Talking CoG

[bomber]

for iyy, it's more of a guess, I assume, nobody calculated that plane in parts (and whether we have enough data to do so) and izz is the same problem.

There's really not that much to a plane that we don't know or can't make a guess at.

Some planes we know the empty weight as well as the weight of lifting parts (wings to you and me)... if you know the wings airfoil you could sectionalise the wing and work out the volume of each section, and from that the weight of each section... and you could use the same techneque to find the weight of v-stab and h-stab.... and therefore fuselage.
We know the weight of a prop and it's engine.. estimates could be made for the seats, controls etc... and before long you'd have all of the plane estimated far better than a wild ase guess that is the interia's at present.

Simon

[jwocky]

Well, but what is the volume and the weight of the tail? Horizontal and vertical stabilizers? Compared to the weight of the nose. And we have to be clear, it can't be calculated exactly on the dry because some two, three or four digits behind the point, it would already change if the pilot has a donut more .

Then there are some old tricks. We build our planes usually with a CoG that actually balances them. But if you look at fighters from WWII for example or certain acrobatics planes, they are intentionally built with a CoG further forward to make them just a little bit tail-heavier, that allows to change pitch faster. So you can't really rely on being it always the absolute sweet balance point.

Thinking about that, I didn't look, but did you implement ground effect in the pups? Because that changes the behaviour in the fist seconds of a flight rapidly and you get a little bit more freedom to play with the CoG which then pays out for you over the whole flight.

[bomber]

We always have a rough idea of where the CoG should be simply by looking at the plane and understanding where the pilot sit,where the fuel tanks are and if military where the wing mounted ammunition boxes are.

As for WWII planes typically british planes comes with a mass ballance chart, this nails the CoG down and defines the actuall weights of most components.

Construction density of parts of a plane are within reason identical from one plane to another within certain era's of plane manufacture.... We couldn't take a WWI bi-planes v-stab, knowing it weight and volume scale it up for a Boeing 747, as that'd be just nonsense. The materials used and manufactuering techneques are worlds apart. But we could take a second bi-plane with a 10% larger v-stab and take a reasonable punt that the weight would also be 10% greater.

So if you've got some data for a cessna 172, as the Beagle Pup is not be miles apart in material used and construction techneques you could extrapolate from one to the other.

And the cessna 172 comes with a lot of data.

Simon

[bomber]

If I apply these techneques using the same construction destiny as I used on the cessna 172 I end up with these values for the separate parts

Splitting the wing up in 8 sections each side.
7.93kg root
4.91kg
5.3kg
4.62kg
3.71kg
3.67kg
2.86kg
2.42kg tip

h-stab each one
4.62kg
v-stab
4.88kg

fuselage
fore 1.8kg
mid 64.3kg
aft 15.8kg

of course these also come with distances from the datum point so as to work out the CoG.

[jwocky]

Nice!

[bomber]

If I scale the cessna's fuel system weight down by a factor of 0.65 based on the fuel volume difference between the two planes.. and the undercarriage down by 0.75 which is the difference between the 2 planes weight and knock 1.5 pounds off the weight of the each pilot controls...

I get a tare weight of 985lbs....

If I then add 2 pilots at 70kg and their parachute at 7.3kg = 77.3kg or 170lbs each plus a 30lb luggage allowance each

total removable weight = 400lbs

The expedable weight is
24 UK gallons of fuel = 108.8ltr @ 0.72kg per ltr = 78.48kg or 174lbs
0.6 UK gallon Oil = 2.5ltr @ 0.85kg per ltr = 2.125kg or 4.68lbs

I get a total all up weight of 1563lbs

and the max load out for this plane is 1600lbs

[jwocky]

Well, you have the better data, but it looks good to me.

[jwocky]

Oh, second thought, what happens if you have only one pilot in there. As light as she is, the CoG should slightly move to the left.

[bomber]

<location name="CG" unit="IN">
<x> 78.00 </x>
<y> 0.00 </y>
<z> -6.00 </z>
</location>

this is the existing tare CoG

here's my calculated...

x = 80.0
y = 0.0
z = -6.0

my all up CoG is

x = 82.4
y = 0.0
z = 7.5

if someone would check what the CoG in the inertia properties section for me.. (I don't have a working copy)

Simon

[sanhozay]

Oh, second thought, what happens if you have only one pilot in there. As light as she is, the CoG should slightly move to the left.

Wow! Thanks for all the interest. I have visitors this weekend and haven't had time to digest all the messages yet.

As far as CoG goes, I thought the value entered for JSBSim was the base CoG and adjustments are made during the simulation for payload weights; so pilot, copilot, rear seat and fuel for the Pup. The computed CoG is shown in the jsbsim part of the property tree somewhere.

Browsing forum posts on the real aircraft, the Pup 100 is considered as practically a single seater, with climb rates labouring well under 500fpm and a long takeoff roll when someone occupies the right seat.