Hello Everyone,
An inside loop defined as the cockpit is facing the center of the loop, an outside is when the cockpit is facing, well to the outside. Why is the outside one so much more difficult?
Because most aircraft are designed to create maximum, controllable lift in an upright position. An inside loop is essentially “all upright,” with the centripetal force outwards towards the center of lift.
Anything but a stunt plane generates far less lift inverted, and an outside loop is, yes, essentially all inverted, with the circular force in the same direction as most lift.
In an inside loop, the plane and its surfaces are doing essentially what they’re designed to do in normal flight. In an outside loop, all the forces and control and balance and so forth are inverted, and both harder for the plane to manage and harder for the pilot to control.
Stearmans and their pilots excepted. 
In an inside loop, wing lift is helping, by pulling the airplane into the loop. An outside loop is done against wing lift.
And :::
With symmetrical wings like acrobatic planes use a lot, the airplane does not really care as long as you have inverted oil & fuel systems.
The human body does care. It is designed and evolved over at least a few weeks to operate in relation to gravity in certain ways. Both biologically, mentally, spatially, etc.
Easiest example, Blackout vs Redout. The average human can do inside loops with their body to much greater forces than they can do outside loops.
Build a plane with the cockpit able to rotate like a ring mounted so as to be movable or rotatable as it were. Now rotate the cockpit so that the airplane is doing an outside loop and the pilot is feeling a inside loop in relation to his body position. Wheeeeee
Loops make no never mind to airplanes as long as the structure can stand the stress. Wing design makes a difference as to angle of attack and stuff & such but with nuff power, the 2X4 airplane will do just fine and the only difference is control position and rate & degree of application.
The average airplane can do more than the human can stand for short times most of the times but if you are trying to break the airplane, many airplanes will break you first.
Acrobatics need one kind of plane, dropping bombs or carrying a lot of peeps long distances need another kind of plane.
Disagree.
Although the lift is not as efficient, a normal wing will fly inverted, straight & level if structurally strong enough and at the right AOA & speed. So an outside loop is not doing anything against lift. But the wind is not really good at producing lift in that position but lift is being produced.
Simple answer. Pilot construction is not as good for moving through the universe with all the blood trying to get into the head.
Most acrobatic aircraft today are mostly aerodynamically balanced for either position. They just don’t have pilots built that way.
If a computer were moving the controls and there were no bumps for peeps to sit in, the modern acrobatic aircraft has no difference in ability as regarding loops.
Remember, aircraft have stress limits. Positive and negative. Positive (rated) are typically close to 2x the negative. Inside loops are positive G, while outside are negative G on the airframe.
No - as GunsNSpot notes, nothing is being done “against wing lift”. Any loop requires using the lift of the wings to make the plane describe a (typically approximate) circle.
An outside loop is harder in part because the wings are being asked to generate lift in a direction opposite to that for which they are normally designed: upright flight.
Anything with a positive angle of attack will produce lift, yes. But a wing is normally built onto the fuselage with a positive angle of incidence, giving it a built-in positive AOA that must be overcome in an outside loop. The negative lift of the horizontal stabilizer must also be overcome. In an inside loop, those things help you.
Well, you’re burying some subtleties in there. Yes, an ordinary wing will fly straight and level etc. - IF the control surfaces are correcting for the imbalance in lift pressures. An aircraft that will fly “flat,” hands-off, in normal flight almost certainly cannot do so inverted; the pilot will have to be maintaining an aircraft-negative, flight-positive angle of attack that has the nose pitched slightly upward (away from ground), more so than it would be pitched up in normal flight.
Non-symmetrical wings are meant to provide positive lift, and flipping them over means you’re going to get ‘negative lift’ unless you correct with the control surfaces.
I assume symmetrical-wing craft like Stearman can fly hands-off level inverted, but that’s a guess.
There are a few things working against an aeroplane flying upside down. I used to routinely fly a Pitts Special upside down. It was slower, needed a LOT of forward stick to maintain level, and the controls were no longer harmonised. Features designed to combat adverse yaw in upright flight create more adverse yaw in inverted flight. Any angle of incidence built into the design reduces drag when upright but creates lots of additional drag when inverted. The Pitts, a purpose built aerobatic machine, didn’t perform anywhere near as well inverted as it did right side up. More modern Extras and Sukhois and the like might sacrifice some upright performance to help achieve better inverted performance but it’s always a trade off.
I don’t think a Stearman has a symmetrical wing by the way.
Sorry, I was visualizing a Pitts and thinking Stearman. Close, cigar, etc. 
What they need to invent is an airplane with wings having a variable angle of incidence – that is, they can rotate up or down about the wingtip-to-wingtip axis. Just make them able to rotate fully 360 degrees, and you have your ultimate aerobaticmobile.
It doesn’t. They’re typical WWII style: flat-bottomed with pronounced upper-surface camber. The incidence is the same on upper & lower wing although IIRC the camber was a bit less on the upper.
It isn’t uncommon for general purpose aircraft to be designed to take far more positive G loading than negative G, since they normally fly around at 1 G positive, and the pilot has a strong positive bias. +6 / -3 G for example. It is possible to save significant weight this way. The struts on a Cessna wing are in tension of positive G, and could be cables. They are rigid to allow some negative G, but not stout enough to avoid buckling at -6G. Similarly, upper and lower spar caps can be sized for the intended load rating, saving weight.