Single engine Cessna tailplane lift

[crazy_aviator]

Ive owned that overpriced tail waggler and wouldnt fly it again for a million bucks !!
When the windsock has to hang like a limp dick to fly this craft,, time to get a "real" airplane

[PilotDAR]

Back from the Rockies, and still trying to get my head around this. With due respect to math, I remain unconvinced. Now I freely admit that math is not my strong point, so I will not attempt to refute math, that's someone else's to do (though I did find an error in the math required by an FAA AC recently!).

I DO know, however, that useful and accurate math is as a tool for working things out, it can be no more accurate than the data going in, and the APPLICABILITY of that data to the problem. I remain unconvinced that the math you have presented includes consideration of ALL of the relevant factors. I'm not knowledgeable enough to be able to assert myself as to which, but I have enough experience flying to question it, and hold "math based" assertions to a very high standard, when they are being offered in place of actual flight experience.

Stick vs Chalk - I'm on the stick side, which the chalk having to make allowance for the actual flight test observation before it is credible.

I acknowledge the examples presented by Photofly, though I see dynamic events in there, which I do not consider applicable to the argument. I have always agreed that during dynamic maneuvering, the tail could lift, to restore the aircraft attitude (pitch down) under pilot control.

I suspect that the Concorde flew "well" because a lot of advanced C of G management in flight, with the pumping of fuel as ballast.

In the book "Cessna Wings for the World" by Thompson (which I highly recommend to Photofly), on page 142, the author provides some applicable information relevant to the Caravan:

"As the C of G is moved progressively rearward, the stability deceases progressively until, at some C of G position, the stability vanishes (becomes neutral), and that position is called the neutral point. The following tabulation shows the effect of flight condition on the stick free neutral point:

Condition Neutral Point
Climb 56.2 % MAC
Cruise 50.2 % MAC
Pwr App, Flaps = 0 58.5 % MAC
Pwr App, Flaps = 30 53.5 % MAC

With the desired rear C of G at 40% MAC, these neutral point locations indicated entirely satisfactory margins."

This is with a stated -.75 degree stabilizer angle of incidence.

I scanned the rest of the book for more information on stabilizer angle of incidence, and loading. There is some, but it leaves me with an incomplete understanding.

There has been talk of power on taildraggers affecting tail load. On takeoff, yes. However, I can assure that in power off landings on land or water, I'll land the Teal, and upon touchdown, move the stick fully forward to hold the tail up as long as I can. When speed decays, the tail with downward deflected elevator no longer has the effectiveness to lift the tail, and it settles. THEN, I'm using full forward stick. Otherwise, in that aircraft, I have never used full forward pitch control, other than pitching over during stall recoveries, which is a dynamic maneuver, and I have agreed may involve momentary lifting of the tail.

For reference, the Teal has a C of G range of 18.9 to 26.6% MAC, and a stabilizer angle of incidence of -3 degrees. I don't have the wing AoI. I can tell you that it is much nicer to fly at -3, than the -5 stab AoI, which was how I first flew it years ago.

However, in general, and in the context of the example conditions I have presented, I remain respectfully unconvinced that the tail lifts in steady flight in GA aircraft, and I offer the foregoing Caravan information as an example as to why I believe this (and I HAVE used momentary full pitch down control on the Caravan many times during flight testing).

I think that there remains too large a gap between stick and chalk here....

I have a lot to learn about the background of why planes fly as they do, but I have reasonable experience as to making them do it, and observing the results. In this regard, I know a non compliant aircraft when I fly it. So far as I understand, when I reach the "neutral point" described my Mr. Thompson, I have a non compliant aircraft. I remain believing that could occur if the tail lifted in steady flight, and therefore that is designed out of the aircraft.

[Colonel Sanders]

I remain respectfully unconvinced that the tail lifts in steady flight in GA aircraft

In straight and level inverted flight:



The tail is most definitely providing "upwards" lift,
relative to the pilot's frame of reference.

You probably don't want to talk about sustained
knife-edge (90 degree banked) flight, which some
people might theoretically assert is "impossible"
because the wings are not providing any lift, and
the horizontal stab and elevator aren't doing much
at all, but I can assure you that it is quite possible.

Some day, it would be fun to talk about the physics
of a tumble.

[Chuck Ellsworth]

Some day, it would be fun to talk about the physics
of a tumble.

Yes, especially a power push over tumble in a high thrust line pusher engine gyroplane.

[bezerker]

Well, my drunken math shows about 1000 ft/lbs of rotational force about the aerodynamic centre of lift on the 182 at 100kts.

The C of G being 2 or 3 inches behind this at the aft limit, most definitely needs need some help with stopping the nose from pitching down.

Pitching Moment = Cmc4 x 1/2V2 x S x c

Found Cmc4 online for 2412 with varied AOA and nice average was -0.036

It just seems to make sense to me that the aft C of G limit corresponds with the elevator running out of downforce. The fact that the C of G limit is almost exactly under the C of L, with an extra few inches added for the rotational forces, seems too much of a coincidence.

Cheers.

