photofly wrote:

dr.aero wrote:

Incidentally, for single engined Cessnas the tailplane is a symmetric airfoil, and for a large portion of the cg envelope the tail is providing lift and not downforce.

Not true at all! Who told you that?

Start a new thread on that, and I'll tell you how I know.

Here it is!

By measuring it.

The C182 has a very boring NACA 2412 airfoil, which has a centre of pressure very close to the quarter chord for a wide range of angles of attack.

Here's the engineering drawing of the C182, from the POH, with the quarter chord drawn in. It's at station 37" from the firewall: The CG limits for the aircraft are 33" aft of the firewall to 48.5" aft of the firewall.

Any time the CG goes rearwards of about 37" (which is most of the CG range) the tail is lifting.

Interesting..... maybe I need to start teaching C of G a little differently.

Confirmation?

You need to think about more than the C of G and C of L. The thrust drag couple produces significant upward torque. Also, lift will not be even on both stabilizers due to spiralling slipstream.

This is what I got in X-Plane with full aft C of G in a 172.



Of course, tying some string at the stabilizer tips and seeing which way they spin would be a sure fire way to find out.

Any time the CG goes rearwards of about 37" (which is most of the CG range) the tail is lifting.

Um... Hang on here... I'm not sure I'm with you on this. Were the force which the horizontal tail exerts around the pitch axis to change from a downward force to an upward force ("lifting"), the pilot would experience control reversal, which is extremely disconcerting, and not approvable.

It would result in very unpleasing stall handling characteristics, and be graphed something like this,



(The reference to "pounds" is control force measured in pitch, with "-" being a push force.)

(and "Tq" is the % engine power being produced)

which I recorded during a flight test in a Cessna like derivative aircraft. This was definitely not certifiable, and downright scary when I first experienced it during takeoff.

Could we take a step back and rethink the aerodynamics and physics here?

The thrust drag couple produces significant upward torque

The analysis works even when the engine is idling; no thrust, no pitching moment from thrust, and no asymmetric slipstream either.

The x-plane shot is interesting: there's positive lift from at least one side of the horizontal stabilizer so I think it's fair to say that the tail is lifting, overall; maybe you could repeat for a power-off dive?

PilotDAR wrote:Were the force which the horizontal tail exerts around the pitch axis to change from a downward force to an upward force ("lifting"), the pilot would experience control reversal, which is extremely disconcerting, and not approvable.

Could we take a step back and rethink the aerodynamics and physics here?

The aerodynamics is fine; pitch stability doesn't depend on a wing up-force and a tail-downforce; it depends on how the wing force and the tail force change with angle of attack, and which changes faster. Buzzwords for future Google searches include "decalage" and "stick-fixed neutral point".

I'm not claiming anything radical, I don't think. Even the aerodyamics-for-pilots books don't claim that the tail always has to generate a down-force (at least, the better books don't) but they say "usually". In this case I don't think it's even "usually".

people,, you are forgetting about the pitching moment the wing creates... There will never be "lift" from the tail in a 182. It will always be counteracting the wing (ie if you try to fly inverted, the tail will still be opposing the wing)

The only aircraft I can think have that have "lifting" tails are canards... mind you, they dont really have tails.


S

Which way does that pitching moment go Strega? Take a balsa wing off a model glider and throw it forward and see for yourself.

But it is irrelevant in a system where there is a tail attached and a C of G that is independent of the wing design, as we already have accounted for the C of L being around the quarter chord. The pitching moment doesn't make good for the tail producing more downforce.... quite the opposite actually.

The wing trys to always "go straight on" or if you are looking at the 182 "engineering" dwg posted above.. it is always counter clockwise.

You will always need "downforce" from the tail to counteract the pitching moment for a given aoa

iflyforpie wrote:Which way does that pitching moment go Strega? Take a balsa wing off a model glider and throw it forward and see for yourself.

But it is irrelevant in a system where there is a tail attached as we already have accounted for the C of L being around the quarter chord. The pitching moment doesn't make good for the tail producing more downforce.... quite the opposite actually.

I think you better double check that...

An airplane with lifting tail is a very unstable airplane, push more air backwards and nose goes down, push less nose comes up. That does not represent how a 172 behaves. The thing is, if I got this right, that even if the CG is behind the CP thus creating a "nose up moment" the force within the wing itself creates a nose down moment. This nose down moment is only created by asymetric airfoil and is greater at samll angles of atack (fast speeds). Adding to this there is the nose down moment of the trust line above the CG. Thats is why there is a need for a tail down force, wich is a need for certification.

