get you into "heavy"
>range quicker than anything else you'll face at this point.
>It may not be too late to move the engine around if you need
>to (or are able to, is this going to be classic legal?).
>
I really didn't intend to make it "classic legal" hence the shortened nose and square wing tips... actually it wouldn't be too terrible to move the engine back a bit more if I do it now (one advantage of the plastic "slip-on" style cowl). However, I can't go very much further than I am now and still have room for a standard COTS type fuel tank. perhaps I should have added some length to the aft fuselage. But, my thinking at that time was to stay as close as I could to the plans/kit. Your comment to watch the balance is a good point.

Does anyone have any "rules of thumb" for estimating the weight of the finish? and how much the CG moves with the finish? even a WAG?

a decent, light finish is in the range of 12-15 grams/sq.ft. Approx. 3/4 of the finish is behind the CG. Just estimating, the Chipmunk has around 1500 total square inches to finish(1000+ for the wing, 300 for the stab, and 200 or so for the fuse), about 10 sq.ft. for a total weight of ~150 grams(5-6 oz.) Excess primer, not enough sanding, and too many coats can easily run that up another 2oz. or so.

Back in 1974 or 75, when the original Van Loo kit was first out, I built two for the engines he used. G21-40's. Neither of the engines worked for me. Both Chipmunks weighed 49 Oz without trim color or canopies, and flew only moderately well at the speeds those 40's insisted on running at (about 65 mph, plus or minus). I gave the first one to a buddy I'd built it for, and he just hung it up to collect dust.

Mine has been flown since then with a G21-46 I acquired later on from John Hill; it flew sweetly with that, and didn't act "heavy", but the tank is too small & wasn't modified to be removable, so mostly it also just collects some dust . .

.

Kiwi

>Back in 1974 or 75, when the original Van Loo kit was first
>out, I built two for the engines he used. G21-40's.
>Neither of the engines worked for me. Both Chipmunks
>weighed 49 Oz without trim color or canopies, and flew only
>moderately well at the speeds those 40's insisted on running
>at (about 65 mph, plus or minus). I gave the first one to
>a buddy I'd built it for, and he just hung it up to collect
>dust.
>
>Mine has been flown since then with a G21-46 I acquired
>later on from John Hill; it flew sweetly with that, and
>didn't act "heavy", but the tank is too small & wasn't
>modified to be removable, so mostly it also just collects
>some dust . .
>
>.
>
>Kiwi
>
>**

65mph sounds pretty fast to my untrained and uncalibrated brain... what kind of speed/lap times did you get with the G21-46? Did you continue to fly it at the +/- 65mph speeds with the G21-46? What line lengths were you using? Was the improved performance a result of more prop/power/thrust?

> ... G21-40's. Neither of the engines worked for me. Both Chipmunks
>weighed 49 Oz without trim color or canopies, and flew only moderately
>well at the speeds (that) those 40's "insisted on" running at (about 65 mph,
>plus or minus).

That was because I didn't have access to much variety of low pitch props. I tried a couple of different 11-4's, and because the 40's I had on hand to use both wanted to rev so high, so I couldn't control the lap speeds with props. Any needle setting would eventually lead to a two-cycle "run away".
>
>Mine has been flown since then with a G21-46 that I acquired later on from John
>Hill; it flew sweetly with that, at normal stunt speeds right around 50 mph, and
>didn't act "heavy", but the tank is too small & the nose wasn't modified for the
>tank to be removable, so mostly it also just collects some dust . .
>
.
>
Kiwi
>
**

(Edited some, to answer Ron).

Take a ggod look at the plane and try to find material you can remove.

Are your wheels te lightest ones available? If not, change to them super duper light ones.

Are you using wheel collars? Remove them and use thin copper wire and epoxy.

If the wing isn't yet covered, you can remove a lot of balsa from the ribs.

If the spinner is metal, put a DuBro plastic one.

Drill some holes in the wingtips and cover them with paper.

If the rudder is solid balsa, put on a buit one.

Use a cheap platic tailwheel instead of a rubber one.

Paint it with a dark color, you'll need less coats. use butyrate dope.

All this together might save you some onces.