Elevator - up force or less down?

Over many beers and margaritas on our trip to OSH, a question came up about the horizontal stab/elevator under different configurations and exactly what is going on. To summarize: standard introductory textbooks show that in most aircraft, the CG is forward of the main wing CL, thus causing a "pitch down" moment which is counteracted with the HS/Elevator's downward-directed "lift" vector.

We all know that loading an aircraft near the aft CG limit reduces the amount of elevator down force required, thus reducing induced drag and (theoretically) allowing for increased velocity.

The question is: on an RV, does the elevator ever generate *upward* lift (assuming one keeps the CG in the envelope)? Or is only a reduction in the downward "lift"?

My understanding was that it was always downward, due partly to the CG-CL relationship and partly to the downwash on the tailplane, but the counterargument was that because the angle of incidence of the RV HS is 0 degrees, any amount of downward elevator beyond neutral means that the HS is now generating UP-ward lift. I submitted that this can't be true unless the CG >= CL (which should be a longitudinally unstable situation).

I'm a systems engineer, not an aero, so I'm sure there are lots of factors involved here...but hoping someone who is can help us settle an Oshkosh "bar bet"

I'm sure a *credentialed* aeronautical engineer will come along in a moment to set me straight, but I am pretty certain that the horizontal tail/elev is always producing "lift" in the downward (relative to the aircraft) direction. i.e.: Tailplane lift vector out the bottom of the aircraft, no matter the attitude of the aircraft.

The angle of incidence of the wing, which is the angle between the chord of the wing and the longitudinal axis of the fuselage, is not germane.

What is germane, is the angle of attack of the wing, and the angle of attack of the tailplane, and how they relate.

OTOH, on the canard planes (ex: Long-eze), both the wing and the canard provide lift in the upward direction.

Standby for a longer and more technically correct explanation.............

I strongly suspect that a tail with a symmetrical airfoil (as appears to be the case with my rv-7 tail) will produce lift in an upwards direction when the stick is pushed forward (elevator deflected downwards). I do not, however, have any idea when the change from a downward force to an upward force occurs.

I'm probably mistaken, but if you can maintain inverted flight (that is, straight-and-level upside down) for any length of time, wouldn't that automatically prove that the HS does indeed pull "upward" from the attach points?

I'm an AE, though it's been a while since I took flight dynamics. Please excuse typos and abruptness; I am typing on my phone.

It is possible that the stabilizer could be producing a small upward lift component during level flight, even in naturally stable aircraft. This could occur from the CG being very slightly behind the wing's center of lift, or because the chosen airfoil has a noticeable nose-up pitching moment. Though we tend to assume that CG behind the wing center of lift automatically means unstable, that isn't necessarily the case. What determines static stability in pitch is the change of pitching moment of the aircraft as a whole when AOA changes. As long as the change is large enough in the right direction, you're still stable. That's how tandem wing, tailless, and canard designs can be naturally stable, too.

Now, you couldn't have your center of lift be too far aft of your CG, or the change in wing lift, even with a tiny moment arm, would overpower the moment from the stabilizer (which doesn't make much lift and doesn't see much change in lift, but has a big moment arm). Other things affect this to, like lift and pitching moment effects of the fuselage, pitching moment from the wing (which is really the center of lift sliding around a bit as AoA changes), and other things. But that's an entire undergrad course.

It should also be noted as a consequence of this that you do not need positive elevator deflection (nose up) to be stable. Some here have argued that having a perfectly faired elevator in trimmed flight means your airplane is dangerously unstable, but that's not the case.

lostpilot28 said:

I'm probably mistaken, but if you can maintain inverted flight (that is, straight-and-level upside down) for any length of time, wouldn't that automatically prove that the HS does indeed pull "upward" from the attach points?

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Ding we have a winner. The physics don't change between positive and neg. G flight, just our perspective on which is the "up" side of the horizontal stab. At zero G, the HS is producing no lifting force in either direction.

Don't forget that in normal upright flight, there's downwash from the wing on the horizontal stabilizer. That means that the HS is operating in a negative angle of attack, and even if the elevator is undeflected, it's producing downward lift.

If you're inverted, the downwash is now upwash to the airplane, so the HS will be lifting up. It takes additional elevator to trim, though.

Dave

luddite42 said:

Ding we have a winner. The physics don't change between positive and neg. G flight, just our perspective on which is the "up" side of the horizontal stab. At zero G, the HS is producing no lifting force in either direction.

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ME/AE speaking.

Hmm. Maybe. "Physics don't change" depends on your point of view. The physics don't change but the results do greatly. The downwash angle on the tail flips and therefore the basic angle of attack of the tail inverts when flipping from 1G upright to 1G inverted flight. You are absolutely right that the tail is lifting the other way when inverted, but it is far from clear that the tail could lift that way in 1G upright flight because of the limiting effects of the downwash angle.

If you are in cruise and right side up, you are producing some amount of downward lift force with the HS because the CG is forward of the CL. If you are producing upward lift in this configuration, you are flying with the CG aft of the CG limits.

RV7A Flyer said:

The question is: on an RV, does the elevator ever generate *upward* lift (assuming one keeps the CG in the envelope)? Or is only a reduction in the downward "lift"?

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Depending on the control inputs, you could have either. A gentle push forward on the controls at the top of your descent could result in just a reduction of the downforce. But the strong forward push to recover from a stall, or to perform an outside manoeuver, can certainly generate enough force to be "lift" rather than "reduced downward lift".

