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Phoenix Tiger Moth 40

CG problems

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barryt13/12/2017 11:22:18
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36 forum posts
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My electric Phoenix Tiger Moth ARF weighs just over 6lbs with a 5000mah battery. This is the manufacturers planned weight. However when I try to set up the CG according to the supplied drawing, the model is extremely tail heavy. When I looked at the drawing again I realised it is contradictory. The plan view shows the CG 45mm back from the leading edge of the (top) wing. The side view shows the CG 45mm back from the leading edge which coincides with the upper mounting of the front strut. Given that the wing is swept back about 50mm at that point there are two CG recommendations about 50mm apart!

Am I doing something wrong? Where should the CG be?

20171213_121828_hdr_resized.jpg20171213_125549_hdr_resized.jpg

barryt13/12/2017 11:56:37
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36 forum posts
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This picture may help.20171213_125620_hdr_resized.jpg

Geoff Sleath13/12/2017 12:17:39
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2475 forum posts
187 photos

I'm not familiar with the Phoenix Tiger Moth but from the size of the battery I guess it's about the same size as my 58" (1.5 metres) DB Moth. I've just checked the drawing and CoG is 4" (100mm) back from the L/E of the top wing which puts just about 20mm forward of the front cockpit opening. On full size Tiger Moths the CoG was reckoned to be over the front compass which is mounted at the front of the cockpit so a bit further back than the DB.

I had to put some lead in my electric version but I've removed some of it and it flies very well with CoG a bit further back. Tim Hooper calculated the CoG for his kit (the Chinese one sold by HK as well as others) and he has his further back still. He detailed his build here at RCGroups, the US forum where several were being built.

Geoff

barryt13/12/2017 14:14:47
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36 forum posts
11 photos

That sounds much more like it Geoff. Yes the model has a 56" span. I'm not sure how 'scale' it is, but with the battery mounted half way through the firewall it hangs with the CG 100mm back from the leading edge of the top wing. I'm feeling more comfortable now. Thanks for responding.

Is this model available in the UK?

Jim Carss13/12/2017 14:21:39
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1965 forum posts
34 photos

Barry

This is c/g point on my 66" moth

img_3433.jpg

cymaz13/12/2017 17:40:11
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7483 forum posts
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With a biplane, take the total span of both wings. So, measure from top wing LE to bottom wing TE. Then do 25% of that. It’s a rough rule of thumb.

Here should help

Tim Hooper13/12/2017 18:15:43
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2815 forum posts
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Posted by cymaz on 13/12/2017 17:40:11:

With a biplane, take the total span of both wings. So, measure from top wing LE to bottom wing TE. Then do 25% of that. It’s a rough rule of thumb.

Here should help

I'd be very wary of that method with an airframe with swept back wings, such as the Tiger.

As Geoff has said, I've built the HK Tiggie, which also sites the CG as being on the leading edge of the lower wing. This looked very wrong to me, so I did the calcs and ended up with a spot that coincided with the point that the rear cabane struts meet the upper wing.

Pete B - Moderator13/12/2017 22:01:49
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I have a Phoenix Tiger Moth and she flies fine with the CG just in front of the upper rear cabane strut - 100mm from the leading edge.

The ailerons have very little authority so I tend to steer on rudder with the ailerons mixed in.

Pete

onetenor14/12/2017 06:08:26
1570 forum posts

Barry T those two positions are the same at 45mm back from the leading edge. You have even written that in your script..Are you asking is it too far forward?

barryt14/12/2017 06:32:56
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36 forum posts
11 photos

Hi onetenor. Yes, I think it way too forward, hence my model seems tail heavy. From the other replies and Pete B's practical example,the CG should be about 100mm back from the leading edge of the top wing. When I hang my model it naturally balances at around the 100mm mark so that is what I am going to use, for starters anyway. The supplied drawing must be just wrong.

Thanks everyone for your contributions. Much appreciated.

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