RE: CD ScaleDesigns Sopwith Snipe build  
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RE: CD ScaleDesigns Sopwith Snipe b... - 5/27/2008 4:09:53 PM   
abufletcher



Posts: 5172
Joined: 2/13/2004
From: Zentsuji, JAPAN
Status: offline 		Thanks, Azzam. It seems like the usual practice is to cut out a hole/slot at the location of the cylinder. I suppose I could do this if that's what it takes. Or would it be enough to cut out the areas between the dummy cylinders (in the ply base)? I'm not sure how I'd manage the "baffles" method of redirectly airflow within the large circular cowl.

No work on the Snipe today, but I did get the Saito 56 installed in the Cub so I should have that flying again in a few days.

I'm still pondering how to handle the adding of the extra weight. As I said I'm planning to build as much weight as possible (as needed?) into the dummy. I suppose I could even use metal for the backing disk on the dummy.

(in reply to phantom_najd)
       Post #: 1526

RE: CD ScaleDesigns Sopwith Snipe ... - 5/27/2008 5:57:40 PM   
abufletcher



Posts: 5172
Joined: 2/13/2004
From: Zentsuji, JAPAN
Status: offline 		Present weight with engine, exhaust, (current) dummy, prop, etc. is 3.6kg (for a 63" equal span and cord scale biplane). This would be pretty much the all-up flying weight minus whatever additional weight is need for balance. So that seems pretty good. We're still double checking the balance point calculation but the CG at the moment might only be a centimeter or so behind the CG as indicated on the plan.

(in reply to abufletcher)
       Post #: 1527

RE: CD ScaleDesigns Sopwith Snipe b... - 5/27/2008 6:11:05 PM   
TFF


 

Posts: 741
Joined: 10/30/2006
From: Memphis, TN, USA
Status: offline 		I would test it without the dummy engine until you are comfortable flying the plane. Like said before, a sagged engine will be like a dead stick and it causes its own problems with control as you will try to use the power avalable. Wallowing around at stall, when you should have dived for the ground to build up airspeed and control, is what happens. Give yourself time with the plane before you get fancy.

(in reply to allanflowers)
       Post #: 1528

RE: CD ScaleDesigns Sopwith Snipe ... - 5/27/2008 6:21:28 PM   
abufletcher



Posts: 5172
Joined: 2/13/2004
From: Zentsuji, JAPAN
Status: offline 		Hmm...that means I'll need to put the balance weight elsewhere. Maybe I can "cast" some lead in a ring that would fit on the inside of the front face of the cowl.

(in reply to TFF)
       Post #: 1529

RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 5:47:10 AM   
abufletcher



Posts: 5172
Joined: 2/13/2004
From: Zentsuji, JAPAN
Status: offline 		At this point it looks like I'll be needing about a pound of lead up front. I reviewed the construction photos for possible ways to reduce the weight of the rear of the fuselage thinking that I could possiby open up the bottom. But after running some calculations for the projected weight savings by lightening the tail at three "stations" (rudder post, stab LE, and rear of turtledeck) there didn't seem to be any point to this.

Basically to achieve around 200g weight saving in the nose, I'd need to be able to remove 20g from each of the three stations. That's 60g removed from the rear of the fuselage which is equivalent to three sheets of 3"x24" 3mm balsa. There's just not that much back there! I think it's just about inevitable that a scale model of such a short nosed aircraft is going to need some weight added.

I thought about adding the weight to the inside of the cowl, but I would have to seriously beef up the way the cowl is connected. So I went back to the idea of adding the weight to the back of the ply plate supporting the dummy engine. The lead will be added in a way so that I add and subtract weight if I want to adjust the CG.

I also did a bit of thinking (and working) on the dummy engine. After looking back at Allan's nicely done dummy, I decided that the "gap" in front of the cylinder doesn't look too bad particularly since the Bentley was the same bright aluminum color of the Saito 72. I also borrowed spikkkkk's great idea of using the bottom of a spray can to form the domed front of the engine. This worked out great and will lend a bit of 3-D look to the photo. I still believe that using a photo of the engine will ultimately provide more realism than building one myself.

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(in reply to abufletcher)
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RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 6:04:39 AM   
abufletcher



Posts: 5172
Joined: 2/13/2004
From: Zentsuji, JAPAN
Status: offline 		BTW, I forgot to say that I'll need to work out a strudy way to attach this ply base to the firewall if it's going to support the weights. On the top it anchors to the battery box. On the bottom I'll work out some strong form of the supports I have in there now.

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RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 7:03:14 AM   
Frank Sopwith



Posts: 106
Joined: 1/20/2007
From: Westervoort, NETHERLANDS
Status: offline 		Don,
I have the feeling you focus too much on the weight issue here.
Of course it is always important to build as light as possible, but in your case the Snipe is oke here; just look at the wingload first.did you calculate it already?
I estimate roughly it ‘ll be between 55 and 65 grams/dm2 if it gets above 80 you should start worrying........

