Weight and balance Hippert's plane

buck toenges · 15 October 2009, 07:24 PM

Quote:

Originally Posted by jumpinjan

Buck,
I'll use your dimensions and get a number.
One thing, when you said "fully loaded" does that include the fuel & oil in the tanks? If so, then those are included in the axle weight readings. If not, then I need the moment arm of the oil tank.
Jan

fuel and oil are included in the axel readings. Oil tank is located 62 inches from front hub.

jumpinjan · 15 October 2009, 07:46 PM

Revised numbers:
CG calculated = 70.8" (with fluids & pilot)
CP calculated = 57.3" (at 23% of virtual monoplane MAC)
difference = 13.5" (tail heavy)
total gross weight = 1856lbs

buck toenges · 15 October 2009, 07:48 PM

Question of figuring gap? I used the height of the vertical portion of the "N" strut from the top of the bottom wing to the bottom of the top wing. In Jeff's diagram it looked like he used the bottom of the lower wing to the mid point of the top wing. What is the correct way?

On the width of the lower wing I measured the leading edge to the trailing edge of a rib. I made the assumption that the lower wing was pretty much "square" in that there wasn't much or any difference between rib lengths.

On the top wing I measured from the leading edge to the trailing edge of the rib closet to the inside edge of aileron as the "average" width.

I was going to re-weigh the plane with additional weights in the nose but we didn't have the scales, they were loaned out for the night.

jumpinjan · 15 October 2009, 08:06 PM

I suggest to measure (using a long straight edge on each bottom surface) from bottom surface of the bottom wing, to the bottom surface of the top wing at the root. On the D7 wings, the thickness taper out to the tips.
(I think if you look closer, that's what Jeff did in his diagram)

Jan

baldeagle · 15 October 2009, 08:12 PM

Well, here's what I get:

Datum is lower wing leading edge

MAC is 57.76" beginning at 15.25" fwd of datum, using 61% of gap as per formula for figuring MAC for biplanes

Main wheels are 7" fwd of datum, tailwheel is 203" aft of datum

Main wheels 1638 lbs. x -7 equals -11,466 moment arm
Tailwheel 94 lbs. x 203 equals +19,082 moment arm

Total 1732 lbs 7,616

CG is plus 4.4" from datum, 19.65" back on MAC, 34% MAC

Buck, those weights for main wheels and tailwheel are without pilot but with fuel and oil?

If so add 200 lbs. at plus 54" aft of datum +10,800 moment arm

Now 18,416 total moment (10,800 plus 7,617) divided by total weight 1,932 and CG is +9.53" from datum, 24.78" back on MAC, 42.9% MAC

Somebody want to check the math?
I don't think gap is really relevant since the CG is a fore and aft measurement so stagger is really the more important number, the formula just shows that MAC is 61% of gap for the D.VII, MAC is then figured using 61% of the stagger both at the front and rear of the wings. Whether the gap is 50" or 60" the MAC will be in the same place fore and aft relative to the datum if the stagger number isn't changed.

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baldeagle · 15 October 2009, 08:33 PM

And if you put the Rousseau numbers in to that computed MAC they match within 1%, so I think I would shoot to match their max aft CG of 38% or 6.7" (thiers is 6.2" for 38%, your numbers for stagger and etc must be slightly different) aft of datum (lower wing leading edge).

So, if you put 100 lbs of lead 50" forward of datum (12" behind the front of the prop hub, if doable, front of engine bearers?) then:

2.032 lbs 13,416 total moment CG is 6.6" aft of datum, 37.8% MAC

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jumpinjan · 16 October 2009, 01:50 PM

Andrew,
I didn't find any computing errors.
(although my gross weight value should be 1,932lbs too)
Jan

buck toenges · 22 October 2009, 06:51 PM

Here is what we got tonight in weight and balance

No weight added: 1912lbs full weight with pilot. cg=70.5" from front prop hub (tail weight 94lb, left wheel 824lb, right wheel 814lb)

1) 97lbs added: 2009lbs full weight with pilot. cg=67.36" from front of prop hub (tail weight 78lb, left wheel 884lb, right wheel 867lb)

2) 157lbs added: 2066lbs full weight with pilot. cg=66" from front of prop hub or 4" aft of leading edge of lower wing. (tail weight 62lb, left wheel 918lb, right wheel 906lb)

We are going to do the following before we attack the prop size issue.

Roger feels we should get the weight on the plane and then check static rpm and do some runway runs to see how plane feels before altering prop.

Von Writter · 22 October 2009, 09:19 PM

Does anyone have the CG of the Mercedes engine its self? If it was known, one could calculate the placment of a replacment engine.

RobW · 13 November 2009, 11:08 AM

Quote:

Originally Posted by Von Writter

Does anyone have the CG of the Mercedes engine its self? If it was known, one could calculate the placment of a replacment engine.

I came across this book today. One may be able to determine the approx cg of a 180 Mercedes engine from the data in this book.

Textbook of aero engines - Google Books

Rob