Single engine Cessna tailplane lift

[dr.aero]

Colonel...

Please explain! Every time I've put a trimmed airplane from steady upright flight into inverted flight, I've needed to hold significant nose down pressure on the stick to keep the nose from dropping towards the ground.

[Colonel Sanders]

What types?

Also, were you flying with a forward or rearward C of G?

Flat-bottom wing with a forward C of G - sure, lots of forward stick

Round wing with rear C of G - not so much

[dr.aero]

photofly...



This guy seems to know what he's talking about. That is what he says on the matter.

Another smart man, Owain, has some things to say on the matter:



It is, I think, clear that if look at the Cp and Cg positions, that will tell you what lift you need to have on the tail to balance it. Or does it? It gets more complicated because you have to take into account the aerodynamic moment of the fuselage and the thrust and drag couple on each specific airplane. Does anyone on here know where the drag force acts on a Cessna 172 in cruise? What about in slow flight? It's important to know that if you're trying to consider if the tail creates upward lift or downward lift. What about the thrust force? Is it inclined to the velocity vector of the airplane? What component of thrust affects lift? How does that change in required Cl of the wing affect the required lift force on the tail?

This isn't a simple question of analyzing 3 forces in a couple and seeing how it balances.

There are airplanes that can create an upwards lift force on the tail and be legal to fly. With reversible controls, that's very difficult because upwards force on a tailplane has a destabilizing effect. Having powered irreversible controls with FBW technology to power them can take advantage of the added maneuverability or increase in efficiency afforded by a very rearward Cg.

[dr.aero]

Colonel...

It was a Pitts S-2B.

[dr.aero]

For the first plane in the video, when the bomb bounces and takes off the tail, the plane rapidly pitches down, so badly that the wings bend down because of negative G. That's because the tail was holding the nose up, until it was broken off then the nose went down!

There was a significant amount of mass that departed that airplane, and being at the end of the airplane, it would have had a large effect on the Cg position which would have contributed to the nose down pitching moment.

[photofly]

It is, I think, clear that if look at the Cp and Cg positions, that will tell you what lift you need to have on the tail to balance it. Or does it?

It depends.

This isn't a simple question of analyzing 3 forces in a couple and seeing how it balances.

But it's a very good place to start, if you have reason to believe that those three forces are the largest contributions.

As far as the tail stalling first, it's not necessary for it to fly at a negative angle of attack for that to occur. It's necessary only for it to fly at a more negative angle of attack than the front wing (this is also known as positive decalage.) But that's a stability consideration, not an equilibrium consideration. We can take the stability of the 182 for granted at all legal C of G.

If the CP is 15 inches forward of the rear CG limit for some reachable AoA, none of the other considerations are going to change the required tailplane contribution from positive to negative or vice versa. If the difference is only an inch then - perhaps.

The mistake I made was to imagine that the CP was much close to chord/4 than it turns out to be. Which was a bad error, and I'm glad to have had the opportunity to correct it.

because upwards force on a tailplane has a destabilizing effect.

It's not destabilizing, just less stabilizing. And it doesn't guarantee instability. The tail force goes upwards when the cg passes behind the c.p. of the wing (+fuselage). But the cg limits for stability are the stick-fixed and stick-free neutral points, which include the stabilizing contribution of the horizontal stabilizer. Moving the c.g. rearward decreases the downforce from the tail, and also reduces pitch stability.

But the zero point of tail downforce is not the same as the point of neutral pitch stability. Owain Glyndwr says the same thing: "if you bring the cg aft of the cp you can trim with upwards lift on the tail".

So those who say "it's impossible for an aircraft to be stable with a lifting tail" aren't right. Canard aircraft do it all day.

[Colonel Sanders]

It was a Pitts S-2B

Gotcha. Have the owner call Dave Schwartz
in Florida. I think that's all I should say about
that here.

PS If you ever fly an S-2C - wonderful airplane -
it has a "square stick", as Eddie at Aviat called it,
where the positive and negative G forces were
identical.

[dr.aero]

photofly...

I really gotta catch a flight... I've skipped two so far today! I'll reply to you later.

Colonel...

I want to buy a S-2C! But I also want to buy a MX-2 and maybe a few other airplanes

I'll pay attention to that if I fly a S-2C.

[photofly]

photofly...

I really gotta catch a flight... I've skipped two so far today! I'll reply to you later.

No problem. Have a good flight.

[digits_]

Apparantly we are not the only ones discussing it: http://www.av8n.com/how/htm/aoastab.html

Read paragraph 6.1.6

Here’s an explicit example. I’ve actually done the following experiment:

I took a Cessna 172 Skyhawk and put a couple of large pilots in the front seats, with no luggage and no other passengers. That meant the center of mass was right at the front of the envelope, so the tail had to produce considerable negative lift in order to maintain equilibrium. There was lots and lots of angle of attack stability.
I took the same Skyhawk and put a small pilot in the front seat, a moderately large mad scientist in the back seat, and 120 pounds of luggage in the rear cargo area. That put the center of mass right at the rear of the envelope, so the tail had to produce considerable positive lift in order to maintain equilibrium. The airplane still had plenty of stability. (As far as the pilot could tell, it was just as stable as it ever was.)

Apparently he tried it by tieing some strings on this tailplane.
who will try this next ?

[photofly]

Apparantly we are not the only ones discussing it: http://www.av8n.com/how/htm/aoastab.html

Read paragraph 6.1.6

Here’s an explicit example. I’ve actually done the following experiment:

I took a Cessna 172 Skyhawk and put a couple of large pilots in the front seats, with no luggage and no other passengers. That meant the center of mass was right at the front of the envelope, so the tail had to produce considerable negative lift in order to maintain equilibrium. There was lots and lots of angle of attack stability.
I took the same Skyhawk and put a small pilot in the front seat, a moderately large mad scientist in the back seat, and 120 pounds of luggage in the rear cargo area. That put the center of mass right at the rear of the envelope, so the tail had to produce considerable positive lift in order to maintain equilibrium. The airplane still had plenty of stability. (As far as the pilot could tell, it was just as stable as it ever was.)

Apparently he tried it by tieing some strings on this tailplane.
who will try this next ?

I will, if I ever get the thing back from the paint shop.

[Strega]

If you loaded a 172, in such a manner the tail had to create "positive lift" ... you would have to be a much better pilot than most as it would be unstable to the point it would be uncontrollable.


You cannot just simply look at the geometry of a static aircraft, and ascertain as to what the loads are in the tail (making assumptions as to where the aerodynamic center of the wing is in relation to c of g ect) It just doesnt work. Stability has to include all of the pitching moments about the the c of g ,including the pitching moment of the tail, wing, fuse, ect) If the tail were to ever become a "lifting surface" the aircraft would be very unstable. Something else thats also very important is a term reffered to as the tail volume ratio, but again, understanding of partial derivatives is required to grasp.

Wiki has some more good stuff...

[PilotDAR]

Having read the posts which suggest that the tail of a certified GA aircraft could carry an upload during unaccelerated flight, how then would the aircraft comply with the design requirement I presented earlier, which reads in part:

"(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."

If the magic plane, whose tail is carrying a part of the aft C of G load, were to be flown more slowly from trimmed flight, without a trim change, what would the force in the pitch axis experienced by the pilot be?

Would the pilot have to pull to fly more slowly?

No, because more slowly equals less "lift" created by the tail, so it will settle because it cannot sustain the load it was carrying at the faster speed, the wing AoA will increase, and the plane will slow more. The pilot will end up having to push to not have the plane rapidly slow down! I have experienced this twice during test flying, and it is scary.

So the fellow who "tested" his 172 by loading it aft (within limits, I sure hope!). When he applied power for takeoff, was the tail tiedown ring on the runway, and it picked up, as the tail began to create lift during the takeoff roll?

It reminds me of the occasion when I flew the owner and his daughter up to pick up his 172, which had been hangared at a farm for some months. Turns out that someone had plowed up the field between the hangar and the runway - the taxiway was gone! So I instructed them to both sit in the back seat, and after I was in the front seat, I had her carefully climb into the baggage compartment. In that configuration, the plane was well behind it's aft C of G limit. I started it, extended 15 degrees of flap to improve the downwash over the tail, added power, pulled the controls all the way back, and the nosewheel just came off the ground as I started to move forward. The tail did not carry any upload as I waddled it out. When I released the controls, the nosewheel came back down, so the tail was not carrying an upload.

I have flown many planes which were precisely loaded to the aft C of G limit for the purpose of handling tests. With the exception of the two I have mentioned, which did require a push during slow flight (were both deficient, and not certifiable), all others required a pull to fly more slowly, which meant that the tail was still providing a down load at all times.

[photofly]

all others required a pull to fly more slowly, which meant that the tail was still providing a down load at all times.

The "push" and "pull" is stick-free stability. The direction of the horizontal stabilizer force doesn't determine the stick-free neutral point. I'm sorry, PilotDAR - it just doesn't.

Here's a first class pilots-eye view of longitudinal static stability:
http://www.flightlab.net/Flightlab.net/ ... 2BA158.pdf

You can compare stick-fixed and stick-free criteria, and their relationships to when the cg passes aft of the the stick-free and stick-fixed neutral points. But those points don't determine the direction of the force provided by the stabilizer that is determined by whether the cg is fore or aft of the cp. The cp is not either of the neutral points.

Bill Crawford, who wrote that document, runs an unusual attitude training course, with an academic ground school attached. Who's up for a trip down to Plymouth?

[Chuck Ellsworth]

Reading this thread has been very enlightening for me.

Two things stand out.

First is I am truly grateful no one knew the depth of my ignorance about flying airplanes during my career, in fact it is disturbing to think they actually paid me.

Second thing that I now understand is my plan to go back to teaching people the finer points of flying will have to be discarded because if anyone ever found out how little I understand the art of flying it would be unbearable to my self worth.

Thanks guys for this thread.

[photofly]

If the magic plane, whose tail is carrying a part of the aft C of G load, were to be flown more slowly from trimmed flight, without a trim change, what would the force in the pitch axis experienced by the pilot be?

Would the pilot have to pull to fly more slowly?

No, because more slowly equals less "lift" created by the tail, so it will settle because it cannot sustain the load it was carrying at the faster speed, the wing AoA will increase, and the plane will slow more.

Here are the correct answers:

Stick fixed: the yoke is held still; does the aircraft have a stable pitch attitude? Yes: if it slows, the lift from wing and tail decreases, the aircraft sinks, and the AoA of both wings goes up because of the downward velocity. Let's say that the angle of the airflow changes by 1 degree. The AoA of the main wing goes up from 4 degrees to 5 degrees and the lift increases by 20%. The AoA of the tailplane goes up from 1 degree to 2 degrees. The lift of the tail goes up by 100%. See Denker, for instance, at http://www.av8n.com/how/htm/aoastab.htm ... -stability

The point is that the change in lift of the tail is proportionately more, providing an extra nose-down moment, which is the correct response for an aircraft stable in pitch. You'll notice this works until the tailplane is trimmed to fly at the same AoA as the main wing, which is the same as the (correct) criterion for neutral stick-fixed stability.

Stick free: the elevator is left to float. The aircraft slows; which direction does the stick move? (i.e. would the pilot have to push or pull to hold it still?) This depends on a lot of things, including gearing on the trim tab. Generally stick-free stability is less than stick-fixed, that is, the stick-free neutral point is ahead of the stick-fixed neutral point.

You can find the maths here:
http://www.aerostudents.com/files/fligh ... ontrol.pdf

The condition that has to be satisfied for correct yoke force in equation 2.16. It's quite complicated; not nearly as simple as determining the direction of force on the tailplane.

[Strega]

Photofly.



Do you have any formal education in aircraft aerodynamics/stability?

[Colonel Sanders]

See Denker

I have to ask, since Denker is referred to as God (or
at least Jesus) as the final authority in aviation ....

Is he an absolute wizard in the cockpit? Or is he
all chalk and no stick, if you know what I mean?

It doesn't really matter - I was just wondering.

[photofly]

See Denker

I have to ask, since Denker is referred to as God (or
at least Jesus) as the final authority in aviation ....

Is he an absolute wizard in the cockpit? Or is he
all chalk and no stick, if you know what I mean?

It doesn't really matter - I was just wondering.

His bio is here:http://www.av8n.com/how/htm/author.html

He's not a God at all; I've caught him out in one big whopper (so far) - but he's an extremely smart guy. You should check his texts on fundamental themodynamics, and special relativity. Readable, and interesting. They're on his website too.

Can he fly? When it comes to explaining aerodynamics, I don't care. Any more than I care if an airline pilot or aerobatics display pilot knows how to integrate by parts or prove Galois' theorem. Or, for that matter, whether a person training pilots on flying boats graduated high school.

[photofly]

Photofly.



Do you have any formal education in aircraft aerodynamics/stability?

To the extent that aerodynamics is just maths and physics, then yes. Decades of formal education. I don't claim it's worth diddly in the cockpit, or even very much down on earth, but it is what it is. How about you?

[Colonel Sanders]

Can he fly? ... I don't care

Just a personal bias of mine. I have found that people
at one extreme or the other (all practical and no theory,
or all theory and no practical) can startle you with some
really strange ideas, that you wouldn't expect them to
have.

aerodynamics is just maths and physics

IMHO physics is applied math, and engineering is applied physics

[Strega]

Photofly.



Do you have any formal education in aircraft aerodynamics/stability?

To the extent that aerodynamics is just maths and physics, then yes. Decades of formal education. I don't claim it's worth diddly in the cockpit, or even very much down on earth, but it is what it is. How about you?

Yes.

[photofly]

Yes.

Awesome. Then I look forward to well thought out, finely argued, concise and precise posts from you, with a high level of technical content, to support whatever point you're making at the time.

[Strega]

BTW Im still waiting to see your work showing the "tail lift" on a 172

[photofly]

I showed it for the 182; you want me to repeat it and see if it's true for the 172 also?