While Flying Jets Nandan introduced me to a colloquial term in RC Flying called 'Kangaroo'.

Kangaroo technically is called PIO (Pilot Induced Oscillation) or SPPO (Short Period Pitch Oscillation)

To understand this we need to know there are two terms that deals with stability of an aeroplane, those are Static Stability (as the name implies) and Dynamic Stability (When it interacts with air).

To Keep it absolutely simple, when you are flying a RC Aeroplane perfectly trimmed, Flying straight and level, you just push the stick slightly and watch what happens, two things will happens (a) Pitch oscillations will dampen out (b) will continue to increase . This is LPPO (long period oscillation) or Phugoid First case is Stable, Second unstable, (See Image) both has no cause for concern because they are easily correctable by piloting



Say you are coming into land you bounce and you push the stick forward, Aeroplane take some time to react and you and the aeroplane get out of phase with each other  , and presence of ground/fast response of the aeroplane cuts of this amplitude and the Long Period is reduced to Short and you get into a dangerous condition called SPPO or PIO

This can damage undercarriage/destroy aeroplane and in real life can kill

A classic case of PIO you can see how the control surface, pilot and the aircraft all of them were out of phase




RC PIO at 5:43

A little more tech explanation on Stability (Kept it as simple as i can)

An airplane in flight is constantly subjected to forces that disturb it from its path. microbursts, turbulence, gusty   winds et all. How does airplane reacts to such a disturbance from its flight attitude depends on its stability characteristics.

Stability is the tendency of an airplane in flight to remain in straight, level, upright flight and to return to this attitude, if displaced, without corrective action by the pilot.

Static stability is the initial tendency of an airplane, when disturbed, to return to the original position.

Dynamic stability is the overall tendency of an airplane to return to its original position, following a series of damped out oscillations.

Stability may be (a) positive, meaning the airplane will develop forces or moments which tend to restore it to its original position; (b) neutral, meaning the restoring forces are absent and the airplane will neither return from its disturbed position, nor move further away; (c) negative, meaning it will develop forces or moments which tend to move it further away. Negative stability is, in other words, the condition of instability. Ball and Cup Analogy is good to understand. See image



A stable airplane , may lack maneuverability.

An airplane which, following a disturbance, oscillates with increasing up and down movements until it eventually stalls or enters a dangerous dive would be said to be unstable, or to have negative dynamic stability.

An airplane that has positive dynamic stability does not automatically have positive static stability. The designers may have elected to build in, for example, negative static stability and positive dynamic stability in order to achieve their objective in maneuverability. In other words, negative and positive dynamic and static stability may be incorporated in any combination in any particular design of airplane.

An airplane may be inherently stable, that is, stable due to features incorporated in the design, but may become unstable due to changes in the position of the center of gravity (Wrong CG  ).

Stability may be (a) longitudinal, (b) lateral, or (c) directional, depending on whether the disturbance has affected the airframe in the (a) pitching, (b) rolling, or (c) yawing plane.

LONGITUDINAL STABILITY

Longitudinal stability is pitch stability, or stability around the lateral axis of the airplane. (See earlier posts on Axis)

To obtain longitudinal stability, airplanes are designed to be nose heavy when correctly loaded. The center of gravity is ahead of the center of pressure. This design feature is incorporated so that, in the event of engine failure, the airplane will assume a normal glide. It is because of this nose heavy characteristic that the airplane requires a tailplane. Its function is to resist this diving tendency. The tailplane is set at an angle of incidence that produces a negative lift and thereby, in effect, holds the tail down. In level, trimmed flight, the nose heavy tendency and the negative lift of the tailplane exactly balance each other.

Two principal factors influence longitudinal stability:

(1) size and position of the horizontal stabilizer,
(2) position of the center of gravity.
(3) Longitudinal Di-Hedral (See earlier posts)


LATERAL STABILITY

Lateral stability is stability around the longitudinal axis, or roll stability.

Lateral stability is achieved through (1) dihedral, (2) sweepback, (3) keel effect, and (4) proper distribution of weight.
Dihedral

The dihedral angle is the angle that each wing makes with the horizontal. The purpose of dihedral is to improve lateral stability. If a disturbance causes one wing to drop, the unbalanced force produces a sideslip in the direction of the downgoing wing. This will, in effect, cause a flow of air in the opposite direction to the slip. This flow of air will strike the lower wing at a greater angle of attack than it strikes the upper wing. The lower wing will thus receive more lift and the airplane will roll back into its proper position.

Since dihedral inclines the wing to the horizontal, so too will the lift reaction of the wing be inclined from the vertical. Hence an excessive amount of dihedral will, in effect, reduce the lift force opposing weight.

Some modern airplanes have a measure of negative dihedral or anhedral, on the wings and/or stabilizer. The incorporation of this feature provides some advantages in overall design in certain type of airplanes. However, it does have an effect, probably adverse, on lateral stability.

Keel Effect

Dihedral is more usually a feature on low wing airplanes although some dihedral may be incorporated in high wing airplanes as well.

