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wood / metal

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Jim Williams

Member
Joined
Sep 17, 2005
Messages
14
Location
Atlanta ga.
The Swiss Colibri MB2 is a wood design with a VW engine. I would like to make a all metal version of the aircraft. The empty weight of the wood Colibri is about 390 lbs, wing span 20 ft with 89 sqft wing area, lenght 15 ft. My Question is , is it possible to build an all metal Colibri and keep the empty weight close to the wood empty 390 lbs weight?
Why not make it of wood? Well I have 45 years experience working with aluminum and none with wood. My shop is set up for aluminum work. What do you think?

Jim W
 

orion

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Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
Yes, there is no reason that you couldn't make out of metal and aluminum just might come out lighter than the wood. However, this is not a simple substituting of metal for wood - you'll have to undertake (or have someone do it for you) the redesign of the structural aspects of the airframe in order to make sure that you still end up with a safe airplane.
 

Jim Williams

Member
Joined
Sep 17, 2005
Messages
14
Location
Atlanta ga.
Thanks Orion for your reply. I have another question. In L. Pazmanys book "Light Airplane Design" I think I read to try to keep the aircraft C/G Between 15% and 30% of the MAC. The maximum forward C/G would be the aircraft at it's lightest weight, light pilot and low on fuel and the maximum Aft C/G would be the aircraft at it's gross weight with a big pilot full fuel. Am I right? Some times the maximum Fwd and Aft C/G will be only a few inchs apart. Am I right on this too?

Jim W
 

orion

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Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
The allowable CG limits of any airplane are not that arbitrary. Although the 15% and 30% are ballpark figures, the numbers change as a function of airplane, its flight surfaces and its control authority. The forward CG is set by the horizontal tail's ability to bring the airplane to full stall in ground effect (full flare with flaps for landing). The aft limit is set by the airplane's stability criteria - the aft limit is generally set between 5% and 10% of the MAC in front of the airplane's Neutral Point (usually the stick free Neutral Point).

The forward and aft limits are defined by the aircraft's designer. Any configurational changes you might make to the design will require a new stability analysis. If you do not change the layout (wing shape, tail configuration, etc.) and only change the material, then you only have to stay within the original designer's limits.

In order to do that, yes, you will have to look at the balance of the airframe with all the variations you mentioned above. And yes, sometimes the CG shift as a function of loading does not change all that much, especially on a small single seat airplane. Just make sure that the shift stays within the design allowable limit.
 
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