I haven't checked to see where the center of lift is on the RV airfoil... It's a good question, as I know i've landed after a long flight with baggage and low fuel and had to maintain light forward pressure on the stick after I crossed the fence... If I eased up the tail was going to drop. That tells me that the CG was at or behind the CL in my landing configuration. It's extremely disconcerting because it felt very wrong to be pushing rather than pulling at that point.

Snowflake said:

Depending on the control inputs, you could have either. A gentle push forward on the controls at the top of your descent could result in just a reduction of the downforce. But the strong forward push to recover from a stall, or to perform an outside manoeuver, can certainly generate enough force to be "lift" rather than "reduced downward lift".

I haven't checked to see where the center of lift is on the RV airfoil... It's a good question, as I know i've landed after a long flight with baggage and low fuel and had to maintain light forward pressure on the stick after I crossed the fence... If I eased up the tail was going to drop. That tells me that the CG was at or behind the CL in my landing configuration. It's extremely disconcerting because it felt very wrong to be pushing rather than pulling at that point.

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We were talking about, in general, level cruise configuration...unusual attitudes could, of course, result in all sorts of other scenarios.

Just straight and level, at normal cruise speeds...I can look back and *see* that the elevator is in a very slightly "nose down" position.

The na?ve assertion is that because the elevator *appears* to be "nose down", it must be generating positive lift. I think what we were missing was the effect of downwash, which puts the HS into a negative angle of attack.

Speaking completely from experience, with only basic (private pilot) knowledge of aerodynamics...

The last time I was flying in an RV-9, while in level cruise, I looked back and noticed the the elevator was deflected slightly downward, and correspondingly, the trim was set in a nose low position, as the trim usually is in any of the planes I have flown during normal to best-power level cruise. In my experience, at a certain airspeed, the wing will be producing a surplus amount of lift, which will need to be countered by nose-down trim/deflecting the elevator downward to maintain level flight. It seems to me that in this state, which in my experience is normal, the elevator/HS would be producing positive lift.

just my $.02, but it seems as if this is being over thought.

Aero-mech engineer here.

The position of the CG with respect to the aerodynamic center of the plane has a significant impact on the nose up/down pitching moment of the airplane.

There is something else not being discussed here. Any airfoil with camber will create a nose down pitching moment during flight. This requires even more downforce at the tail. On a low wing plane, like a cherokee, ever notice how when you extend the flaps, the nose wants to drop and more tail downforce is required? You introduced more camber, so there is more nose down moment.

Planes with symmetric airfoils (ie NACA 0012) do not have a pitching moment. Nor do reflex airfoils where the trailing edge curves back up. That is what Maule does with its -7 degree flap setting, to eliminate some pitching moment in cruise from a highly cambered airfoil. This reduces the negative lift (drag) from the tail, and reduces the overall lift required from the main wing (also reducing drag) from the downforce on the tail it would otherwise have to counteract.

So to your question on the RV9. There may be some instances in which rearward CG needs to be counteracted by the tail to create positive lift, and the plane could still be dynamically stable. That would take a whole bunch of calcs to figure out. An easier way would be to look back at the tail in cruise in different conditions, and see if the trailing edge of the tail is ever in the low position, creating positive lift.

BONUS POINTS HERE!
In a C172, when you lower flaps, the nose pitches up, opposite of in the cherokee. Why is this happening if the increased camber should be forcing the nose of the plane down?

PilotRPI said:

Aero-mech engineer here.
(snip)
BONUS POINTS HERE!
In a C172, when you lower flaps, the nose pitches up, opposite of in the cherokee. Why is this happening if the increased camber should be forcing the nose of the plane down?

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My understanding is that the lowered flaps create a substantial downflow alteration aft of the wing, which changes the angle of attack of the horizontal tail, increasing it's lift (in the downward direction) which pulls the tail down, raising the nose.

And, commenting on QC1016's post, just because the elevator is precisely in trail with the horizontal stabilizer doesn't mean that the horizontal tail is producing zero lift. It all depends on the angle of attack of the horizontal tail. The AoA of the wing and the AoA of the tail are not the same.

Anyway- "Pull back, houses get smaller. Pull all the way back, and houses get bigger again, and appear upside down."

BONUS POINTS HERE!
In a C172, when you lower flaps, the nose pitches up, opposite of in the cherokee. Why is this happening if the increased camber should be forcing the nose of the plane down?

Center of Lift is moved forward.

PilotRPI said:

Aero-mech engineer here.

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Ditto.

So to your question on the RV9. There may be some instances in which rearward CG needs to be counteracted by the tail to create positive lift, and the plane could still be dynamically stable. That would take a whole bunch of calcs to figure out. An easier way would be to look back at the tail in cruise in different conditions, and see if the trailing edge of the tail is ever in the low position, creating positive lift.

Click to expand...

I think that's the expensive question here. If you look back and can see the counterweights sitting above the horizontal stab, ie. elevator deflected slightly down, does it switch to creating lift, or does it just create less downforce?

I maintain it's the latter, due to the downwash from the wing. But it would be interesting to hear from Van's on this. Scott, are you following along?

If it's deflected in cruise, it's creating drag. That suggests that you could reduce some drag by adjusting your HS angle of incidence until the elevator sits perfectly in trail in cruise.

PCHunt has it right. In the high wing Cessna, the flaps create more of a downwash on the tail, than in a low wing plane. This increased downwash decreases the effective angle of attack on the tail, creating more of a downforce, which more than compensates for the nose down pitching moment of the added flaps.

Some think the nose up from the flaps in a high wing is due to the higher drag above the CG of the plane. This may have some effect, but not as much as the downwash effects.