[photofly]

I remain unconvinced that the math you have presented includes consideration of ALL of the relevant factors.

Well, that's fair. I've included all the ones in the traditional aerodynamics texts (and/or said that I'm not including them, and why.)

You're absolutely right that stick trumps chalk, so I will go and test it.

The quote from Thomson's book (which I shall order, thanks) is consistent. Unfortunately it doesn't tell us where the centre of pressure is, for any of those flight conditions. But it does reinforce that knowing where the neutral point is is very important, not the cp.

@bezerker:

Using your preferred value for Cmc4.

Pitching moment is:
Cmc4 * 1/2 * ρ * v^2 * S * c
(I think you forgot the density.)

In SI units:
-0.036 * 1/2 * 1.2kg/m3 * (51.4 m/s)^2 * 16.2m^2 * 1.75m
= 1617 Nm of torque
= 1192 ft.lbs

Weight of aircraft = 2950 lbs

So if the cg is more than (1192/2950) ft = 4.8 inches aft of the chord/4, the tail needs to lift.
The cg range goes from 33 to 48.5", with the chord/4 at 37".

By your own figures (with which I agree, -ish) any time the cg goes aft of 41.8" the tail has to lift, and it's got another 6.7" to go.

[bezerker]

Well, I am not sure I understand your chord4 number of 37 (hence the diagrams earlier). Are you saying 25% chord distance is 9 inches, for a total chord of 36 inches?

Taking the leading edge as your station 28, and adding 1/4 of a 64 inch chord, does not equal station 37.

28 + 16 = 44 + your calculated 5 inches for torque, and I get 49. Outside your aft C of G.

I'm too lazy to figure out MAC and 100 inches of 64" chord before taper was good enough for my calculations so far.

[photofly]

From the lower scale diagram at the front of the POH, the chord/4 line passes exactly through the wingtip strobes.

When I print it out, on the upper scale diagram, the distance from the wingtip strobe to the firewall, measured perpendicular to the firewall, is 17mm. The nose-to-tail distance, which is 28' +3/4" = 336.75", measures at 152mm on the page. By scaling I deduce that the chord/4 distance from the firewall, which is station zero for cg measurements, is 37.63".

Other measurements from the diagram:
The leading edge is at station 22"
The root chord is 67"
There is no sweepback.
root chord/4 is 16.6"
leading edge+ root chord/4 = 38.6, which is close enough to the direct measurement for me.

I think you're looking at your diagram and deducing the leading edge is at the top of the windshield line. It's not. The leading edge is set forward of the top of the windshield by about 10".

[Shiny Side Up]

Some day, it would be fun to talk about the physics
of a tumble.

Yes, especially a power push over tumble in a high thrust line pusher engine gyroplane.

I know very little about gyro flying, but would that be what is referred to as a "bunt"?

[Colonel Sanders]

I want to know very little about gyroplanes,
but yes, tumbles generally begin with some
variant on an outside snap roll:

www.pittspecials.com/movies/tumble.wmv

[PilotDAR]

tumbles generally begin with some
variant on an outside snap roll:

Well, you can do it with jumpers too. Four of them, trailing from the right wingstrut, suddenly yell "Yahoo' and let go, while you have lotsa left rudder and aileron in trying to keep the 185 level....

[Chuck Ellsworth]

I know very little about gyro flying, but would that be what is referred to as a "bunt"?

Yes and it is always fatal for the people in the gyroplane.

[Trematode]

I'll just leave this here.

[PilotDAR]

There are a lot of thought provoking posts on this thread - and a picture of a bicycle.

[crazy_aviator]

TREM,,,NOW i know where the idea of a taildragger came from and they have been nosing over ever since

[photofly]

This might be of interest to someone.

<file removed>

Comments and corrections appreciated.

Next I will include contributions from the pitching rate dα/dt, which should demonstrate the existence of the short period longitudinal mode (and we can look at its damping) and also the features of manoeuvring stability and how it differs from cruise stability.

[digits_]

I contacted Cessna in the hope to get some experimental data or to fill in the missing numbers from the equations in the first pages. They refused to divulge that information, they did however email this.

Airspeed increases the effective force the horizontal stab will provide

Engine power or prop wash will also have that effect

An aft CG does tend to increase the cruise speed of the airplane by reducing the down force required to maintain level flight

Down force is wasted energy as it increases drag with no added lift

The ideal is to position the CG so as to require neither lift or down force from the tail and to have the fuselage as close to parallel to the relative wind as possible, as the wing is more efficient at producing lift and the fuselage would produce the least possible drag in this configuration, therefore the airplane will operate at the highest cruise speed for a given power setting.


Hope this helps to sum up your questions.

Note the line in bold, which may persuade some people that photofly's theory is in fact correct.

[photofly]

The ideal is to position the CG so as to require neither lift or down force from the tail

I don't believe that to be exactly the case.

If you can reduce the lift of the wing and increase the lift on the tail by the same amount (total overall lift the same) while reducing the drag from the wing by more than you increase the drag from the tail (overal drag) decreases, you get an overall gain. Thus the optimal AoA of the horizontal stab. depends on the relative slopes of the lift vs. drag curves of the two airfoils. I doubt the optimal point is exactly zero AoA on the tail.