No, it goes the other way Strega, clockwise. Propeller blades do the same thing.

The only exception is during stall when the C of L moves rearward.

iflyforpie wrote:No, it goes the other way Strega, clockwise. Propeller blades do the same thing.

The only exception is during stall when the C of L moves rearward.

nope....

people,, you are forgetting about the pitching moment the wing creates.

If you want to examine stability criteria, you need to be a bit more sophisticated, and analyze all the pitching moments and how they change with AoA.

An unreflexed bare wing with positive camber has negative pitch stability; the pitching moment coefficient Cm increases with AoA. The same goes for the fuselage. However, add a tail on a long arm, arrange the tailplane to be of a sufficient size and have it fly at a more negative angle of attack than the main wing - so it has stiff positive pitch stability - and now you've achieved the basic criteria for pitch stability of the aircraft as a whole ("trimability"). But the way the mathematics works out, and the way it works out in real life, is that you can indeed have a stable aircraft where the tail lifts. It's not unstable.

The only aircraft I can think have that have "lifting" tails are canards... mind you, they dont really have tails.

An excellent example. There's no aerodynamic difference created by shrinking the wing at the front of a regular aircraft into a canard and enlarging the tail until it becomes the new main wing. The same analysis applies, and will tell you that the canard in front flies at a higher angle of attack than the main wing, which makes the system stable in pitch even though both surfaces are lifting.

Thats is why there is a need for a tail down force, wich is a need for certification.

There's no certification requirement for a tail down force, only for pitch stability.

Hard to get that pitch stability on a single engine cessna like airplane without tail down force.....

Strega wrote:

iflyforpie wrote:No, it goes the other way Strega, clockwise. Propeller blades do the same thing.

The only exception is during stall when the C of L moves rearward.

nope....

Uhm.... yes. Try and keep up Strega, I am not talking about the C of G C of L pitching couple..... simply because that is what we were talking about the whole time here before you showed up.

Then you added this.

Strega wrote:people,, you are forgetting about the pitching moment the wing creates...

And by wing I assume you mean wing, like the wing itself, not the wing in combination with the aircraft C of G..... which would create a downward moment except..... IF THE C OF L IS IN FRONT OF THE C OF G...... which was the whole reason for this discussion in the first place.

So, I go on to assume that you mean the forces on the wing itself.... those forces with the wonders of calculus come together to form our C of L. Fine..... but how does one force by itself produce any moment? Simple, it isn't one force, it is the sum of many forces, not all acting on the same point or in the same direction.

You will find, Strega, that if you separate the forces that produce the C of L only, they will produce a pitching moment that will increase the angle of attack. Again, go take a balsa wing glider, take the wing off, throw it away from you lengthwise and observe the direction of rotation.

It is not supportive of increasing the downforce on the tail of the aircraft.

You will always need "downforce" from the tail to counteract the pitching moment for a given aoa

I got Strega's back....

In the case of a standard configuration properly loaded plane, the tail will always be counteracting a downforce by "lifting" downward, never upward. That downforce will be a combination of C of G vs C of L position, and pitching moment of the wing. If it is a very far aft C of G, it is still a combination, but effectively only the pitching moment which is being balanced off by the C of G force, so less trim required than for forward C of G.

I do not have the academic aerodynamic qualification to argue this from that side, but practically:

Load the 182 any way you want within it's C of G limits, set the elevator trim to the "nose down" position, and it will always require an elevator pull force to get airborne during the takeoff roll. Irrespective of the loading, you must overcome the pitching moment of the wing, which will increase with speed (and is affected by flap position). As the plane accelerates after takeoff, the C of G position is not changing, but the pitching moment is, so you're going to have to trim nose up - no matter how you are loaded. This is because the tail is having to "lift" more down to counteract a pitching moment which is increasing with speed.

Consider that all flying wing aircraft airfoils have a "reflex" upturned trailing edge to counteract the wing's pitching moment, because they have no tail to do it.

https://www.google.ca/search?q=flying+w ... 1024%3B791

I fly a certified tailwheel plane, whose photo appears to the left. When I begin my takeoff (with any C of G loading case), I will do so with the elevator held full down. I will continue to hold full down until the tailwheel lifts off the runway. THEN the tail IS lifting up = up force, not downforce. But, before the plane will lift off the runway, I will have to raise the elevator to change that lift into a downforce. The pitching moment has entered the equation due to an increase in airspeed ('cause the C of G did not change during my takeoff roll), and I must now use elevator = tail downforce to overcome it. (If I don't pull back, the plane begins to take on a very alarming attitude on the ground!)

