Why a plane goes UP in a dive if it's NOSE heavy?

[Flying Cat]

In the article tittled "The Art of Low-Power Aerobatics" by Keith Shaw in the R/C Pilot's Handbook, Keith describes a very effective way to test the center of gravity of a plane:

Once the plane is at a safe altitude, you do a 30 degrees nose dive, once it has gained speed, you release the elevator, then:

a) if the plane is nose heavy, it will gain altitude; i.e., the dive angle will decrease
b) if the plane is tail heavy, the original diving angle of 30 degrees will increase
c) if the center of gravity is correct, the plane will continue its 30 degree dive.

I have done this test and it works great. However, I will appreciate if someone could explain why when the plane is nose heavy, it will GAIN altitude when I release the elevator.

The nose heavy case, which works exactly as Keith Shaw describes, seems to be counter-intuitive; if the plane is nose heavy, shouldn't it increase the dive angle due to its heavy nose?

Thank you in advance for your explanation.

Flying Cat

[maciek]

You trim your plane to fly level at a cruise speed. For a nose-heavy plane, you have to give it some up-elevator. Now when you put the plane in a dive, the "nose-heavyness" exerts the same amount of nose-down torque on the plane as before, but the downward force from the up-elevator increases with speed, and that's why the plane levels out.

[senor_paco]

Well, you have other things to consider. say if you have a flat bottom airfoil, the plane will have a tandency to rise, since with the increased speed of the dive comes increased lift. Likewise if your wing has positiveincidence, i.e. the leading dege is higher than the rear. this allows many planes to ROG safely on their own. Insofar as tail heavy goes, that's a new one to me. Ususally, and if I am remebering my model racketry and physics correctly, you can only move the cg back to the center of pressure (cp). any further and you remove the stability. That's why some of the experimental aircraft like the x-29 could only fly by computer. they are designed unstable on purpose, and a person annot control them. I would like for someone to explain the tail heavy dive statement.

[uscra112]

Not that I presume to be as eddicated as Keith, but this IS how I learned to trim the CG of gliders. You HAVE to have the thing trimmed out for level flight first, just on the edge of porpoising, before you do the test, and with a truly tail-heavy plane it's almost impossible to do this because it's already unstable. But if you DID get it steady, you'd have to have some UP-force coming from the stabilizer. For most situations, as speed increases the upward force from the tail and the nose goes down, sure enough. I to recall a theory for towline Nordics back about 1960 that said you could use airfoils for the wing and tail that increased lift with increasing speed at different rates, so that even with a rearward CG the increase in speed would increase wing lift faster than tail lift, so that the CP would move forward as speed increased. It never worked for me, but I was only fifteen at the time. Maybe I should try it again now that I have radio control to keep the thing away from the ground.

[vintage1]

I think you can fly a plane with the CG behind the center of pressure - since anyway that is a 'moveable feast' and will be at different chord positi0ns depending on angle of attack, and speed...

The fact however, that a model with rearward C of G will tighten into a dive, is in itself proof of the very instability you mention :-)

My short and limited (:0) experience with too far aft C of G , is that you can just about fly the plane, but it tends to be a switchback ride.!

It was a bit easier with control line, where there is some negative feedback so as the model rises on the lines, the elevators automatically flap down, but severe porpoising followed by a loud silence was often the result :-)

With RC, you are having to use skill to fight the natural instability. Up to a point, you can, and the 'tightening in a dive' point is really as far as you ever want to go.

As far as gliders go, and duration free flight. some at least used to balance with a very aft C of G - up to 60% of chord. or more (I seem to remember some with CG *behind* the wing altogether), with almost positive incidence on the stabiliser. How these achieved stability I do not know. But they did. The extra lift generated by the stabiliser increased duration.

I assume that somehow the wing generated more lift in a dive than the tailplane, proportionately...and so generated the righting moment needed to level the plane out.

Under power, there was an effective climbing moment generated by the enigne thrust line being below the pylon mounted wing - which comprised the center of drag, mainly.

We (a bunch of kids at the time) built one of these, and were rewarded by finally getting the trim right on a full tank (2 minutes when it ought to have been 15 seconds)...and watching it circle slwoly downwind out of sight....

