For some reason I had dropped out of the system. So I'm a new member again as Collin and Steve fixed the problem fast

But for the problem; As much as I've been dealing with RF4D's I have actually never done or even seen weighing of one. I am a certified guy to do it, and thus very interested as I should be able to do it next year. However, a friend called me yesterday as he was going to do the same thing for another RF4D.

The instructions in the manual are very limited. So, how should the procedure be performed in correct way with RF4D. We have the MTOW (or is it MTOM nowadays...), and the C of G limits. But I can't even find the location of datum, not to mention when the airplane is considered to be level. Is it the fuselage, or main rib or what? Are there even any instructions available? I know this can be done with the info available (after finding out the datum), but I would be interested in doing it the way it was supposed to be done... And how about the outriggers and balance?

-A-

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Somewhere in the manual we are told the datum is the wing leading-edge where it contacts the fuselage (actually there's a gap!) and the longerons on either side of the cockpit give us the level.

We put pre-weighed chocks on a single, central scales in front of and behind the main wheel, then lifted up the tailwheel on to a box with series of blocks on it and another scales until the fuselage was level. During this time, the wing tilts alarmingly!

Then, I got one friend to hold one wing-tip exactly level, between two fingers, while another read the readings on the scales.

We first did this with no fuel, then with me in the cockpit and no fuel, then with me in it and full fuel (since those two are the limiting cases).

However, none of this makes allowance for the retracted wheel (it moves forwards). I can only presume that all the manual's limits are with the wheel down, since it is not mentioned.

Since the C of G of both aeroplanes was in the mid range, I just theft things there.

Good luck Antti, and enjoy the process!

Yours, Bob

I think I have to read the manual again, then So it doesn't seem to be an easy task to do. I can remember the wing/fuselage joint and have to guess if it is supposed to be a straight line from leading edge to fuselage or a line parallel to leading edge... Then I need to unscrew the fairing and find the actual line. Interesting.

It might also be possible to add two more scales under both wingtips and thus balance the whole system without two fingers. For some reason I believe it's pretty hard to keep it level and stable. I have done the weighing procedure with a couple of gliders but don't remember exactly how the wings were kept level.

EDIT: I read the manual and now have to think it again. It actually says:

-reference: upper stringer of fuselage horizontally (so, it's not the cockpit longeron after all)
-datum: wing leading edge root chord (this is easier, too. However needs to be determined w/o the fairing)

The thing with gear is ablolutely true. Should the bird actually be weighed gear up for accuracy if the c of g with pilot needs to be calculated...?

Have you determined the moment of pilot, Bob? How about fuel or baggage? Would be interesting to do some maths, but I am not able to do the weighing at the moment just for fun

Exciting, indeed!

-A-

[Edit by andy1 on Thursday, August 7, 2008 @ 02:00 PM]

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If someone of you wish the weighing form, just has to e.mail me and I'll send it.

cheers

Eugenio

If you think about it for a moment, you will realise that the upper fuselage stringer IS the cockpit longeron (the bit the canopy rests on) because there is no other, higher stringer/longeron.

I removed a wing root fairing and took a straight-edge along the front of the inboard leading-edge and marked a line on the fuselage where this intersects, making that my datum.

I think you will find elsewhere on this site, my calculations for the moment/arm of fuel and pilot, derived by weighing the aircraft with and without fuel and with and without pilot (me), and already knowing the weight of the pilot and fuel. I did not calculate the baggage arm.

I can only presume that, since it would be very difficult to weigh the aeroplane with the wheel retracted, all the limits in the manual are with the wheel down.

Have fun with your weighing.

Yours, Bob

Thanks for the info. You are right with the longeron, Bob. Sorry, I got it wrong. I have actually heard that someone has thought that upmost longeron is the rear fuselage line and weighed an RF4D by leveling that. It of course lifts the plane to a very abnormal position. But great. Now I know all the details and try to figure out the limits for pilot, too.

-A-

[Edit by andy1 on Saturday, August 9, 2008 @ 02:29 PM]

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But then, I added the given reactions and arms into my table and got horrible readings. By those results it would be allowable to fly by a max 70 kg pilot with almost full tank or the c of g would be over its rear limit. And no baggage, of course. (You had actually counted the arm of baggage, too, Bob)

So, I had to think the given arms had to be incorrect. I changed them one by one to have the same readings as the weighing had given. (Of those calculations I got the same results).

I have now come to conclusion that arm of pilot is at 0,627 m aft of datum and arm of fuel is at 0,322 m front of datum. This is of course too accurate to be true and absolute, but in theory, anyway. If we move the 80 kg by almost 28 cm forward and the 27 kg almost 4 cm forward, it won't surprise that the center of gravity moves quite a lot forward and thus being in the correct area.