If the plane did not fly this way, it would require immense piloting skill, because it would not meet the design requirement which reads: (my bold)

Sec. 23.173

Static longitudinal stability.

Under the conditions specified in Sec. 23.175 and with the airplane trimmed as indicated, the characteristics of the elevator control forces and the friction within the control system must be as follows:
(a) A pull must be required to obtain and maintain speeds below the specified trim speed and a push required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained, except that speeds requiring a control force in excess of 40 pounds or speeds above the maximum allowable speed or below the minimum speed for steady unstalled flight, need not be considered.

That requirement is applicable to all C of G positions

Consider this photo:



I took that, while riding in the back seat, checking out the company pilot of the aircraft. At the point I took that photo, the aircraft was quite aft C of G ('cause I was in the back seat), and the pilot had the stick held full nose down, which equates to the 50 KIAS, Torque 80% 30 flap position on this graph:



Notice that on this graph, there is no control force change between 55 and 50 KIAS, and it is a push force. If you were to allow the plane to enter a turn, you could build up G, but have no corresponding increase in stick pull force - you'd probably have to push, but I don't know, I did not try that!

I was checking out this pilot, demonstrating what I had found while doing the testing to get the data to plot this graph. He was rather started by the control reversal. But the C of G had not changed during flight.

This WAS a case of the tail lifting during flight, which is really unsafe. It is manageable, with great pilot skill, but not certifiable, In this configuration, it have a large pitch force null zone = no feel. I had first found it just after coming airborne on takeoff, when full nose down was required to maintain control - scary! This plane is not Canadian Type Certified.

In pitch, planes have "feel". It is this "feel" which allows the pilot to feel the approach to a stall, or a G build up. This "feel" is produced by the variation of the downforce required (pilot action by control or trim) to keep the plane aerodynamically balanced. If there were any configuration where the plane was flying with an upforce (lift) instead, that feel would either not be there at all, or would be reversed. Very hard for a pilot to interpret to fly by feel. I did experience this control force reversal very close to the approach to the stall in another type which CS would like to check out in. Not a GA type though, so the average GA pilot is safe from blundering into that.

So, Instructor Mike

Interesting..... maybe I need to start teaching C of G a little differently.

Confirmation?

No, please keep teaching this properly!

@PilotDAR
I don't know about a Cessna specifically, but the discussion "is it generating lift upwards or downwards" vs "a pull must be required" vs trim arguments is totally unrelated.

For example, theoretically, if a tailplane generates downward lift, then we might have (fictive numbers):
PULL YOKE - elevator fully up - downward force of 100 N
PUSH YOKE - elevator fully down - downward force of 10 N

If you trim it "in the middle" (at downward 55N), you would feel a downword force of 45 N when you pull the yoke and an upward force of 45 N when you push the yoke fully.

On the other hand, if a tailplane generates upward lift, we might have
PULL YOKE - elevator fully up - upward force of 10 N
PUSH YOKE - elevator fully down - upward force of 100 N

If you trim it "in the middle" (at upward 55N), you would feel a downword force of 45 N when you pull the yoke and an upward force of 45 N when you push the yoke fully.


In whatever situation you are, you will still need to execute a pull force for the nose to go up/fly slower when trimmed.


Regarding your example:

I fly a certified tailwheel plane, whose photo appears to the left. When I begin my takeoff (with any C of G loading case), I will do so with the elevator held full down. I will continue to hold full down until the tailwheel lifts off the runway. THEN the tail IS lifting up = up force, not downforce. But, before the plane will lift off the runway, I will have to raise the elevator to change that lift into a downforce. The pitching moment has entered the equation due to an increase in airspeed ('cause the C of G did not change during my takeoff roll), and I must now use elevator = tail downforce to overcome it. (If I don't pull back, the plane begins to take on a very alarming attitude on the ground!)

This is also irrelevant regarding downforce or upforce. When you are accelerating, your wings will start to generate lift, hence your CoL will start to form. Depending on its location in comparison with CoG , this will cause (if you do nothing) a nose down or nose up effect. Your tailplane , at the same time , will also start to generate lift (upward or downard is determined by the manufacturer to keep it stable). It's positioned so that it will counter this effect and it will +- neutralise the nose down or nose up effect. In any case, you will use your yoke to 'disturb' this neutral point and make sure that, in any way, the tail goes as high as possible, be it by generating as much upward lift as possible, or by generating as less downward force as possible.

