Pyro, inverted lift and drag?

[Kieren]

Written out loud to all, not directed at anyone.  
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Things I can see that would impact the ability to climb as well inverted as upright:[list=1]

• Airfoil selection. With a semi-symmetrical airfoil the center of lift would be located in a different location, possibly affecting the AoA necessary for level flight. Inverted the airfoil would possibly generate less left even under optimum conditions due to the amount of curve applied to the lower surface. Again, a higher AoA would be needed to compensate for the decreased lift ability, increasing drag.
• Dihedral. Dihedral is the upward sweep of the wing that increases stability in the upright, normal flight regime. The high wing's lift vector is not pointed vertically up as much as the lower wing's lift vector, therefore the high wing tends to drop. While inverted, the reverse would be true. This would force more control input to maintain control, therefore more drag.
• Washout. Washout is the turning of the tips of aircraft down (AoA) a few degrees relative to the root of the wing- the purpose is to help maintain control at slower speeds (delaying the stall of the wingtips a few mph). Inverted, wingtip stall would occur faster.
• Thrust line. In many cases the thrust line of an aircraft will not be set at 0 degrees. The Hellcat had something like 2 degrees down thrust, so inverted that converts to 2 degree positive thrust.
• Angle of Attack (AoA). The position of the wings relative to the centerline is chosen with many factors in mind, but in our case it is important to know that if an aircraft carries 1 or 2 degrees positive AoA in upright flight it translates to 1 or 2 degrees negative AoA in inverted flight. This would increase drag over regular flight significantly. Don't forget the horizontal stab can have its AoA set too, and this must be figured in as well.[/list=a]
  
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  In the end, inverted flight should probably be harder to maintain, show decreased performance, and generally feel unstable. Maybe someday I will take a "dream ride" and find out for myself- what do I know!  
  
  I eagerly await the holes that will be punched in this.  
  
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   http://www.hpcisp.com/~kieren/calendar/calendar.htm
  
  [This message has been edited by Kieren (edited 08-07-2000).]

[Pyro]

 

Originally posted by Daff:
The increased drag will probably be directly related to the increased AoA.
Increased AoA = increased downwash = increased drag.

This is what I was getting at by saying that you really have to make the comparisons as if it were two different airfoils.  The part you said above about AOA is a general statement that is true for most discussions dealing with an individual wing at different AOA's.  But the reality is that it's really based on Cl.  Looking at this example, the plane weighs the same, has the same wing area and planform, and will require the same Cl whether inverted or not.  

Example: 2 planes have the same weight, wing area, aspect ratio, etc.  The only thing different are the shapes of the airfoils.  Plane 1 has a lift co of 1.0 at 10 degrees AoA.  Plane 2 needs 11 degrees of AoA to get a lift co of 1.0.  Which plane has more induced drag?

Anyway, there are differences between an inverted and normal climb, I'm not trying to argue that.  I just don't know that they will be that big of a deal when it comes to climb rate.  But then, I don't even know how much difference in RoC it makes to have your gear down.  



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Doug "Pyro" Balmos
HiTech Creations

[Waxer]

Is radial G modeled in AH?

[Daff]

Well, I know that on a typical asymmetrical winged plane, the inverted stall speed is about 50% higher than upright. That, too me, indicates that lift is a fair bit lower and drag a fair bit higher.
You dont have an airfoil analyzer to get the general numbers for inverted lift and drag?

Daff

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CO, 56th Fighter Group
"This is Yardstick. Follow me"

[Andy Bush]

What a thread!!

Regarding laminar flow airfoils...they are usually cambered just like any other unsymmetrical airfoil (the P-51 had a unsymmetrical, but laminar, airfoil).

An inverted climb under 'normal' conditions would be an uncomfortable experience in a WW2 fighter. 'Normal' in this case means a sustained climb where the aircraft is at or near one G.

The unsymmetrical shape of the airfoil would tend to produce a lifting force off the 'top' of the wing regardless of the attitude of that wing. If that attitude is down, then that lifting force is also down...and would have to be offset by an equal and opposite force to keep the aircraft from going down.

In any aircraft, the pilot gets that equal and opposite force by pushing forward on the stick How much? He needs to see minus one G on the G meter.

The difference between a symmetrical and unsymmetrical airfoil aircraft is that the pilot in the unsymmetrical aircraft has to 'push forward harder' to get the needed angle of attack on the wing to obtain the minus one G. Generally speaking, the unsymmetrical aircraft will require a greater AOA than its counterpart. This AOA may or may not be attainable...or sustainable (the aircraft may 'stall' first).

Anyway, the real point is that I can see no reason for an inverted climb in RL...now if the sim programming gives a particular advantage under these conditions...well, that's a different story!

Andy

[Daff]

"Anyway, the real point is that I can see no reason for an inverted climb in RL"

Why not?..it's fun!

Daff

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CO, 56th Fighter Group
"This is Yardstick. Follow me"