By these rates it is totally impossible to get the c of g to go out in front and needs a 110 kg pilot with full baggage and minimum (8 l) of fuel to get it out in rear.

Correct me if I'm wrong!

The copy of Excel table looks ugly here, but try to find out where I made a mistake

Mass (kg) Dist from datum (m) Moment (kgm)
Main gear 259,8 0,068 17,6664
Tail wheel 25,6 4,1 104,96
Fuel 8 -0,322 -2,576 -0,2846
Pilot 110 0,627 68,97 0,9059
Baggage 10 1,357 13,57
Sum 413,4 202,5904
Center of Gravity 0,4901

Limits
Pilot 50-109 kg
Baggage 10 kg
C of G 300-490 mm
MTOW 390 kg

-A-

[Edit by andy1 on Saturday, August 9, 2008 @ 06:04 PM]

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If there is any discrepancy between your figures and mine, it is yours that will be correct, so ignore mine. My calculations did not use a computer or calculator, just my fingers, a pencil and a scrap of paper in the dark corner of a hangar, so it is very possible there were many mistakes. I never was any good with numbers!

Yours, Bob

You have done so much calculations that I have to trust you know very very much about these things - and Fourniers, of course But I got these results based on your data and I believe thay are accurate enough for the interest I had on the movement of center of gravity with different loads. Perhaps someone has the correct measures or I will try to calculate them next year when I have to do the actual procedure.

However, this results shows that an RF4D is easy to load as there are no "difficult areas" when staying within limits. Planes like G109 are very difficult when the pilot's arm is next to nothing - 9 cm if i remember correct - and big fuel tank is located behind the crew, as is the baggage, too. I once made a table for different loads in G109 and there are quite a big surprises.

-A-

[Edit by andy1 on Sunday, August 10, 2008 @ 06:01 AM]

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I am no good at sums (math) so I have little confidence in the following. Can anybody help?

When I weighed HDO (Aussie Blue) I got these figures:

Prepared for service, with empty fuel tank, but with a normally dressed pilot aboard:
Weights Mainwheel 328.8 kg moment = 22,358.4 kg mm.
Tailwheel 36.7 kg, moment = 150,470 kg mm.
Total 365.5 kg, moment = 172,828.
Thus: C of G = 472.85 mm (33.8%) aft of datum (limits are 300 to 490 mm, 21.5% to 35%).

From my weight measurements, I figured:
Weight of full fuel = 26.3 kg = 37 lit (manual quotes 38 lit)
Arm of Fuel = -284.6 mm

Then I calcluated that with aerobatic fuel (15 litres)
weight should be 376.2 kg (max aerobatic weight is 370 kg)
C of G is at 387.9 mm AOD (27.7% MAC) (limits are still 300 to 490 mm).

Does that seem right?

Now...

I have camcorder and battery pack. They weigh 700 grammes, and they fit neatly under the fin fairing.
They get info from a small 'lipstick cam' with a wide-angle lens I've fitted to the top of the fin to film my antics this weekend.
I had been going to put the camcorder in the locker, with a long lead through the fuselage, but that is difficult to arrange without the possibility of fouling the controls under +6/-3g.

By my calcluations, even with the camcorder and battery pack right back there, the aeroplane is still within its permissible C of G limits before I fly, and after I've used all the fuel.

Does anybody agree?

Yours, Bob

According to my calcs for HDO, you're fine even with empty tank and camcorder as long as you do not carry any baggage behind the seat. I have an Excel file with all known measurements (VH-TKD, VH-HDO, G-AWGN) which I can e-mail you offline.

To all: Following Bob's earlier comments, this is what I have done for VH-TKD:

1. Set up glider as per Eugenio's comments. You do not have to remove the wing root fairings, since the leading edge is straight, you can extrapolate exact position of datum on the ground.

2. Weigh empty with oil.

3. Weight empty with fuel. From this you can derive the fuel weight and fuel arm. The fuel arm is negative (see comment below).

4. Weigh with pilot. From this you can derive pilot weight and pilot arm.

5. Weigh with 10 kg of baggage. From this you can derive the baggage arm.

For normal pilots (up to 85 kg), the limiting scenario is no fuel and full baggage. Whilst this is unlikely due to reserve fuel, it could happen…

For VH-TKD I have to limit baggage to 6 kg, but this is mainly due to Australian regulations, according to which the rear limit for CoG is 481 mm. Kinda safety factor for the safety factor…

BR

Tim

I just found this old photo of us weighing HDO five years ago.

It looks really scary in retrospect.

Yours, Bob

(sorry, Bob)