Again, this is in theory, I have no idea what the situation is for your specific airplane.

rob-air wrote:Hard to get that pitch stability on a single engine cessna like airplane without tail down force.....

Once your center of gravity moves behind CoL, a down force on the tailplane would create an unstable airplane.

Once your center of gravity moves behind CoL, a down force on the tailplane would create an unstable airplane

I disagree. The downforce on the tail is what creates a stable plane. If there is an upforce, no force, or a transition between down and up force, the aircraft will be unstable.

I agree that the trimming out of the control forces is not a direct indication of there being a downforce, but it is built into the design of the airplane, with either or both of inverted camber of the horizontal stabilizer, and negative angle of incidence.

To satisfy the design requirement:

"Sec. 23.201

Wings level stall.

(a) It must be possible to produce and to correct roll by unreversed use of the rolling control and to produce and to correct yaw by unreversed use of the directional control, up to the time the airplane stalls.

(b) The wings level stall characteristics must be demonstrated in flight as follows. Starting from a speed at least 10 knots above the stall speed, the elevator control must be pulled back so that the rate of speed reduction will not exceed one knot per second until a stall is produced, as shown by either:


(1) An uncontrollable downward pitching motion of the airplane; ..."

To achieve the required uncontrollable downward pitching, the tail must be exerting a downforce all the time during controlled flight, so that when the tail stalls (is no longer aerodynamically capable of exerting the required downforce) it "releases" that downforce, and the nose pitches down. Otherwise, the aircraft would have an unrecoverable stall characteristic. You could get the plane so both the wing (lifting) and the tail (lifting) were stalled at the same time, the plane was headed down, and there was nothing you could do to change the situation.

The aft C of G limit is established during certification flight testing at the point 5% ahead of where the intolerable lack of tail downforce begins to be apparent. I have found this with both the Cessna 206 and 208, when during flight testing spins, it required full forward and held elevator control to initiate recovery, not just the let go and wait which is typical of the 150/152/172.

We know that your most efficient cruise configuration is with an aft C of G, because there is the least drag for the given weight, because the tail is causing the least induced drag, because it is creating the least downforce - but it is not creating an up force, or the induced drag would be going back up again as the C of G moved aft!

If the tail were ever creating an upforce, it would also not be possible to achieve the following design requirement: (my bold)

"Sec. 23.155

Elevator control force in maneuvers.

(a) The elevator control force needed to achieve the positive limit maneuvering load factor may not be less than--
(1) For wheel controls, W/100 (where W is the maximum weight) or 20 pounds, whichever is greater, except that it need not be greater than 50 pounds; or
(2) For stick controls, W/140 (where W is the maximum weight) or 15 pounds, whichever is greater, except that it need not be greater than 35 pounds.
[(b) The requirement of paragraph (a) of this section must be met at 75 percent of maximum continuous power for reciprocating engines, or the maximum continuous power for turbine engines, and with the wing flaps and landing gear retracted--
(1) In a turn, with the trim setting used for wings level flight at VO; and]
(2) In a turn with the trim setting used for the maximum wings level flight speed, except that the speed may not exceed VNE or VMO/MMO, whichever is appropriate.
[(c) There must be no excessive decrease in the gradient of the curve of stick force versus maneuvering load factor with increasing load factor. ]

That means that there must be positive elevator (downforce) control force required to pull G. As per (c), it must not be permissible that the elevator control force decrease as you increase G. This requirement protects the plane from being overstressed by a careless pilot, because the pilot will have to put the muscle into pulling the G, so it won't happen unknowingly.

If ever, the tail were lifting, this could not be achieved - the elevator control forces would be all wrong, and would not increase as G increased. Look at my control force graph again, this unacceptable characteristic is plotted there.

It's sure been a long winter.

@PilotDAR:

I see 2 elements in your post I'd like to address.

I disagree. The downforce on the tail is what creates a stable plane. If there is an upforce, no force, or a transition between down and up force, the aircraft will be unstable.

Do you agree that it is possible that the center of gravity moves behind the center of lift ? If no, please explain. Because from the schematic in one of the posts above, it is possible that the CoG moves behind the CoL.

