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Calculate the CG on Old Time plane

Old 05-04-2015, 11:36 PM
  #1  
abborgogna
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Default Calculate the CG on Old Time plane

Anybody know how to calculate the CG on a under cambered wing with a lifting horizontal stabilizer? I am in dire need of some help.
Andy
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Old 05-05-2015, 12:09 AM
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fhhuber
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Gets very complicated SOMETIMES.

True lifting stab you can push the CG back well past 60% MAC. Before even attempting that we need photos of the aircraft.

Its far more common for a plane, such as the current Telemaster series, to have a lifting airfoil at the tail but to have significant "decalage" forcing the tail plane to actually be pushing down rather than lifting. Then you'd go back to the standard of appx 25% to 33% MAC of the main wing.
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Old 05-05-2015, 12:54 AM
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abborgogna
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Here is a picture. The plane is Sal Taibi's power house with a 36" wing from Bob Holdman. Bob and his helper both suggested a 50% CG starting point and yes the horizontal stab does lift. They also suggested a 1.5-2 degree incidence between the stab and the wing. It is currently built into the design go give the wing a 1.5 degree positive incident relative to the stab. This maybe backwards of what I really want. Yet that is how the plans produced the fuselage sides. Maybe I am reading the incidence meter incorrectly, it is a digital incidence meter and maybe it is really negative incidence from wing to stab.

As you can see I am confused and could use any help you provide. If you need to see a side view of the wing rib, or a close up of the horizontal stab let me know and I will post a picture in this thread.
Andy
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Old 05-05-2015, 01:27 AM
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fhhuber
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Go with the recommended CG.

I've pushed CG back to 40% on a plane with a flat plate tail plane. 50% is not extreme for a lifting tail. I've seen CG at 80% of main wing before for a FF model.

Expect the elevator to be sensitive compared to what you would expect with a more forward CG and stall recovery might be difficult so avoid stalls.
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Old 11-01-2018, 10:35 PM
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E-Challenged
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My electric Mambo has a lifting stab and I added elevators to it. The model does not go into a nose down attitude with high throttle. I had to increase the sized of the elevators to give enough elevator authority for a flaired landing, suspected that the thick lifting stab blanked out the narrow elevators at landing speeds.
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Old 02-01-2019, 11:15 PM
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smithdoor
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I used 30% no fuel and 25% with fuel

Dave

Originally Posted by fhhuber View Post
Gets very complicated SOMETIMES.

True lifting stab you can push the CG back well past 60% MAC. Before even attempting that we need photos of the aircraft.

Its far more common for a plane, such as the current Telemaster series, to have a lifting airfoil at the tail but to have significant "decalage" forcing the tail plane to actually be pushing down rather than lifting. Then you'd go back to the standard of appx 25% to 33% MAC of the main wing.
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Old 02-02-2019, 10:55 AM
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solentlife
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The design here in question has the old timer advantage of a banded tail as well as main wing. It was not uncommon to adjust CoG and then to shimy the tail to adjust the relation of incidence tail to wing. Cigarette packets were often to be seen with part of the card packet torn out !!

I think we have to also remember in those days - power was limited ... often really just power assisted glide. So incidence was usually greater than we use today. Also why lifting tails were popular. If we apply the CoG / figures of the old days - we would have a seriously climbing model today with todays power offerings unless we restricted ourselves to same power as old days. Therefore I suggest putting the CoG forward of the original to allow for the better power we have to use today. I would actually start as we would with most models today ... then work back if needed.

But not to forget the fact is - ALL horizontal surfaces provide lift otherwise lifting body modern fighters etc. wouldn't exist.

Nigel
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Old 02-03-2019, 11:39 PM
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quorneng
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abborgogna
You really can't do better than set it all up, including the CofG, as per the plan and then its just glide test from there. When you have achieved a stable glide by adjusting tail incidence you can start apply just a bit of power and see what the tendency is. Adjust the motor down and side thrust as appropriate. Time consuming and fiddly to do as well as risking damage to the plane but then it always was!
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