All non-canard types: elevator downforce

[stanfield]

Hi together,

I´m just curious: how much downforce would an elevator produce on a conventional, aerodynamically stable aircraft in unaccellerated level flight, relative to the A/C gross weight? I would expect some low single digit percentage, but has any of you confirmed figures on this?

[Zaphod Beblebrox]

I don't know how to answer that question specifically. I went a did a little search and found two FAA publications online. Look up "tail down force" and you will get some examples.

The material says there are two components of tail down force.

1. Static - the aircraft is generally nose heavy because the center of gravity is ahead of the center of lift.
2. Dynamic - there is some value where the aircraft is at it's maximum forward center of gravity where the tail must exert the greatest possible force.

The actual value of the force is somewhere between those two positions.

I don't have an answer as to the actual value in pounds of what ever other unit is used. It would obviously depend on the design and the size of the aircraft.

[Goldenrivett]

I would expect some low single digit percentage

Interesting assumption.

Assume A320 dimensions
Nose to tail = 37.57m
Wing span = 34.1m
Wing area = 122.6 m↑2
Average chord = 122.6/34.1 = 3.6m

Fwd C of G limit = about 15% MAC
Aft C of G limit = about 45% MAC.
Diff = 30% chord width = 1.08m

Assume aft C of G is at approximately 55% of fuselage length and elevator C of Lift is at 95% of fuselage length. (Diff = 37.57*40/100 = 15 m)
Therefore when C of G is at fwd limit, extra down force on tail = Mass * 1.08/15 = 0.066 * Mass (6.7% of Mass of Aircraft).

I don't know what the down force is on the tail at the Aft C of G position, but if the load is an extra 6.7% of the aircraft mass at the fwd limit then I'd guess the load is about 3% of M at the aft limit giving a total of about 10% M at the fwd limit.

[dubbleyew eight]

we worked it out once by measuring up the aeroplane.

for this type of aeroplane...


weight 590 kg
lift 590 kg
thrust 64kg
tail download 37.6kg in cruise.

depending on the speed and position of the cg, the tail download varied between 37kg and 63 kg.

to fly my aircraft (similar but not the one in the photo) at Vne in straight and level flight requires full forward elevator (so obviously an upload) to get enough incidence off the wing.
ymmv.

[VinRouge]

2. Dynamic - there is some value where the aircraft is at it's maximum forward center of gravity where the tail must exert the greatest possible force.

Not forgetting additional dynamic load as a result of effect of controls on the aircraft.

[Mozella]

...............how much downforce would an elevator produce on a conventional, aerodynamically stable aircraft in unaccellerated level flight,..............

This is not easy to answer. Calculating the force on a tail is a complicated matter. For one thing, elevators (I assume you mean horizontal stabilizers used in conjunction with elevators) don't always produce downforce in spite of what you may read on the Internet.

It may be important to mention that not all aerodynamics experts fly aircraft, but everyone who can fly considers himself an aerodynamics expert.

When you look at conventional aircraft (non-canard) and consider the combination of the horizontal stabilizer and elevator you can find examples of tails which produce downforce as well as tails which produce lift.

Most aircraft have positive longitudinal stability and generally speaking, this feature is desirable; very desirable. This stability can be achieved with a tail providing downforce, providing lift, or in the case of flying wings, no tail at all. Or, given a sophisticated enough flight control system, an unstable aircraft can fly just fine too.

Stability and control (they're two different things) go hand-in-hand. This discipline can be a rather complicated and specialized area of aerodynamic study. Furthermore, some of the laws of aerodynamics aren't easy to understand. When it comes to the most simple explanation of longitudinal stability, using an example of a tail producing downforce makes that explanation easier to understand.

Those who study aerodynamics in a serious way nearly always refer to something called the Aerodynamic Center; Google it. The aerodynamic center is the point at which the pitching moment coefficient for the airfoil does not vary with lift coefficient. Wrapping your head around this concept and what it means takes some thought; perhaps a lot of thought. Especially when you realize that it involves the pitching moment COEFFICIENT, not the pitching moment itself. Google is your friend here. Try searching on pitching moment, aerodynamic center, tail downforce, aircraft stability, etc. You'll need to understand the AC before you get too deep into stability.

Of course, stability is not the only factor that must be considered. For example, an arrow with a heavy arrow-head is stable, but it will fly in an increasingly nose down attitude. Aircraft must be not only be stable but must be able to be trimmed too, as we all know.

By selecting the right airfoils (usually this means a horizontal tail with a different lift slope from the wing), the correct plan-forms, the correct CG, and the correct incidence of the wing and tail, one can design a stable aircraft which can also be trimmed while at the same time producing positive lift from the tail. This idea isn't nearly as easy to grasp as the chalk drawing dealing with stability which we all had shown to us before we got very far in flight training. But if you dig deep enough you'll find the answers you're looking for on the Internet.

[Owain Glyndwr]

The number(s) you are looking for vary quite a bit with aircraft configuration. If you only want ballpark figures. the tail download might be as high as 35 percent weight for full flaps, forward CG limit and approach speed.

For a clean aircraft in cruise at aft CG limit the load might be as low as 3 to 5 percent weight depending on the stability designed in. Aircraft with fuel tanks in the tail will be at the lower end of this range.

All that for level flight as you specified in the OP, and by "elevator" I assume you mean tail.

[stanfield]

Thanks to everyone of you, ballpark figures are close enough for me. And indeed, I meant the whole tail, so stabilizer and elevator. The point with momentum coefficient sounds very interesting, since I heard a couple of times about conventional RC planes generating lift at the rear end yet flying stable... I´ll try to get mental access to that ;-) But as on airliners I usually see the horizontal empennage with a profile "upside down" I assume that they do produce a reasonable amount of downforce.

Thanks again!