Author Topic: JRollon CRJ-200  (Read 1983 times)

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Offline Adrian Chitan

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JRollon CRJ-200
« on: October 28, 2014, 03:31:32 PM »
Hello hello, guys!



Just wanted to point out an excellent plane that'll make you read read and again read even before getting into the flight deck. I know it is a bit "older", but it is one of the best simulator planes out there and I've had my sights set on it for quite a while now. In this description I'll also present the procedures to calculate the takeoff trimm setting for this plane because it is so real life-like! And maybe this will help some of the new comers into flight and aviation understand a little bit more about the world of aviation. Needles to say this is not an advertising material because if I point somebody to a B737-500 landing cockpit video on youtube, I am definitely not expecting him/she to buy a B737.

Ok, so JRollon is a guy named Javier Rollon who's spending "more time in the hangar creating superb models than actually flying". He created the Jetstream 32 and the CRJ-200. Both are extremely complex studies (usually JRollon calls in real life pilots to test or tell him about the usage and the workings of different systems on a plane).

The Bombardier CRJ-200 is a 50 pax airplane used for regional/domestic flights. It has a maximum range of around 1644 nm and a usual cruise Mach number of .74. It is a rear mounted engines plane so you will notice a slight difference in handling from the normal under-wing engines models that you are used to. I felt a longer duration wobble of the airplane while in heavy flights because of the smaller distance between the center of lift and the center of gravity and the fact that the engines are closer together so the momentum of the stabilizing force given by an under-wing configuration is negligible or nonexistent (as in real life). That's why these models are more prone to the dutch-roll.

Now let's see how real pilots calculate the takeoff trim setting in less-automated planes (this method applies to lots of regional jets and smaller aircraft).

1)
So, first thing's first, you have to know the number of passengers (number of tickets sold :D) and the weight of the bags (weighted on the airport scale at check-in). As with any airline, the weight of the passengers is unknown but estimates are used. In the case of the CRJ-200 the manufacturer is considering 80 kgs (176 lbs) per adult passenger and 40 kgs (88 lbs) per child. Let's say for our flight we have 33 pax so this adds up to:
people - 80 kg x 33 pass = 2640 kgs (5820 lbs)
bags - 429 kgs (946 lbs)
---------------------------
Traffic Load - 3069 kgs (6766 lbs)

The plane has a dry operating weight of 13835 kgs (30900 lbs) so we have a zero fuwl weight of 16904 kgs (37666 lbs). For this flight we will need about 3175 kgs (7000 lbs) of fuel. Not considering the APU burn-off or the taxi burn-off we arrive at a takeoff weight of 20079 kgs (44666 lbs).

2)
The next step is to find the position of the center of gravity. In aviation, this is usually calculated as percent of the mean aerodynamic chord (%MAC). The MAC is a 2-D transformation of the actual wing into a single rectangular shape. So, practically you transform the plane into a rectangular board and you then pin-point the position of the actual center of gravity on this board. This was introduced into aviation because it standardizes the center of gravity position (3 meters from the nose on a B747 is different than on a ATR-72, but 7%MAC on both is the same thing from an aerodynamic point of view).

The center of gravity position is computed by using manufacturer given indexes for every weight position in the plane. These indexes tell the pilot how much a certain weight in a certain position on the plane pull or push the center of gravity towards the start or the end of the MAC. The standardization of these indexes is possible because the manufacturer already knows everything about the plane :D and that the weights are standardized (mean passenger weight) or measured (cargo). So it is just a problem of force momenta translated already into indexes to ease the life of the pilot.

To get the indexes in our example we have to also know the actual position of every passenger. This is taken care of by the flight manifest. The CRJ-200 was divided by the manufacturer in 4 passenger areas: A (first 4 rows), B (next 3 rows), C (next 3 rows) and D (last 3 rows). So let's say we have 3 passengers in section A, 12 passengers in B, 12 in C and 6 in D. Looking at the table (pilot's handbook) we get these indexes:
A: 4.9 (this would pull the most on the CG to the front of the plane but we only have 3 passengers here)
B: 10.2 (this is a full section but it doesn't pull that much because it is almost over the wing - the empty CG)
C: 1.1 (this pulls the less on the CG because it is over the wing and over the normal empty CG position)
D: 3.7 (this is backward and as we'll see it pulls back on the CG, though it is still quite close to the empty CG - don't forget this is a back-mounted engines plane)

Next we'll get the cargo index because the cargo hold also pulls on the CG. This depends on the weight of the cargo (exactly measured by the scales in the airport). We have 429 kgs of luggage. We find a cargo index of 6.5 at 400 kgs in the pilot's handbook (we round the weight lower to account for the APU and taxi burn-off ;)).

There is also a fuel index also depending on the fuel weight. At 7000 lbs of fuel we find an index of 6.3 in the tables.

Now we calculate something called Loaded Index at Zero Fuel Weight (LIZFW) by using:
A+B+C-36.84-D-Cargo = -30.84 (the minus sign is irrelevant so we'll use 30.84 from now on)

I guess I have to explain the 36.84. This is the actual intrinsic index of the empty plane. It is the actual index number that will give the position of the empty CG on the MAC. As you can see it pulls back hard on the CG because the plane has the engines quite far towards the back. Of course it was calculated by the manufacturer and it is a constant in CG calculus.

The only thing we have to add to the index is the fuel pull on the CG:
LIZFW-fuel = 24.54

3)
Now comes the graphic part. Aviation works a lot with performance plots so here is what the CRJ-200 manufacturer gave us:



We know the takeoff weight (right side in kgs): 20.079 thousand kgs
We also know the index we've obtained: 24.54

Starting from the intersection of these two values and following the parallel lines towards the %MAC on the upper axis we arrive at a %MAC of 12.

4)
To get the takeoff trim setting we use a final manufacturer table given in the pilot's handbook and we arrive at a trim setting of 7.7.

And that's it. I love it when things get complicated especially when that's how real life procedures are :D.

While the procedure is not complicated, as in real life you just have to follow manufacturer's tables and plots, I hope the explanation is helpful. For now, the only sim plane I've found that doesn't calculate the %MAC automatically or the actual T/O trim is this superb study from JRollon, that's why I've put this in the X-Plane folder.

Cheers
« Last Edit: October 29, 2014, 07:07:27 AM by Adrian Chitan »

 

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