Most high wing airplanes are laterally stable simply because the wings are attached in a high position on the fuselage and because the weight is therefore low. When the airplane is disturbed and one wing dips, the weight acts as a pendulum returning the airplane to its original attitude.
Sweepback

A sweptback wing is one in which the leading edge slopes backward. When a disturbance causes an airplane with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises and the airplane is restored to its original flight attitude.

Sweepback also contributes to directional stability. When turbulence or rudder application causes the airplane to yaw to one side, the right wing presents a longer leading edge perpendicular to the relative airflow. The airspeed of the right wing increases and it acquires more drag than the left wing. The additional drag on the right wing pulls it back, yawing the airplane back to its original path. 10 Deg of Sweepback = 1 Deg of Dihedral is the thumbrule


DIRECTIONAL STABILITY

Directional stability is stability around the vertical or normal axis.

The most important feature that affects directional stability is the vertical tail surface, that is, the fin and rudder. Keel effect and sweepback also contribute to directional stability to some degree.

The Fin

An airplane has the tendency always to fly head-on into the relative airflow. This tendency which might be described as weather vaning is directly attributable to the vertical tail fin and to some extent also the vertical side areas of the fuselage. If the airplane yaws away from its course, the airflow strikes the vertical tail surface from the side and forces it back to its original line of flight. In order for the tail surfaces to function properly in this weather vaning capacity, the side area of the airplane aft of the center of gravity must be greater than the side area of the airplane forward of the C.G. If it were otherwise, the airplane would tend to rotate about its vertical axis.

One of the best article on "Right Thrust" and "Prop Effect" I have recently read. Written by Blucor Basher (Ben Fisher owner of 3D Hobby Shop).

http://www.3drcforums.com/content.php?216-Right-Thrust-and-Prop-Effects

Great write up there, must read

quoting the sum up verbatim
"So, to sum up:
Props are thrust producers, but they also impart a lot of other forces into our airplanes, like gyroscopic force and many others.
These forces are strongest when the prop RPM (engine power) is high.
At high airspeeds, the tail of the plane counteracts these prop forces effectively, but not at low speeds.
Therefore, at high power at low airspeed (like takeoff and hovering) we feel prop forces most.
The net effect of these forces is usually to make our aircraft turn left.
To take some work off of our left thumb, we mount our power system and prop at an angle pointing right so that at high power it balances the left turning force with a right turning force.
This angle is a compromise based upon the need for the airplane to handle well at a variety of airspeeds.
Any decent quality aerobatic ARF already has an adequate right thrust angle, changing it is only for advanced pilots.
Applying the classic test to check right thrust can be difficult on a modern 3D airplane since our planes power right past the limits used in the classic test.
So, we might need to experiment to find perfection, and this is easiest done by experimenting first with rudder to throttle mixes before taking your plane apart."

gusty sir nyc write up u have gone totally technical on them haha .....wing types ..drag types ...lift to drag ratio ,types of flaps lets have it all 

Gusty sir,


Why is the lift considered always behind the weight? Is it because the airflow essentially separates at the thickest aerofoil section and the weight being concentrated at CG, which mostly lies around the spar?

If that is correct, in the case of a Mugi (http://www.rcindia.org/electric-planes/mugi-1-5-size-build/msg139700/#msg139700), the weight would be at the CG, and the lift behind (where) the thickest section of aerofoil? I am unable to identify the location of the lift line so I can visualize the lift-weight couple.

Am I also correct in understanding, that in an ideal setup (practically undoable) the Thrust Drag couple should nullify each other and so should the lift weight couple?

Lift Behind weight is a stable design, whenever there is increase in lift, natural tendency of the aeroplane is to come back to the original state (like the ball in a cup),. IF the lift is ahead of the CG it will continue to depart (Like a ball on top of the cup)




Generally, yes lift is slightly aft of the thickest portion, CG slightly ahead of the thickest portion is good enough



No! how can lift weight couple nullify each other? since they are a 'Couple of Force' there will be a residual moment, depending on the quantum of force and distance between them. since, by design lift weight is nose down, thrust drag should be nose up so that they cancel each other and there is very little work for the tail plane. remember tail plane is called Horizontal stabilser !! why, whatever is residual, it balances, what changes drastically? it is the lift weight, weight being constant lift changes due to (a) Speed (b) manourvre. Who balances this residual force ? tailpane

Ideally all the force should act on the CG, there will be no residual and no requirement of tailplane (except to initiate the disturbance) that is not possible, both theoretically and practically. Fulcrum-Load_effort ? visualise

Thanks chief. All makes sense now. I see pulleys and loads and fulcrums clearly now. Think the mugi will need a little downthrust about 2 deg, since this is a pusher, the prop shaft will be pointing upwards, but yes thrustlines make most sense now.
Many thanks
GS

 Question on Thrust....
1. Is the 2deg right & down angle applicable on all engines - irrespective whether they are electric or glow or gas?

