I've read that the Pfalz DIII had a (mild) inverted airfoil shape to the horizontal stab. This was said to improve its ability to come out of a dive. Can someone explain why this would be so?

--Don

First things first:

In a typical tandem configuration the tail produces negative lift to counteract the moments produced by the wing. This is essentially what gives the plane stability (and yes - for you engineers this is GREATLY simplified).

The lift produced by an airfoil is a function of the geometry (CL), the density and airspeed (q), and angle of attack (alpha). In a dive, the airspeed increases. Thus for the Pfaltz, the (negative) lift of the tail increases which pitches the nose up.

This is a simplified analysis, but I hope it gives you an idea of what's involved.

Thanks, I think I get it. So the faster the plane goes (for example because of the dive) the more lift the tail surface would have thus allowing it to more easily "pull out" i.e. to adapt a nose up attitide. I'm assuming the type of airfoil used on the tail is such that at normal level flight speeds it wouldn't have much effect.

That's correct. The relationships between the tail and wing would be based on some design speed. You can expect trim changes with speed, but a Cessna changes trim with speed too. I don't know if the Pfalz had trim tabs on the elevator, but I would assume it did. So you could trim for different flight speeds. My guess is the tail was designed so that the speeds where the tail started to self correct would be higher than the plane could obtain in level flight but (obviously) lower than the max for the airframe.

The typical WWI airplane had no trim on any surface.
Pilot brute force kept the pointy end at the front.
With the Eindecker, this required a lot of muscle ALL the time, with both the vertical and horizontal full-flying, and the drag of the motor support shaft in its bearing forcing a continuous right turn which had to held off by the pilot.
Very late in the war the advantages of trim were being looked at.. The SE-5 had a trimming horizontal.
Fokker went from that awful comma-shaped all-flying rudder to a fixed vertical and a movable rudder, thereby immensely reducing the physical demands on the pilot.
The inverted horizontal, which is still in use today, acts to reduce the trim demands. Its nose up force increases directly with the nose-down moment of the wing as speed changes.
The Lockheed L-1011 has a full-flying horizontal, with an inverted airfoil. The C-130 horizontal is inverted, with a conventional fixed horizontal and moveable elevator.

on today's planes, does the inverted airfoil also help rotate the aircraft for take off? The L-1011 with the full flying tail would be increasing the angle of attack in relation to the airfoil, therefore causing it to produce more lift and raise the nose quicker/easier to get the plane flying with less tail forces, and also less drag. Pulling up elevator on the C-130 would be in effect putting the flaps down on a wing. This would cause the tail to produce more lift, negative in this case, to rasie the nose....

Typically, the greatest effort on the part of the horizontal stabilizer is during the landing flare. Here you’re flying very slowly, and so the stab needs to rely on airfoil shape, and it’s angle of attack in order to produce the required downforce (Note we’re talking horizontal stab AOA… not wing AOA). This downforce is used to raise the nose of the aircraft so that it can assume the two point landing attitude (nose high). If you put an inverted airfoil on the tail, you can create more down force at the horizontal stabilizer to do exactly this, raise the nose. The aircraft designer needs to make sure that the horizontal stabilizer doesn’t stall during the flare process. If the horizontal stab were to stall during flare, you’d suddenly lose almost all of the downforce at the tail, and the nose of the aircraft would violently drop. Not a desirable situation on landing for obvious reasons.

If I think back to my airplane design courses in university, it was always the flare condition at landing that we used to size the horizontal stabilizer. If it could handle the landing flare, then it was a fairly safe bet that it could handle any other flight condition.

Tom

Read Tall Paul's post again. The inverted airfoil doesn't "help" any particular condition. It reduces the trim demand to get the plane in trimmed flight (no stick forces required). It does this by biasing the tail lift downward (relative to a symmetrical airfoil), which is usually helpful, as the tail is usually "pushing" down.

One exception would be if you want to fly inverted. This hurts a lot, because you now have to put extra force (directly or with trim) to keep the plane level- it's (the stab) also going to stall sooner, which isn't too severe unless you're close to the ground! It's the same effect as flying something like a trainer, with say a flat bottom airfoil, inverted. It takes much more AOA to get the necessary lift, and you will stall sooner (at a higher speed).

Models which use inverted profiles on the horizontal are typically the "lifter" type, which have no need to fly inverted, although when empty they can be forced upside down.
If inverted flight is part of the flight requirement, then a conventional symmetrical horizontal should be used.
Worrying about flaps while inverted.. Why?
Or spoilers?
It's wasting resources to be concerned about transient situations which may never occur.
Don't overanalyze this situation, go and build and fly.
It's much more productive.

Take a look at all Boeing models starting with the 707.......horizontal stab is inverted.......as is the 727, 737, etc.....as Paul said....this makes the trim less sensative.....and allows autopilots to use a "Cruise Trim Motor" which moves the stab.....which can hold altitude much better......also helps control mach tuck.....with a very small actuator that trims the elevator.....which in turn.....with the neutral shift mechanism....will trim the stab.........and keep airplane @ selected altitude........Bill...

As noted above by the other writers it is strongly dependent on aircraft type. Don't read the above and apply the results to a standard fully symmetrical airplane (I know it was not in question). A lot of guys assume that since those kinds of airplanes being discussed require a down tail load that it is needed for all airplanes.

If there is no fuselage pitching moment and the airplane uses a symmetrical airfoil the tail is actually lifting at all angles of attack. It is only when you have moments due to airfoil, flaps, fuselages, gear, etc. that the load shifts downward. Of course 98 percent of the things flying fit in the later catagory!

The primary reason for using an inverted airfoil on an airplane that is of a configuration that normally balances with a download on the tail is really quite simple. It has less drag when producing the required down force than an equivalent symmetrical section.