Cargo doors for dedicated freighters - aft or fwd of the wing

[CE-HAM]

CG with front/rear doors

I may have misunderstood this, but in the concept phase of the A320 P2F they decided to put the cargo door behind the wing, because the added structural weight of the door mechanism was beneficial for the angle of attack in cruise. Usually, the tail must be aerodynamically pushed down a little. This was then done by the added weight in the back - not the front.

[DaveReidUK]

Originally Posted by CE-HAM

Usually, the tail must be aerodynamically pushed down a little. This was then done by the added weight in the back - not the front.

If it was as easy as that, why didn't they build the A320 with the wing a bit further forward from the outset ?

[CE-HAM]

Over-simplified

It's not easy. Moving wings or control surfaces forward and aft has many more consequences than moving "dead weight" as a door mechanism. For pax ops, the wing was/is in its optimum position. Now Airbus wanted to add some weight (the door) and it was better to put it aft of the wings and not forward. Thus the aircraft can be trimmed less nose-heavy and drag is reduced. In case of the B737 freighter the decision was different which proves that there are many factors more factors going into these decisions than I know of.

[Ex Cargo Clown]

Easy on a 74F, unload the rear MD and load Cpt 1&2 LD. Helped being able to put Q6 pallets through the nose as well.

[DaveReidUK]

Originally Posted by CE-HAM

It's not easy. Moving wings or control surfaces forward and aft has many more consequences than moving "dead weight" as a door mechanism. For pax ops, the wing was/is in its optimum position. Now Airbus wanted to add some weight (the door) and it was better to put it aft of the wings and not forward. Thus the aircraft can be trimmed less nose-heavy and drag is reduced.

Now you've lost me completely, I'm afraid.

If we agree that the 4,500 A320s built to date had their wing correctly positioned(!), then adding the additional weight of a cargo door either forward or aft of the wing would require compensating changes in the weight distribution on the other side of the wing in order to maintain the CofG in an acceptable range relative to MAC.

That's no different to what happened, for example, when they stretched the A320 into the A321 by inserting fuselage plugs both forward and aft of the wing - in fact the forward plug is around 50% longer than the aft one.

So it's hard to see how there would be any specific aerodynamic advantages resulting from putting an A320 cargo door aft, rather than forward of the wing.

[Lancelot de boyles]

If it's anything at all like a B737 freighter, the forward cargo door plus a bulkhead tends to bias to a forward CG in a vast majority of loads. Very forward, in some cases.
Flown empty, or with light loads, or with a container load each with similar masses, a ballast container aft is required to achieve an in trim configuration.
With a cargo net instead of a bulkhead, the forward CG is alleviated.
Add the longer fuselage of a -400, and you start to encounter greater considerations of fuselage bending (Banana).

[Rwy in Sight]

Just a question some one with very limited experience on cargo ops. How do you secure ballast in short notice? I have read once in PPRuNe that sometime in the past bricks were used for ballast but I don't remember how they were procured.

[FlyingStone]

Airbus had the "advantage" of not considering A319F when positioning the doors, as they only planned A320 and A321 conversions.

I don't think it's possible to put the cargo door aft of the wing on the 737-300, let alone -200 where everything started.

[Owain Glyndwr]

When I googled A320 P2F I found the following extract from "Air Cargo World":

>The technical challenge for EFW was resolving the issue of where to put the cargo door without aggravating the A320’s forward centre of gravity issue.

Aircraft stability involves balancing a number of factors including the centre of gravity (CG) and centre of pressure.

EFW’s head of aircraft conversion sales, Andreas Mayer, spoke to ACW and says: “We had the problem of a design decision to put a cargo door in the back, the A320 has a forward centre of gravity issue and putting structural reinforcement in the rear we would have cured that problem, but we created new problems by putting in a door at the rear because the structure was very weak at that area and we had to reinforce to a much greater extent and we were required to do a full fatigue test. Then we realised the aircraft is going into an unstable state, flutter, and it created…new technical challenges.”<

>Mayer explains: “We now have the door in the front. Our cargo door location is at two different locations for the A320 and A321, so we do not have the door in the same position.

“On the A321 it is further back due to the forward CG issue. We are trying to put the door as far back as possible without reducing loading clearance for the engine nacelle.”<

Does anyone know what the forward CG issue was/is? The text suggests it may be specific to a freighter version

[dixi188]

Maybe there is a lot more galley and toilet weight at the rear on a pax A320 that causes the fwd. C of G issue when it is removed.

[Owain Glyndwr]

Yes, I understand that removing the furnishings could move the empty CG forward, but why would that be a problem?

[barit1]

Pardon my dandruff while I scratch my head.

Seems to me the job of the cargo dispatcher is to plan the pallet etc. loading to optimize CG for the loaded aircraft. Given a random selection of heavy vs light pallets, he's got some pretty powerful levers to pull. And a loadmaster-type computer package to do 80-90% of the work.

I realize that a positioning flight gives him fewer tools - so be it. But what am I missing?

[Vessbot]

Originally Posted by DaveReidUK

Now you've lost me completely, I'm afraid.

If we agree that the 4,500 A320s built to date had their wing correctly positioned(!),

That's not a necessarily true assumption... at least in the sense that there are no CG problems. Such problems are common.

For example, the CRJ 200 at full or nearly full loads is often outside the forward limit, requiring the movement of passengers backwards. If it's totally full so that no one can move, then you have to add ballast. Now if a 190 pounds jumpseater wants to get on, he's really a 400+ pound weight addition.

So it's hard to see how there would be any specific aerodynamic advantages resulting from putting an A320 cargo door aft, rather than forward of the wing.

Do you see it now? If you're more commonly bumping against one end of the allowable range, and you're about to add weight, naturally it's better to add it closer to the other end.

[DaveReidUK]

On converted, stretched bizjets, I can believe.

But on a clean-sheet airliner design? Did Airbus really get it that wrong?

[Vessbot]

Well I'm talking about the general case, I can't speak specifically to the Airbus. It could very well be perfect. And you're right that the CRJ isn't the best example, for the reason you said. But it shouldn't take too much imagination to consider that even a clean sheet design could fix the wing in place at a stage where it would take too much re-engineering to move it, and subsequently something else happens that moves the CG.

[Ex Cargo Clown]

Don't know if there any loadies/drivers on here but doesn't the MD11F have some serious ground CoG issues: