Elevator and Rudder - Drag or Lift devices?

[old man emu]

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?

[kasper]

Simple answer - both. 
 

tanstafl - you can’t produce anyincrease in lift without producing drag so both exist for any change in lift being produced down the back - if that’s where your control surfaces are - when you deflect a control surface to produce an effective change in camber or angle of incidence that is adjusting the lift force back there. 
 

however it is principally lift forces being adjusted at the end of a long lever arm to the centre of lift that is inducing pitch and yaw changes.

[facthunter]

Application of most controls will cause more drag.  As does turbulence The most efficient producer of lift is the wing, but add flap which gives more lift but much more drag as a % . Lets you fly slower sometimes by 100 knots.  Nev

[old man emu]

7 minutes ago, facthunter said:

Application of most controls will cause more drag.

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 

[Yenn]

It depends upon what you call lift. If lift in your eyes is a force acting against gravity, then the elevator usually produces no lift. But if lift is a force perpendicular to air flow, then the elevator usually produces lift, but it is usually acting with gravity.

The rudder is easier to envisage. There is drag when it is deflected, but the movement in yaw is caused by the lift sideways.

The elevator lift, drag is highly influenced by C of G location.

[facthunter]

Lift is not quite the right word but we probably know what is meant. It's all  aerodynamic forces. Some flaps are predominantly DRAG but they still help and spoilers are lift destroyers of great effectiveness even though they only work on the top wing surface. Nev

[facthunter]

OME differential spoilers will kill lift and create drag at the same time such that they work quite naturally and normally show no adverse YAW. EVEN differential aileron generally shows adverse Yaw which Frise aileron often corrects by adding drag on the inner wing reducing the need for Rudder often . I'd prefer to just use rudder myself as required. Nev

Edited December 5, 2020 by facthunter

[Thruster88]

3 hours ago, 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 

I would guess about 95% lift and 5% drag based on my internal g sensor.  

[RFguy]

I like fully flying tailplanes.

[spacesailor]

The wrights used " twist in the wing " to produce bank.

Not much drag there .

spacesailor

 

[facthunter]

 Scary, RF. Efficient but  too elastic and inclined to flutter. Best is adjustable horizontal stab and it acts as another pitch control if one fails. How do I know that?  Also the pole stays in the same place at all trimmed speeds/angles. Plenty of range also to cover high speed. It was the ONE thing I didn't like on my twin Comanche Nev

[Jase T]

I assume you use the word 'lift' in your original question to mean a force in the upward (in relation to gravity) direction? Because in reality they work with the fixed part (or they change their AOA for a flying tailplane / fin) to make a variable  aerodynamic surface and the production of lift in one direction or the other is what they do, and what they are designed to do. 

 

In a lot of ways the direction of lateral lift depends on your CofG.. Whilst the tail plane should (I hope) be producing 'lift' in a downward direction normally (god help you if it isn't and you stall) there may be times when a large nose down movement is required and the stab is producing lift in an upward direction. Dont forget you need to include this downward lift in the total upward lift the aircraft is required to produce... Yes they produce drag (you get nothing for nothing) the production of lift always creates drag, but I believe they are designed to create lift in one direction or the other to change the attitude of the aircraft in the way you want them to. Then there are the effects if there is a disturbance away from straight and level... The stab/elevators and the fin / rudder dont care what direction they are pointed they will produce lift relative to the angle between their chord line and the relative airflow....

Edited December 5, 2020 by Jase T
I cant spell

[RFguy]

Hi Nev

Fully flying tailplanes.

So you say Scary because of the redundancy issues ??

 

Yes,  efficient !

OK downsides- flutter. But these are construction defficiencies? Or is it hard to build to do the job?

I see many air transport planes use them.

 

Ahhh I read- good for when a control surface gets too heavy. But used on small planes, does not provide feedback of control surface pressure.

 

 

 

Edited December 5, 2020 by RFguy

[Jase T]

10 minutes ago, RFguy said:

 

OK downsides- flutter. But these are construction defficiencies? Or is it hard to build to do the job?

I see many air transport planes use them.

 

Many go very very fast aircraft use them as well.....

 

 

 

[RFguy]

Now reading about twin comanches and their fully flying tailplane / stabilator. 

flutter, attachment bolts problems.. etc oh these are not people's favourite parts of that aircraft......

 

Golly there is a lot of this topic. geeeez wizz

 

Aeroprakt Vixxen also has full flying tail. Most likely contributes to its speedy performance (blended fuselage aside) 

But doesn't have a very high range of speed, say compared to the twin comanche.. 