[Sparky Paul]

"the "nose-heavyness" exerts the same amount of
nose-down torque on the plane as before, but the downward force from the up-elevator increases with speed."
.
"if you have a flat bottom airfoil, the plane will have a tandency to rise, since with the increased speed of the dive comes increased lift. "
.
"You HAVE to have the thing trimmed out for level flight first, just on the edge of porpoising, "
.
"I think you can fly a plane with the CG behind the center of pressure .."
.
Interesting.
Wrong, but interesting.
.
Moving the c.g. aft alters the stability of the airplane by reducing the static (stability) margin. Positioning the c.g. at the neutral point results in a plane with no stability. It goes where it's pointed. Moving the c.g. aft of this results in a plane which might or might not be controllable.
The airplane responds to a "dive test" because the speed increases in the dive. Since the premise is "hands-off", a plane trimmed for level flight will be out of trim because of the speed increase. The natural effect on the plane will be to climb.
If the plane is "neutrally stable", nothing will happen, it will continue in the dive. If the plane is just a hair on the wrong side of "neutral", the dive will increase.
If the dive is permitted to increase, the pitching moment of the airfoil results in the wing structure twisting, forcing the nose down even more. Many times the plane cannot recover from the dive, due also to the tail boom bending down under the increased load on the horizontal, which result s in more "down" elevator, without moving the control stick.
.
Sparky Paul
http://www.angelfire.com/indie/aerostuff
PJB's Seriously Aeronautical Stuff
http://home.earthlink.net/~pjburke1/aindex.html

[AirBornOne]

, I will appreciate if someone could explain why when the plane is nose heavy, it will GAIN altitude when I release the elevator.

According to Martin Simons...*
"For stability,it is neccesaary that if there is a disturbance of equilibrium,
causing a nose-down or nose-up pitch,then a corrective pitching moment
is required.
If the CG is aft of the NP (neutral point or aerodynamic center) a
nose up disturbance produces an increase in lift and this produces a nose
up pitching moment amplifying the original nose up influence.Vice
versa for the nose down disturbance; the lift is reduced and the nose down
pitch is worsened.
...For static stability the CG must be in front of the NP(static margin).Then a nose-up disturbance produces an increase of lift behind the CG and this tends to restore the normal trimmed and balanced flight attitude."

"Flight" involves a very complicated interaction between "pitching moments" (leverage) aerodynamic forces,gravity etc.
Stability generally means the plane goes where you want it to when you
want it to and will fly straight and level without pilot input and occurs when the CG is forward of the NP providing an "offsetting" of the forces
acting on the plane to equal level flight.
With the CG at the same point of the NP, the plane will react to every
influence.A wind gust,change in wind direction etc. and will require the
pilot to Constantly adjust to all the influences to achieve level flight.
When looking for thermals,a rearward CG (reduced static margin) lets
the pilot "read" the air more accurately because the plane is more sensitive to the movement of the air.

*"Model Aircraft Aerodynamics" by Martin Simons ,Argus Books 1991.

[Flying Cat]

I want to thank you for your replies. As I expected, the physics and aerodynamics behind a flying plane are complex and intriging at the same time. But now I understand why a plane behaves the way it does during the "dive" test.

Thanks,

Flying Cat

[Mike Rolls]

As far as gliders go, and duration free flight. some at least used to balance with a very aft C of G - up to 60% of chord. or more (I seem to remember some with CG *behind* the wing altogether), with almost positive incidence on the stabiliser. How these achieved stability I do not know. But they did. The extra lift generated by the stabiliser increased duration.<<
F/F gliders seldom have CGs that far back - it was power duration models in the days before timer controlled surfaces (and still in sonme classes which ban such devices) which had CGs way back - in a few cases evena percent or two behind the wing TE, although most went with 90-100%. The aft CG was linked to a large tail - soemtimes a tad over 50% of wing area although again, most didn't go that far. Reason was to enable the difference in rigging incidences on wing and tail to be as small as possible to reduce the looping tendency whic occurs when a model trimmed for glide with a forward cg and several degrees of difference in incidence is then made to fly much faster under power.

Mike

[uscra112]

Again my memory is very hazy, but I think the idea behind large tail surfaces and very rearward C.G. might have been a product of rules which limited WING area. If tail surface wasn't counted, and if it could be made to contribute lift, it in theory got you a better glide. As I said earlier, the theory depended on getting the wing to increase net lift faster than the stabilizer when speed went up, hence a lot of attention to the relationships between airfoils used. Generally less camber on the tail than on the wing. I don't recall that shifts in the C.P. with speed had anything to do with it. I've read somewhere that current knowledge discounts all this as a net advantage due to the drag from the large tails, but it was a common theory when I was in my teens. I remember trying it, but never got it to work properly.

[Sparky Paul]

Free-flights with 50% horizontals and c.g.s aft of 100% are "single-speed" airplanes, once gliding.
The decalage and area distribution are optimized for the lowest sink-rate.
Current free-flights put smaller horizontals much further back, but for the same reason. Once the plane is gliding, the lowest possible sink-rate is desired. And it's not selectable, once the airplane is launched.
The areas etc result in super stability to reduce the effects of gusts, and keep the plane in a predetermined glide circle.
Because of the large speed difference in the short powered section of the flight and the glide, the engine typically required a significant of offset from straight ahead for a pylon-mounted wing, less so for a engine mounted in line with the wing.
These planes are quite a compromise of flight conditions to get working together.

