Elevator - up force or less down?

[RV7A Flyer]

Originally Posted by Snowflake

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.

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.

[gc1016]

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.

[PilotRPI]

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?

[PCHunt]

Originally Posted by PilotRPI

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?

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."

[RidiculousM]

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.

[Snowflake]

Originally Posted by PilotRPI

Aero-mech engineer here.

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.

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.

[PilotRPI]

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.