These are some design notes for anyone taking part in the @Pilotmario Close Support/Attacker Challenge. Having submitted a couple of designs that have been found lacking, I've gone through the test reports that have been posted and have come up with this information packet to guide future designs.

General design:

Try to keep the design relatively simple. Large, over-complicated designs are viewed as problematic.
Ensure the landing gear has quite a wide stance for good stability on landing.
A short nose and good visibility from the cockpit are ideal. Avoid having a long nose.
If carrying lots of fuel, make sure it is protected from being shot. If possible, add an extra layer of armour.
Consider adding extra armour protection around the cockpit as well.
Being able to cope with damage is desirable. Designs seem to be being tested on how they cope with the loss of an engine and possibly other parts as well.

Weapons:

Mount the rocket pods on hardpoint pylons, not directly on the plane.
Ideally, all hardpoints (pylons) should be usable for bombs and drop tanks as well as rocket pods.
Having hardpoints high off the ground is undesirable.
If using a biplane or triplane design, avoid putting hardpoints anywhere other than the lowest wing.
Having anti-armour guns is highly desirable. Pilotmario's Boar mounts a modded minigun that works for this, or you could use the Mk8/9 wing guns, also from Pilotmario.
It is expected that all bombs will be dropped before returning from a mission.

Engines:

The T2000 prop engine is highly thought of for its reliability.
Using 2 x T2000 engines to power contra-rotating propellers is acceptable.
The T3000 prop engine is acceptable.
J15 engines are not recommended due to logistical issues.
J50 jet engines are acceptable.
J90 jet engines are acceptable.
VTOL engines are not recommended due to maintenance issues.
If using propellers, we know that these are among the standard Pilotmario air force ones:
The Mk 2 propeller has four warbird-type blades and is 110 inches in diameter, and employs 3x chord length. This is the type employed on the Pummeler Mk 2 and the Mk 4 as well as the Provider transport.
The Mk 4 propeller has four Cessna-type blades and is 115 inches in diameter, and employs 3x chord length. This is the type employed on the Provider II transport.
The Mk 5 propeller has six Scimitar-type blades and is 120 inches in diameter, and employs a 2.5x chord length. This is the type employed on the Pathfinder airliner, which has been adapted into a gunship and a maritime patrol aircraft.

Control:

Be sure to include a rudder.
Having manually-adjustable air brakes is good, ideally using the VTOL slider to control them.
Ideally, include flaps on the design. The Pummeler and the Boar both include manually-adjustable flaps on their wings, so you could save a wing from one them as a sub-assembly for use on your own design.