How to calculate the C of G

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Postby ozzie » Mon Feb 09, 2004 4:35 am

OK the attempt to post the information as it was, was a big failure. wrong button no doubt. so i will just type it here word for word and have a go at the small drawing. i do not know who prepared this but i assume that it was part of the data package that had to be supplied when the lazair was presented for approval in 1986 when certification and compliance became mandatory here in Australia. (good luck)


DETERMINATION OF CENTER OF GRAVITY
POSITION FROM FIRST PRINCIPLES

it is possible to determine whether the C OF G is in aproximatley the correct position simply by checking the load on the tail spreader bar T11. However, doing this makes a number of assumptions about the positions of the wheels in relationship to the wing mounting brackets.

The following procedure will allow you to determine the C of G position exactley in the relation to the wing. For normal flight characteristics it should be between 30 and 38 cms back from the leading edge.

1. With the pilot seated in the normal flying attitude, the fuel tank half full, and the aircraft fully rigged for flight, place the hook of a spring balance under F4 or a pair of bathroom scales under the tail spreader T11.

2. Raise the tail until the fuselage tube T23 is horizontal (spirit level). Record the weight shown on the balance
See note (1)

3 With the fuselage held in the same horizontal position, drop a plumb line from the front assembly(the one that pushes into the very front of the fuselage tube). Measure the horizontal distance from the plumb line to the centre axis of the axil tube- usually about 8-12cms.

4 Measure the total length of the fuselage tube T23 from F4 to the front fitting from which you suspend the plumb line (Normally close to 295.5 cms).

CALCULATION

A. Weight W = Wt of aircraft plus pilot

Add your own weight to the empty weight of the aircraft. you may assume 140lbs for the series 1 Lazair (63.5 kilos).

Then G = distance C of G is behind wheel axis calculated as:

Load measured at tail x distance x - y
OVER
Total Weight W

The answer should be a small number(about 0-5cms) if it is much more you may have a C of G problems.

B. Add distance calculated above to the distance (y) measured earlier.

C. Add a further 24cms to this total - this being the distance from the front wing mount to the leading edge of the wing.

D. The total in © above is the distance from the leading edge of the wing to the C of G.

Note 1
it should normally be necessary to lift the tail. If you find a tendency for the aircraft to sit nose down, ie you have to hold the tail DOWN, the Cof G is in FRONT of the axle, and you need to make some changes to the calculation:-

1. Calculate G as given above in Calculation (A). the answer is the distance the C of G is IN FRONT of the wheel.

2. SUBTRACT this distance G from the plumb line to the axle distance "Y".

3 To the number calculated in (3) above is the distance from the leading edge of the wing to the C of G.


hope this provides some entertainment while you are waiting for summer..
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Postby Chappy » Mon Feb 09, 2004 11:50 pm

I still have the letter I received from Peter Lawrence at Ultraflight dated June 17, 1981 in response to my inquiry concerning the location of the center of gravity on the Series I. I'll try to attach an image of it.

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Postby ozzie » Tue Feb 10, 2004 4:59 am

try again
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Postby Chappy » Wed Feb 11, 2004 1:24 am

Well, that didn't work so I'll just paraphrase the letter. The letter was in respose to a letter or call from me to Dale Kramer concerning the location of the Center of Gravity on my Series I, serial #25. I had gotten carried away and made a bunch of mods to it (reduction units on Pioneers, 44" pine props, fibreglass seat, wide axle assembly, luggage boot and 5 gal tank) and became concerned because no C of G info had been published by Ultraflight up to that point (June 1981) for me to do a weight and balance. The response was written by Peter Lawrence, Ultraflight's Chief Engineer, and he stated he had discussed my question with Dale. He listed the following details and checks:

1) With the aircraft on the ground (with the tail on the ground) and the pilot setting in it, you should have about 3 to 4 pounds on the tail.

2) (Assuming your reduction drives have not changed the thrust line), the aircraft should trim out to approximately level flight at normal cruise power with hands off.

Additionally, me supplied some numbers that he thought might help (Which is exactly what I was looking for!):

1) With the aircraft on the ground, the C of g will be about 1.2 inches aft of the centerline of the axle and about 22 inches above it.

2) With the aircraft in a normal flight attitude (boom slightly tail high) the C of G will be on a verticle line which cuts the chord line 13 1/2 inches aft of the leading edge. This puts it at about 23% of the chord at the root.

Remember, this letter was in reference to a Series I. Not all of these details will hold for later models, with the probable exception of the last item. I would think that the C of G on any Lazair would still be located at the same place on its wing's chord.

Incidentally , because my Lazair is so heavy (195 lbs) for a Series I, even with 55 lbs of static thrust from each of my re-drive Pioneers, climb isn't too teriffic. I've found that I can operate it with the C of G rather far aft to improve its climb rate. Taken too far, it will definately begin to get somewhat squirrelly feeling at Vx (best angle of climb). I can easily adjust my C of G by changing the tilt of my seat. It pivots at the bottom mount, and is attached near the top of the back with a turnbuckle.

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Postby ozzie » Fri Feb 13, 2004 4:50 pm

have re entered the C
of G notes and Diagram any comments
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Postby ozzie » Tue Feb 17, 2004 1:16 am

C of G info rentered at top of this page
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