 

[old man emu]

3 hours ago, Jase T said:

I assume you use the word 'lift' in your original question to mean a force in the upward (in relation to gravity) direction?

Actually, I was trying to imply a reaction to forces generated when a fluid moves over a surface that has more curvature on one side than on the other. We accept that generally we talk about Lift as the resultant force of air moving from fore to aft across a wing. Let's not get into the Bernoulli -v- Newton debate just now. 

 

Now the horizontal stabiliser is a wing. The elevators are moveable parts of that wing and when they move, is the result of that movement a change the camber of the wing and therefore change the Lift produced, or do they simply expose more surface to the airflow, creating Drag.

 

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?

[pmccarthy]

The word deflection would be more appropriate than lift. Many control surfaces have no significant aerofoil , but can be considered as flat plates. So they produce forces due to deflection of the airstream and drag.

[Jase T]

I have been using Rod as a goto guy to explain stuff for a while now. This is the best explanation out there on how it all works in my opinion. I  think it is clear from this that most surfaces produce lift as their primary purpose the drag is a byproduct. 

 

 

[aro]

I don't understand the question. Of course they produce lift.

 

Drag is an unavoidable byproduct of lift, but the drag does not produce a force in a useful direction (with the exception of odd configurations, like aircraft with the rudder on the wingtips).

[old man emu]

15 hours ago, Jase T said:

I assume you use the word 'lift' in your original question to mean a force in the upward (in relation to gravity) direction?

Actually, I was trying to imply a reaction to forces generated when a fluid moves over a surface that has more curvature on one side than on the other. We accept that generally we talk about Lift as the resultant force of air moving from fore to aft across a wing. Let's not get into the Bernoulli -v- Newton debate just now. 

 

Now the horizontal stabiliser is a wing. The elevators are moveable parts of that wing and when they move, is the result of that movement a change the camber of the wing and therefore change the Lift produced, or do they simply expose more surface to the airflow, creating Drag.

 

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?

 

In aircraft of low Mass, if you put your hand and arm out of the cockpit into the airstream, you can induce a movement (usually yaw) to the side your arm is located. That has to be completely due to Drag.

[aro]

"Lift on one side" is an inaccurate picture.

 

Lift is the result of a difference in pressure between one side and the other.

[old man emu]

1 hour ago, aro said:

Lift on one side" is an inaccurate picture.

I'd say more simplistic than inaccurate. You have to allow a little bit of leeway in how things are expressed in written form. Too technical and the entry will go on for pages. Too simplistic and it trivialises the entry.

 

1 hour ago, aro said:

Lift is the result of a difference in pressure between one side and the other.

This statement puts you in the Bernoullian camp, facing off against those in the Newtonian camp. The true answer is still hotly debated, but until that truth is defined, then both camps will contribute to the explanation in various situations. 

 

14 hours ago, old man emu said:

Let's not get into the Bernoulli -v- Newton debate just now. 

 

[Yenn]

OME your question about lift and drag on the rudder makes me think of applying a burst of throttle and full rudder to start a turn on the ground. Full left rudder and a burst of power moves the tail to the right. If there was a lot of drag it would move to the left, so the answer is clear. It is lift that causes the movement. Bournelli would say that was negative pressure on the right side of the rudder. I say, as would Newton that it is deflecting the air to the left, that causes movement to the right.

[aro]

 

39 minutes ago, old man emu said:

I'd say more simplistic than inaccurate.

No, it's fundamentally wrong. I don't even know which side you are suggesting the lift comes from.

 

 

39 minutes ago, old man emu said:

This statement puts you in the Bernoullian camp, facing off against those in the Newtonian camp.

 

Dividing people into Bernoulli and Newton camps is completely misunderstanding what Bernoulli and Newton are about. They are both correct* - they are just different ways of looking at the same thing. It depends on whether you choose to view air as a fluid or a stream of molecules.

 

* Some people - usually those who divide people into Bernoulli and Newton camps - use Bernoulli's principle incorrectly. But that doesn't mean Bernoulli was incorrect.

 

[APenNameAndThatA]

On 06/12/2020 at 12:45 PM, aro said:

 

Dividing people into Bernoulli and Newton camps is completely misunderstanding what Bernoulli and Newton are about. They are both correct* - they are just different ways of looking at the same thing. It depends on whether you choose to view air as a fluid or a stream of molecules.

 

* Some people - usually those who divide people into Bernoulli and Newton camps - use Bernoulli's principle incorrectly. But that doesn't mean Bernoulli was incorrect.

 

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