Top loading an aircraft vs bottom loading

Question

When an aircraft carries a load, it could be mounted on top:

This is the Myasishchev M-4 carrying the Energia rocket fuel tank.

This is the American side.

Other times it could be mounted below:

My questions is:

What determines the reason where to mount the payload?

Attempt to answer:

The initial thought is, it might be easier to mount a large payload on an aircraft on top, which does not have a purpose built ground clearance. There simply may not be enough space under the aircraft.

Another reason (a concern for the FA/18) is that the ordnance mounted on the hard points generally will have gravity applying a tensile force on them, and when the aircraft is accelerating against gravity, the total force will increase on the mount point.

A top mounted load, in contrast, will lead to a compressive force in this situation.

Tensile force often might lead to a failure of the hard points faster than a compressive force. All ordnance have a maximum allowed G-limit. (Aircraft accelerating in any other direction where gravity is normal to it will not have to worry about gravity summing up to change the total compressive/tensile force.)

What I am left wondering is, are there other (aerodynamics or control/stability related) reasons, why a load may be mounted on top or to the bottom of the aircraft? How does the answer change for low, slow flying UAV?

I saw this post on reddit, and it talks about bringing the prop plane closer to the horizontal plane where COG is for quadcopters. What are the general principles?

Please also point me to books or resources.

Answer 1

The payload is mounted on the bottom if possible for improved stability and ease of attaching.

The pendulum effect doesn't apply in a coordinated turn. But roll without turning with a heavy top mounted payload will increase unless countered. This isn't desirable even for jet fighters - they prefer "relaxed stability", which is basically instability, without strong stabilizing or destabilizing forces in absence of pilot input.

Underwing weapons on jets are basically mounted on the centerline. Unless loaded with large gravity bombs, a fighter retains broadly similar control response between different AoA loadouts. For any detachable load (combat or not), bottom mounting allows for gravity release, which is a massive safety improvement.

Underslung payloads, often used by helicopters, don't create stability or instability per se, but in practice they can do both, slowing down the maneuvers or creating instability through pendulum motion (unrelated to the pendulum effect).

For a cargo plane, bottom-mounting the payload is always safer for most purposes, but only if there's room for it, which is usually not the case. Top-mounting does work, but it does reduces your margins of error.

Answer 2

A couple of things. The Soviet aircraft above transporting the fuel tank or NASA’s 905 SCA 747 are examples of how to use existing aircraft to move physically large or awkwardly shaped payloads that will not fit inside of the internal cargo space of existing aircraft. In this case a “piggy pack” configuration works well. The disadvantages to this is that it produces a lot of parasite drag as well as causes lateral and yaw stability problems, generally addressed with larger tail surfaces.

Some aircraft are designed specifically for oversize loads. These include Airbus’ Beluga conversion of the A300 airframe, the Boeing DreamLifter 747-400 conversion and the Aero Spacelines Super Guppy modified from a Boeing 377 StratoCruiser airliner. The specialty airplanes have been used to transport fuselage sections from other aircraft and even rocket boosters and individual stages of space rockets and ballistic missiles.

Large cargo aircraft can carry standard size pallets and shipping containers in their interior. They can even accommodate small vehicles or other aircraft. The Lockheed F-117 Nighthawk was designed such that its wings could be easily removed, and the fuselage and wing set loaded into a Lockheed C-5 cargo aircraft for transport. Said cargo aircraft can also be used to paradrop large pallets of equipment or even vehicles out of their rear ramp. Some very large munitions such as the Vietnam era BLU-82 ‘Daisy Cutter’ or GBU-43/B Massive Ordnance Air Burst (MOAB) launch that way from the C-130 cargo aircraft aboard specially designed sleds which separate for the munition once clear of the aircraft.

For smaller combat aircraft and underslung external payload for fuel tanks and ordnance works well, provided the payload can be centered very close to the aircraft CG. This is one of the reasons that most weapons hard points are located on the wings of an aircraft as well as waist and centerline stations.

