# Elevator and Rudder - Drag or Lift devices?

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It's simple . Basically, if you change what's going on to a parcel of air, you are adding or subtracting energy from it. If it slows you 've impeded it. Drag. If it escapes to a lower pressure it cools and expands  and will propel the container in the opposite direction to where the gas is escaping from even in a vacuum. If it follows a curved path the high pressure will be on the outside of the curve and the lower on the inside always. Air flows FROM higher to lower.. If you push on something it will react and push you..

Pulsed smoke trails in a wind tunnel and also manometer tubes located all about an aerofoil will show what's going on with pressures and flow patterns.. Wool tufts on the top of a wing  will show flow separation commencing as the Angle of Attack to the relative airflow increases to around 14 degrees.

Bernoulli just states an effect.  With Newton you have an explanation.  It's just a different approach.  LIFT depends on Viscosity of the gas too.  but that will make your head hurt. Nev

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4 hours ago, APenNameAndThatA said:

I agree entirely. They can't not both be correct.

Oh but they can ... both can contribute to the experienced lift with neither able to explain it all by their lonesome.

ive always been Happy with logically Saying to my self the wings are both being sucked up and chucking air down in their wake.

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BUT

If total vucuum above airfoil if only 14.7 psi. Does the underside of airfoil need the total weight of a 2000 lb aitcraft to maintain lift ?. ( 2000LB PREASURE  )

spacesailor

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Here's another example of a rudder being used to change direction.

Say you were in a boat going across a mill pond (no tidal effects or currents) in a straight line at 5 kts. You decide to make a quick change of heading of, say 25 degrees to port. So you push hard on the tiller which causes the rudder to move out of alignment with the fore/aft axis of the boat, exposing more of the rudder to the water on the port side of the boat. The boat begins to change heading.

Was it drag or something akin to the results of Bernoulli's Principle that caused the boat to  change direction?

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Would you be able to turn to starboard if the rudder was attached to the extreme port side of the boat?

Of course, you would, so it’s not drag doing the job.

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54 minutes ago, spacesailor said:

If total vucuum above airfoil if only 14.7 psi. Does the underside of airfoil need the total weight of a 2000 lb aitcraft to maintain lift ?. ( 2000LB PREASURE  )

There are a lot of square inches of wing. The pressure difference required works out to around 0.1 psi for a typical light aircraft.

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38 minutes ago, aro said:

Would you be able to turn to starboard if the rudder was attached to the extreme port side of the boat?

Here's a Viking longship with its rudder attached to the extreme side.

If anyone really wants to drift off this topic, here's a paper of marine rudder design. https://www.tandfonline.com/doi/full/10.1080/17445302.2016.1178205

Interestingly, the paper discusses the performance of rudders in terms which we are familiar with in aeronautics. Seems a fluid is a fluid, it's only the density that changes.

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Infinitely variable lift and drag and drag with this split rudder.

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That split rudder is dumb. The fin wouldn't be designed for the extra load and there would no doubt be some buffeting. Nasty. You can't beat spoilers for dumping lift. Glider pilots know that as do drivers of Commercial jets. You can extend them at high speed as they will blow down if they are overloaded. If they are differential you get incredible roll control. As ground spoilers you get the plane pinned on the ground instantly and high braking effect possible because the planes weight is on the wheels. Descent rates of around 10,000 fpm easily done. Naturally you don't want that at 500 feet. Nev

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Stick something out into the airstream and it creates Drag.

The Fairey-Youngman flap was patented in 1941. These flaps were large, around 1/3rd of the wing chord. Their movement went in two phases, controlled by a linkage.

Firstly the flap lowered below the wing and approximately parallel, making the aircraft almost a sesquiplane. This gave improved lift, but with little extra drag, and was used for landing. The flaps could be extended further for landing, now rotating downwards to 30° as a slotted flap. With the use of flaps, wing loading was reduced and also gave a gain in lift coefficient. The Fairey-Youngman flap and its initial downward parallel movement was superseded for other aircraft by the Fowler flap, which too had an initial parallel movement, although rearward sliding.

Fairey Aviation used this flap design for the Fairey Barracuda dive bomber, as the design could be modified to also tilt the extended flap upwards, acting as a dive brake.

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On 06/12/2020 at 12:54 AM, old man emu said:

It's probably clearer with the Vertical Stabiliser/Rudder. Usually the vertical stabiliser is a symmetrical wing - same curvature on both sides. When the rudder is moved from its neutral position, does it create lift on one side by changing the camber of the symmetrical wing, or is it just a sheet of material thrust out into the airstream to cause drag?

When the aircraft is an a steep bank angle the rudder will produce lift.

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Forum member JG3 wrote a nice review of the inverted stabiliser/elevator as found on the Zenith CH701 aircraft. Hans at Kilkoy replaced this with an ICP Savannah horizontal stabiliser/elevator.

This article shows real world experience with empennage lift and drag: https://www.stolspeed.com/tail-feathers

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On 05/12/2020 at 11:20 AM, old man emu said:

Leaving out changes to power settings, we move the elevator to induce pitch changes and the rudder to induce yaw changes. But how do these moveable surfaces at the rear of fixed surfaces produce the forces necessary to change pitch and yaw angles? Are these devices increasing Drag, or increasing aerodynamic Lift?

A simple way to describe what's going on. The airflow pushes on the deflected surface (elevator/aileron etc.) and the reactionary force pushes the surface in the opposite direction of deflection. For every action there is an equal and opposite reaction. The downward pointing arrow is a vector representing the deflection force and the drag.

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On 05/12/2020 at 12:27 PM, old man emu said:

Granted. But is it the additional force of Drag that does the job, or an increase in Lift force?

Leaving out any discussion of the placement by the designer of the tailplane and vertical stabiliser for "straight and level" flight, do these two control surfaces work by reducing speed on one side (Drag), or increasing Lift on the opposite side to the direction of deflection

The drag generated by the control surface deflection will tend to have a restorative influence on the diversive force generated by the lift element.

Take for example a rudder deflection. As detailed above in Jabiru7252's post, deflection of the rudder generates a lateral component (lift) and a rearward component (drag), although he only shows the resultant combined vector in his diagram.

The lateral force will cause the plane to yaw (which is the desired effect). The rearward drag force will act about the centre of drag (roughly on the centreline), but being displaced progressively more with increased yaw, this induced drag will represent a restorative couple, and attempt to prevent the tail from yawing.

Same applies to elevator & aileron deflections: the induced drag acts counter to the lift, but the value is lower, so the lift wins.

Bruce

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Aileron adverse yaw effect should be well known to all pilots.. IT is caused by drag. Getting extra lift by flaps or or ailerons gives a greater % of drag for the lift gained.  (worse L/D ratio). The wings are the most efficient providers of lift. Flaperons have a lot of adverse Yaw and you fly those planes with a different technique or you will get into strife some day..  Nev