2. How does one check the angle of the firewall - whether it is perpendicular and pointing straight or at an angle?

Very good question, i did think that one day someone will question this logic. Not to complicate the matter (as it involves complex explanation of Gyroscopic effect, Fin wing position and vertical stabilizer interaction and the mathematics to go with it)

Suffice to say NO, small prop high rpm pusher doesn't require as much compensation. (Yes it requires compensation for being a pusher and being above the drag line) as a big prop tractor does.

This general thumb rule 2 deg right and down (for a tractor) works for most models, in fact once you gain proficiency and graduate onto big gassers, you will learn how to fly and see if the compensation is adequate and how to correct the same, physically, electronically (explanation of which i guess can be avoided for now)

You say small prop pusher do not need much compensation, but the Easystar and all clones/variants like Bixler, Skysurfer etc have a very steep angle. I would think, these models could qualify for high rpm and small prop.....

Higher the mass, bigger the diameter and higher the RPM of spinning Powers system then higher the gyroscopic effect.

However having said that higher RPM is more often than not coupled with smaller dia and lesser mass to achieve high RPM so less over all gyroscopic effect.

However pusher/ high pod mounted tractor /a simple tractor need different compensations due to difference in relation between the angle of thrust- angle of CG-angle of CP.

Gusty Sir,

Please have a look:-
(Me Flying MXS-R)

Is this a PIO (Pilot Induced Oscillation) or SPPO (Short Period Pitch Oscillation) ??

http://www.youtube.com/watch?v=hU58GOXjCxM&feature=youtu.be

Tanmay..

Gusty,
A question on Lift....

For a semi-symetrical aerofoil, with increase in speed, the lift increases. So net result if the lift + thrust is greater than weight + drag, then the plane goes up?
Now the question is, if this is correct then there can only be one speed at which the plane will fly level, for all other speeds we would have to give inputs - up/down elevator. Is that so?

Reason I am asking is that I used to do scratch build, then I decided to do a kit build. On all I notice that if I give full throttle, the plane starts to climb on its own. So the problem for me is to go flat out and be level at the same time.

Pankaj

it is PIO
has two types of oscillations, Short and long period, long period isn't dangerous and some aeroplanes inherently has it and people dont even know they exist, because you have time to correct.

It is the sort period oscillation that is dangerous, because pilot, instead of control, can aid it because there is a finite time delay in you giving the control input to the time the aeroplane reacts, say nearly 0.2 secs, when this time reduces then it is not possible for the humans to perceive it and correct it, therefore he ends up aids the oscillations, the timing of this oscillation closer to the ground is reduces because the amplitude of this oscillation is cut off by the ground itself therefore frequency increases you end up bobbing up and down, this is particularly dangerous because it breaks up the aircraft and the undercarriage. Cut off the energy is the only solution, throttle back and let the beast die down

that is in a nutshell PIO, SPPO happens at supersonic speeds too, it is very violent and can have catastrophic results. aeroplane from being perfectly to completely broken up takes only seconds

mid eighties we had autopilot and auto stabilisers protecting one from this menace, (It is essentially a design compromise, for performance, a neccessary evil, like steroid)

Modern aeroplanes have fly by wire controlling it

Pankaj
Lift > weight and Thrust > Drag are the requirement for initiating a climb. It is not additive. Meaning thrust need not be (And in most cases) more than weight, that's how wright brothers flew, their engine was producing very small amount of thrust compared to the weight.

open power and she climbs is because of the couple, thrust drag couple is a nose up couple.

in steady climb and descent the lift is less than weight.

in a truly vertical climb and descent lift is zero

think about it, we will discuss further

Some questions...
Does sweep back and dihedral produces same effect?
Where is the cg of a commercial airliner located?

Sweep back and dihedral do not produce the same effect:
Dihedral: It gives the aircraft a self correcting tendency. Dihedral causes an airplane to level out, so if you are in a slight bank and you leave the sticks, dihedral will cause your plane to level out again.

Swept Back Wing: A swept back wing is mostly used to reduce drag. When an airplane is travelling at high speed, a concept known as wave drag comes into play which is caused by air flowing over a curved surface at high speeds. This drag not only requires more power from the engines to counteract, but also causes shockwaves to form above and below the wing. Sweeping the wings at an angle reduces this drag.

The CG of commercial airliners is usually between 25-30% of the mean chord length of the wing.

I  hope this helps you. I'm sure senior members would be able to help you more (and correct me if I'm wrong).

Thanks

maahinberi, you are wrong about the sweepback


topalle and mahinberi please read the whole post

I'm sorry sir. Thank you for correcting.
Sir, could you also explain the effect of forward sweep?

Post no?

An airplane with forward sweep is unstable in yaw. Additionally there are structural problems associated with it (i don't want to get into the details). However, since the wing is swept forward, the chances of tip stall are generally low. But forward sweep is not very popular.

click on where it says "Quote from: rcpilotacro on January 24, 2013, 10:08:04 AM"

Hi Gusty, would you mind throwing some light on canards and related aerodynamics and why has it not made much headway in to the model world? Thanks.