I'll try to work out the exact maths later.

[trampbike]

I doubt the optimal point is exactly zero AoA on the tail.

I doubt it too, but I guess it must not be very far from it, since a tail generally has quite a low aspect ratio.


BTW, this thread is awesome, thanks to you photofly.
Althought I very much appreciate your contributions on avcanda, I feel I have to say this to you (as CS told you earlier):

[photofly]

Here's the paper I wrote on the way the elevator/horizontal stabilizer works, with an additional section looking at the stability of the equilibrium. It goes into more detail about where the cg must be for the aircraft to be controllable in pitch, and It goes on to derive and solve a second order ODE which describes the longitudinal short period mode.

I sent it to some professor-type people for comments but I haven't heard anything back. I'll update it in the light of comments if I get any.

stability.pdf

As usual, comments and corrections are most welcome.



I'm working on the condition for the cg position to give minimum drag. I have some nice results, i.e. that the AoA of the horizontal stabilizer should always be a fixed fraction of the AoA of the wing, and that fraction is determined only by the aspect ratios of the wing and stabilizer, which is quite cool. For instance, for a 172, regardless of the airspeed, minimum drag would be obtained when cg is such that the AoA of the stabilizer is 42% of the AoA of the wing, and the stabilizer provides 3.3% of the lift. But I want to check through the algebra a few times before posting the details.

[crazy_aviator]

From Wikipedia

On some airplanes built before World War I, such as the Bleriot XI, the center of gravity is between the center of lift from the wings and the tailplane, which instead of providing a downward force, provided an upward one. However there are severe handling issues with this arrangement that were beyond the capabilities of designers at the time to fix[citation needed], and the approach was eventually abandoned. Examples of aircraft that had this setup include Charles Lindbergh's Spirit of St. Louis, the Sopwith Camel and the Gee Bee Model R Racer - all aircraft with a reputation for being difficult to fly. With computer controls this is no longer a problem and aircraft as different as the Airbus and the F-16 are flown in this condition. The advantage to this is a significant reduction in induced drag caused by the tailplane, and in the case of the F-16, improved maneuverability.

A note: With POWERED controls this can be accomplished easily

Also, with powered controls, Control balance is NOT necessary, disconnect an aileron from the PCU ( hydraulic ram) and your aileron will leave the plane PRONTO !!

[photofly]

Oh dear. Someone needs to put whoever wrote that article in touch with the author of any reputable aerodynamics textbook - or teach them some maths, so they can stop misleading the vulnerable! Oh, and read the talk page behind that article. It's hilariously similar to this thread.
http://en.wikipedia.org/wiki/Talk:Tailplane


See section 3.3.8 (page 204) of this text for a reputable, and correct treatment:
http://books.google.ca/books?id=vROcxJj ... &q&f=false

[photofly]

I found some much more reliable engineering data on the C172, so I've updated the calculations to match. The results I think are a lot closer to what I'd expect to see for the real aircraft.

If I get a chance I will try to use the "standard" experimental methods to find the position of the stick-free neutral point next time I'm out flying.

stability.pdf

(555.3 KiB) Downloaded 105 times

Here's the work I've done on trim drag; consider it a draft at the moment.
Edit: updated with some more data for induced drag at different airspeeds.

min_drag.pdf

(289.53 KiB) Downloaded 111 times

[guy]

There is a continuity of aircraft wing configurations from conventional through Delanne tandem, equal-area tandem, small-foreplane tandem to canard wing.

Clearly, on all but the conventional type the rear surface is always lifting and all these types can be designed to fly stably.

Conversely, we also know that many conventional tailplanes spend a lot of time pushing down (and causing trim drag).

The question boils down to, do (m)any conventional tailplanes spend some, or even most, of their time with positive lift?

Given the continuum of designs, this clearly depends to some extent on the size of the tail plane (i.e. how far to the extreme end of the continuum it is) and on the momentary relationship between CG and centre of lift (both of these typically move to and fro during flight). A categorical "never" would be a dangerous statement to support.

I have also noticed that some analyses of conventional aircraft stability consider the zero-lift condition. here, it is obvious enough that in any stable the tailplane must exert a downforce. When lift is appled by increasing the AoA, it is sometimes forgotten that this downforce remains relative to the wing lift which, quickly becoming much stronger as the nose rises to the cruise position, may quite possibly sum to a small positive lift from the tail.

[digits_]

Given the continuum of designs, this clearly depends to some extent on the size of the tail plane

No. The size of the tail plane will only influence the required angle of attack necessary for this tailplane the generate the correct amount of lift/downforce created. It will however not change the requirement to either need a lift or a downforce.

Also:

Conversely, we also know that many conventional tailplanes spend a lot of time pushing down (and causing trim drag).

As stated many times in the previous posts, the feeling of "pushing down" is completely irrelevant. You can not derive whether your are generating lift or not by just feeling the stick.