Frank


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RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 7:30:52 AM   
abufletcher



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From: Zentsuji, JAPAN
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quote:

ORIGINAL: Frank Sopwith
Don,
I have the feeling you focus too much on the weight issue here.
Of course it is always important to build as light as possible, but in your case the Snipe is oke here; just look at the wingload first.did you calculate it already?
I estimate roughly it ‘ll be between 55 and 65 grams/dm2 if it gets above 80 you should start worrying........

Frank



That's good to hear. I think it's mostly the "aesthetics" or "philosophy" of the thing. In a perfect world and on a perfect model there would be no need to add weight. There's a certain "purity" to this. As a second best solution, the weight could be added in the form of actual additions to the model (for example, an all-metal dummy engine). The least pleasing solution in my aesthetic sense is to have to add lumps of lead.

Frank, did you get my email about the CoG calculation?

(in reply to Frank Sopwith)
       Post #: 1533

RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 9:48:18 AM   
camdyson



Posts: 828
Joined: 7/20/2005
From: Bairnsdale, AUSTRALIA
Status: offline 		Look at it this way Don - if it's to be REALLY scale it needs to have scale weight distribution too, so a big chunk of metal SHOULD be in the nose

It'll handle the weight fine, just be sure it's well tied into the airframe. Glad to hear you're not attaching it to the cowl - whoever thought that one up should be forced to fly ugly sticks.........

Keep going, it's looking brilliant.

Cam

(in reply to abufletcher)
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RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 11:56:10 AM   
Frank Sopwith



Posts: 106
Joined: 1/20/2007
From: Westervoort, NETHERLANDS
Status: offline
quote:

ORIGINAL: abufletcher
Frank, did you get my email about the CoG calculation?


Chris and I calculated the CG for the Snipe on 96mm from the leading edge of the top wing.
This is 25% MAC and is a good location to start with; worked fine for the pup!
Later you can decide to move it backwards a bit ( max 28%MAC !)

Frank

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RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 1:33:57 PM   
abufletcher



Posts: 5172
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From: Zentsuji, JAPAN
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quote:

ORIGINAL: Frank Sopwith
Chris and I calculated the CG for the Snipe on 96mm from the leading edge of the top wing.
This is 25% MAC and is a good location to start with


I'd be happy with this...but it's a bit different from the number I came up with (which could of course be wrong). I checked around online and found several sites stating that the CG on a equal cord biplane is based on the "total wing cord," i.e. the cord measured from the top wing LE to the bottom wing TE. I measured this distance on the model by placing a t-square under one of the lower wing ribs and then using a large triangle to touch the upper wing LE. The measurement I got was 32.6cm. The same measurement on the plan comes to 32.3cm.

So using the total wing cord method that gives the following values:


For 32.6cm measured on the model:

25% = 8.15
28% = 9.128
30% = 9.78

For 32.3 measured on the plans:

25% = 8.075
28% = 9.044
305 = 9.69

So either there is some discrepancy in our measurements or you guys are using a slightly different calculation method. I did find one formula that also includes the area of the stabilizer/elevator and a factor for the rudder design but that required more math!

Anyway, could you tell me how you came up with 96mm? Not that I don't trust you but...

(in reply to Frank Sopwith)
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RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 2:51:15 PM   
TFF


 

Posts: 741
Joined: 10/30/2006
From: Memphis, TN, USA
Status: offline 		With your measurements, I think you have proven that on something as complicated as this plane there will be differences from plan to plane and that the first flight CG will not be the final CG. That is why I would err on the side of nose heavy. And something I have noticed, but not scientific, is that the CG on same platform wings, with normal amounts of stagger, seems to be at the bottom wings leading edge or so.

(in reply to abufletcher)
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RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 2:56:18 PM   
abufletcher



Posts: 5172
Joined: 2/13/2004
From: Zentsuji, JAPAN
Status: offline 		At Frank's calculated CG for 25% MAC I only need about 280 grams (about 10oz) to balance! This is with the dummy in place. That would put to total weight at just under 4kg.

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       Post #: 1538

RE: CD ScaleDesigns Sopwith Snipe ... - 5/30/2008 2:58:19 PM   
TFF


 

Posts: 741
Joined: 10/30/2006
From: Memphis, TN, USA
Status: offline 		Also in the Warbirds section someone was complaining that they did not want to add a lot of lead to balance their spitfire so they were going to give up on it. I figured out that the actual Spitfire's engine was around 50% of the total weight of the plane, so we are at a disadvantage from the start we have to add weight.

(in reply to TFF)