Underslung payloads also facilitate ease of loading, unloading, or reloading ordnance during combat operations not to mention making it much easier to release freefall weapons such as bombs. There have been some rare cases where combat aircraft have used overslung weapons. The SEPECAT Jaguar, for instance, had overwing missile rails for Sidewinders. I suspect this was done to free up more real estate on the bottom side of the wing for air to ground ordnance.

Externally mounted payloads or weapons also have a consequence of vastly increasing an airplane’s radar cross-section, which is why current low observable designs are opting for stores carriage inside internal weapons bays. This both preserves the aircraft’s stealth signature as well as reducing parasite drag.

Answer 3

If the load is meant to be released during flight, hanging it below the airplane is much preferred to ease separation. Bombs, fuel tanks and missiles are almost always mounted below, with the notable exception of the overwing tanks of the EE Lightning or the short-range air-to-air missiles of the SEPECAT Jaguar (pictured below, source).

This position was chosen because all other places were already taken, and a missile will separate more easily than a bomb (which is visible hangng below the wing in the photo above). If you want to learn what could possibly go wrong when releasing something in flight, watch this video.

The next reason, of course, is the much easier replenishment of those underwing or underbelly positions. Only when the load becomes too large for hanging it below the airplane will it be mounted on top.

For the Me-109 even a jettisonable auxiliary wheel was tested to allow take-off with a 1000 kg bomb which would not have fitted between fuselage and ground without raising the tail by means of that extra wheel (picture below, souce). After take-off the wheel simply fell off and landed hanging on a small parachute.

Also, the underwing position has aerodynamic benefits: Since airspeed above the wing is higher than below when lift is created, a body stored below the wing produces slightly less drag than one above. Ease of separation can also be added to the aerodynamic realm since releasing an overwing load needs plenty of angle of attack on that load right after release so lift will carry it up and away from the airplane. For an underwing load, some nose-down attitude is helpful, but weight will already help with separation. In total, however, aerodynamic considerations have much less weight when selecting a storage position.

The kind of stress in the hard points is even less of a factor.

In terms of flight mechanics, the lengthwise position is much more important than the vertical position. Since most external payloads behave like slender bodies, they are destabilizing the airplane. They cannot be mounted too far aft in order to keep the center of gravity in the desired range, so this destabilizing effect must be compensated either by oversizing the vertical tail from the start, or by adding auxiliary vertical surfaces at the tail later.

Paul Allen's Stratolaunch uses a raised profile at mid-fuselage to allow a high enough wing position such that the massive booster it is to carry will fit below. The B-52 from which the X-15 was launched even accepted a wing cutout to make an underwing position possible.

Only when a payload is sure to stay attached until landing will an overwing position even be considered. The Ju-87 could be fitted with overwing pods to ease the relocation of personnel to a new base (picture below, source). For that purpose the position on top of the wing made more sense.

Answer 4

Loading considerations are as follows: relation of center of thrust to center of drag (front), sideslip force to vertical center of gravity, and relationship of added weight to existing center of gravity.

Last first, obviously the center of mass of the added structure should be placed at or near the existing center of gravity (near the center of lift).

If possible, try to avoid excessive trimming by having the new center of drag (forward) as close to the original one as possible. The farther above or below the center of thrust it is, the more trimming will be required when changing airspeed.

The side slip forces can be a bit more detailed, as the object being transported on the M-4 will raise both the center of gravity and the center of (sideslip) drag!

Judging by the modified vertical stabilizers$^1$, the Energia tank raised the center of drag slightly more.

Finally, the consideration of attaching cargo above or below the wing (if ground clearance allows) would be favoring below the wing as lift production is less affected. This is because lift below the wing is more from a simple ram effect of a surface at an angle of attack to the relative wind, causing an increase in pressure, whereas lift production from the upper wing surface works best with undisturbed airflow.

$^1$ similar to the Heinkel 162