If 'yes', draw the forces on a line. You'll get

(nose airplane) -- Lift (UP) -- CoG (DOWN) -- Tailplane (X)

Now, the airplane rotates around the CoG. If the Lift goes up, the X needs to go upward as well. If not, you'll create a moment which will make the airplane rotate like a windmill. It's like 2 children sitting on a swing: for the swing to be in balance they both need to sit down or stand up.


Regarding the stall: I do not know enough about it to discuss it in depth. However, the tailplane stalls later than the wings, which allows you to keep the aircraft stalled or recover from it (but that's more Colonel Sanders' expertise )


Regarding the elevator control forces:
as far as I know there is no way you can deduct which way the tailplane is lifting (up or down) by the stick forces. The stick forces are an indication of how far you are "away" from the equilibrium point. This equilibrium point is roughly set by horizontal stabilizer and finetuned by your elevator trim. This is the equilibrium. Let's say the tailplane generates a force X on the airplane in equilibrium. You won't feel anything in your yoke. It doesn't matter if this force is positive or negative. What you will feel is an increase in lift (which is the same as a decrease in downforce) when you push the yoke or a descrease in lift (increase in downforce) when you pull the yoke and move away from the equilibrium.

The nose down moment created by the wing (the forces within the wing itself), conteracts and overcomes the "nose up moment" created by a CoG behind CL. Thus requirering a tail down force even with the CoG at the aft limit.

If the C of G is behind the C of L of the main wings then theoretically you have (up) lift from the tail.

If based on that assumption the wings stall first, then the tail would still be providing lift as long as the critical angle for the tail hasn't been reached. If that's the case the loss of lift from the wings plus the continued lift behind the C of G would still lead to the aircraft pitching down in a stall. In theory anyway. It does depend on what the critical angle for the tail is in relation to the main wings.

And don't worry PilotDAR, I'm not going to change the way I teach on a whim. If however, this is anything like the way Bernoulli and equal travel used to be taught for theory of flight, I need to consider the possibility of change.

I'm on the edge of my academic qualification in discussing C of L and C of P topics, but, I will assert that the C of G of the aircraft will not ever be aft of the center of total lift provided by the wing, including any pitching moment for the wing in that configuration. The drawn schematic does not include the pitching moment of the wing.

The higher the G loading acting on the plane, the more load acts downward through the C of G. Therefore, to remain in equilibrium, the higher the lift must be from the wing, through its center of lift. Therefore the greater the downforce must be from the tail to balance the increased lift produced by the wing, from the combination of increased AoA and/or speed. This is how Va is determined, the speed at which the aircraft stalls before it is overstressed. The tail stalls, and releases the download, which reduces that angle of attack of the wing, thus preventing overstressing. Faster than Va, the tail is capable of exerting so much down force that, the wing can be overstressed.

The wing may stall before the tail, but not necessarily. When you are in unaccelerated slow flight, and you pull back too much, the nose drops. That could be because the wing has stalled and the front of the plane goes down, or it could be because the tail stalled, and the back of the plane went up. It's kinda hard to tell sometimes. However, when you consider aircraft like the Cessna 177 Cardinal, which have a stabiliator, it is definitely possible to stall that, and drop the nose before the wing stalls at all - at any C of G position. This is the reason why when you look at one, it has an inverted slot in a part of the leading edge. The early ones did not, and were found to be wanting in low speed handling, resulting in some banged nosewheels. The tail had a downforce all the time, though sometimes not enough relative to the whole plane's forces.

It's not the force on the elevator control, but the change in force per change in G (with no change in trim) which matters. In the graph I presented, the equilibrium point is 70 knots, where I have trimmed out the pitch force to zero, from there, I did not retrim, but measured the force on the stick as I slowed, and changed power. When you see my results showing a push force as the plane slowed, and worse a pitch force which did not change as the speed further reduced, you are seeing a plane which is very unstable, and difficult to fly. I cannot tell you if the tail was actually lifting at that point, or just providing a very small down force relative to all other forces, but it was not approvable. If it actually was lifting, it would just be the same or worse than what I measured, and certainly not approvable.

Remind yourself that pilots who have only ever flown type certified GA planes have a sense of "normal" which has been very carefully designed in to conform to the certification requirements. There is a whole other realm of aircraft that do not fly as well, and may have very different characteristics. Military and amateur builts being the most common examples. Sometimes the instability is purposefully designed in (aerobatic types), sometimes it's just a poor design (what I have graphed).