[Mike Rolls]

Again my memory is very hazy, but I think the idea behind large tail surfaces and very rearward C.G. might have been a product of rules which limited WING area. <<

Not so. The FAI rules defined TOTAL are, not wing area, from 1951 onwards. Barry Wheeler's Eliminator, which won the 1952 World Power Champs had a 52% tail, and a rearward CG - although not IIRC at 100%. Ditto the late Carl Wheeley's 1954 winner had a tail of around 50%. George Fuller, 2nd to Wheeley was using an open model (no area restrictions) a couple of years later, the famous Dixielander with CG ranging (many versions built by many people) from 95% to 105%.
Don't know about elsewhere, but FAI rules post 1945 have never limited wing area - just total area - whether for F/F power, glider, or rubber, or for C/L team race and speed. The only rules which limited WING area were the Wakefield rules prior to 1951 (which were not set by FAI) which limited the wing to 200 sq. ins +/- 10 sq. ins. but at the same time limited the tail area to 33% (not 1/3) of wing area.
Mike

[Mike Rolls]

These planes are quite a compromise of flight conditions to get working together.<<
Absolutely. The trick is to balance the loop - caused by the "excess" speed, and the roll, induced by wing warp, to give a stable spiral climb. Good when you get it right...
Mike

[uscra112]

Mike, weren't the A-1 and A-2 Nordic towline glider rules based on area? That's where I was coming from.

[Tapio]

As far as gliders go, and duration free flight. some at least used to balance with a very aft C of G - up to 60% of chord. or more (I seem to remember some with CG *behind* the wing altogether), with almost positive incidence on the stabiliser. How these achieved stability I do not know. But they did. The extra lift generated by the stabiliser increased duration.


The combination of aft CoG, minimal incidence and large taiplane was not to increase duration, nor to maximize area (as the total area is limited), BUT used in power models (either IC or rubber) to kill looping in the climb. With such setup the models do have a large range of speeds where the pitch stability remains rather similar, thus the models could race up the skies and still have a floaty glide, without changes in the incidence. The slight tendency of looping - required for stability - was tamed by trimming the model into a spiral climb, during which the model - from pitch point of view - loops all the time.

These days the same trim requirement is mostly taken care with variable incidence tails. This has enabled the models to be with larger incidence, smaller tailplanes and more forward CG. All this means much improved stability and efficiency in glide, but with the cost of increased complexity of gadgets.

There is a nice book about the older trim setup, called "Circular airflow". Written by Carl Zaic (who else), it is a true classic of model aerodynamics!


-Tapio-

[Mike Rolls]

Mike, weren't the A-1 and A-2 Nordic towline glider rules based on area? That's where I was coming from.<<

Both were/are based on total area of wing and tail combined. A/2 is 495-525 sq.ins, can never remember the A/1 figure. Intersting that the A/2 is the only major category which has never changed its spec (towline length reduced from 100 metres to 50 metres back some time in the 50s). In the good old days you got 20 secs moto run, and, for a .15/2.5cc a minimum weight of 17 1/2 ozs (app) with an minimum area loading (wing and tail combined) of 3.93 ozs per sq. ft (amazing how the numbers stick - must be over 40 years since I flew power duration!). The dear old Wakfield went from no restriction on rubber (which meant 5-6 ounces of rubber in an ariframe of 3- 3.5 ounces) to today were you are allowed (IIRC) just 30 grammes in a model of the same size and weight - and the experts are getting performance of similar levels!
Ah, for the good old days:)
Mike

[Sparky Paul]

Nordics (I had tons of fun with mine) had a somewhat atypical area distribution, due largely to the lack of the engine run requirement which affected power models.
The ideal high aspect ratio wing, and small tail way back there was found early on to be optimum, as free-flighters today are learning.
My FAI power models were seriously overweight (by the rules) compared to AMA free flight. The "THUMP!" when they landed dethermalized was awesome!
Nylon covered wings, lots of spruce.

[uscra112]

Thanks Mike. I stand corrected. Serves me right for opening my yap after having done no flying for thirty-five years. ;)

[Mike Rolls]

Serves me right for opening my yap after having done no flying for thirty-five years<<
Don't worry about it - I remember myself as young and handsome!
Mike:)

[Mike Rolls]

My FAI power models were seriously overweight (by the rules) compared to AMA free flight.<<
Sparky
Was that back in the 17 1/2 oz days or the later lead sled variety?
Mike

[Sparky Paul]

Originally posted by Mike Rolls
My FAI power models were seriously overweight (by the rules) compared to AMA free flight.<<
Sparky
Was that back in the 17 1/2 oz days or the later lead sled variety?
Mike
.
Ought 58 to '60, as I recall.
I flew an Ehling designed (and built..shhh, don't tell anybody) Cox Olympic 15 powered high thrust line.. published in American Modeler as "Green Bird" (plans by yers truly), although it was covered in pink nylon.
Also flew Wakefield and Nordic trying for the US free-flight team way back then. Too many good fliers on the East Coast! Drat! :D

[Mike Rolls]

Too many good fliers <<
Yeah, curse aren't they!
Mike:)