The Stall Part 1: What is a Stall?

RC pilots who have been flying for a while have heard the term “stall” and most know that it is a bad thing.  Most RC pilots understand that a stall causes the airplane to “stop flying” and is usually the result of some mistake on the part of the pilot or builder.

However, I believe much of what I hear at the RC field regarding stalls is based in tribal knowledge (knowledge passed among a community by word of mouth…in this case the “tribe” of radio control fixed wing aircraft pilots).  This lack of depth to the knowledge causes problems for an RC pilot as he or she advances in the hobby.

With this in mind, RCGS is starting a new series on the stall.  This will, again, be a multi-part series with weekly or bi-weekly updates until we feel the topic is sufficiently covered to move on to other topics.

With this first post we are going to define the stall:

A stall occurs when the smooth airflow over the
airplane’s wing is disrupted, and the lift degenerates rapidly. This is caused when the wing exceeds its critical angle of attack. This can occur at any airspeed, in any attitude, and with any power setting. -FAA Airplane Flying Handbook

The FAA definition of stall is suitable for our discussion despite the oversimplification of a complex aerodynamic (and, more generally, a fluid dynamic) principle.  One key point to emphasize for our ongoing discussion:  “This can occur at any airspeed, in any attitude, and with any power setting”.  Put a pin in that and we’ll get back to it in a later post.

Another term we need to define, angle of attack, is the angle between a line from the center of the leading edge to the center of the trailing edge (referred to as the ‘chord line’ of the wing) and a line parallel to the wing’s path through the air, or in other words, it is the angle between the chord line and the direction of the surrounding undisturbed air.  This is a relatively hard thing to explain with words and a picture is always better (from http://smoknjoe.tripod.com/aviation.htm):

download

Finally, the first part of the definition: “A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly”

There are two things we need to discuss in this statement:

1.  Smooth airflow disruption is the key to a stall.  A wing operating efficiently has smooth, laminar airflow over the top and bottom of the airfoil.  As angle of attack increases it becomes more difficult for the air to stay attached to the top of the wing, and it begins to separate.  Since lift is created by the difference in pressure between the top and bottom of the wing, and the difference in pressure is created by the airflow over the top of the wing smoothly moving faster than the airflow on the bottom of the wing; when the airflow begins to separate lift will decrease in the local area of the wing where that separation is occurring.  Once enough of the wing experiences this condition and lift is no longer great enough to overcome gravity, the wing essentially “stops flying” and we have a stall.

2.  Lift degenerates progressively as angle of attack increases, but it begins to degenerate rapidly at or near the critical angle of attack.  Essentially, at high angles of attack (i.e. slow flight speeds while maintaining altitude), some of the wing is experiencing separation, but the occurrence is gradual.  Once the critical angle of attack is exceeded, the separation is rapid and even sudden…accompanied by a nose drop and often a wing drop as well.  The point is that stall is progressive and it accelerates approaching and passing the critical angle of attack.  This will become more important as our discussion moves to wing design, stall recovery, and the spin.

We have covered a lot of ground in a very short time, but the RC pilot doesn’t need a degree in aerodynamics to have a suitable depth of knowledge.  If these concepts have piqued your interested, the FAA is a great source of free information on basic aerodynamics.  In addition, Aerodynamics for Naval Aviators is an excellent resource for those who really want to dig in.  There are also free courses in fluid dynamics available from M.I.T. and other renowned institutions.

Adverse Yaw Part 3: The Importance of Coordinated Flight

Coordinated Flight can be defined as flight with no presence of side slip or skid.  Another way to is to picture yourself sitting in the aircraft…if you are not feeling any side-ways force on your body…you are experiencing coordinated flight.  This IS an oversimplification (as are most teaching aids) to illustrated a point…as there ARE exceptions.  Still, for our purpose the illustration is functional.

We know that Adverse Yaw causes uncoordinated flight (a slip) if it is not compensated for by rudder.  But, beyond the aesthetic (coordinated turns look better from the ground), what benefit is there to learning to use rudder to compensate for adverse yaw and attain coordinated flight?

I’ll start by saying:  The benefits are subtle.  They are not experienced in most situations an RC pilot encounters.   They are more about the skill of rudder coordination than they about material benefit to the airplane (most of the time).

1.  Efficiency:  Adverse yaw creates drag, this drag slows the airplane down and decreases it’s efficiency.  Coordinated turns are the most efficient turns (as opposed to the slip caused by adverse yaw or the skid caused by over compensating for adverse yaw) an RC pilot can accomplish.

2.  Safety:  Properly compensating for adverse yaw reduces the risk of spin entry, especially at slow speeds or during high load-factor (high g-load) maneuvering.  We will talk about spin entries in another post (often mis-labeled ‘tip stalls’), but suffice to say, learning to use rudder properly will increase the margin above stall and reduce the chance of a spin entry or fully developed spin.

3.  Skill:  As one of the best RC pilots in our club often says, “you paid for that rudder, why not use it?”.  Learning to use rudder as an independent flight control will benefit the RC pilot greatly.  Of the three benefits we are discussing, this is the greatest.  Teaching yourself to coordinate rudder will give you a foundational skill that could save your airplane (or a friend’s airplane if your skill gives you a reputation as a safe ‘trim pilot’).  Learning to use rudder will expand into the ability to side-slip in a cross-wind or forward slip to increase angle of descent during a dead stick into a short runway (both skills we will expand on in later posts).  Teaching rudder coordination will help you as you advance to heavier, higher performance models with short fuselages and big engines (some race planes, many warbirds) and you have the skill to properly compensate for engine torque, P-factor, and other right turning tendencies (yet another preview of future posts).  In essence, practicing coordinated turns will ‘wake up’ your left finger and thumb (or thumb) and lay a foundation for added rudder skill.  Precision aerobatics, freestyle (3D) aerobatics, and smooth, scale flights all require good rudder usage.  Teaching yourself to use rudder now while learning the basics of RC will pay off with an easier road of advancement as you move forward into these and other areas of RC flying.

Adverse Yaw Part 2: Airframe and Mixing

Last week we defined adverse yaw and talked about our primary way of correcting for it, the Rudder.  This week we will touch on a few of the ways to correct for it with airframe design and computer mixing.

Airframe Design to Correct Adverse Yaw

Adverse yaw has been a problem for aircraft designers ever since the invention of the aileron.  Since adverse yaw results from the lift created by the down-going aileron and the profile drag of the down-going aileron, counteracting it with small changes to the airframe is possible.  None of these design characteristics will completely eliminate adverse yaw but added together then can significantly reduce it.  Most RC pilots are not designing their own airplanes so this discussion might seem irrelevant.  However, at RC Groundschool we believe in “depth of knowledge”…which means we believe the more the RC pilot understands, the better pilot he or she will be.  This will be a general discussion that touches on a couple of the more popular techniques utilized to fight adverse yaw.

If the objective is to reduce the drag generated by the down-going aileron, the obvious choice is to deflect that aileron less relative to the up-going aileron.  This can be accomplished by hinging the aileron at it’s top, so that the down-going aileron presents less surface area than the up-going aileron, or via some sort of mechanical travel reduction via the flight control system.  This technique is used on many light General Aviation aircraft.  A general term for these techniques is aileron differential which refers to the differential or difference between the up going aileron and down-going aileron travel and hence the amount of surface area presented to the airflow.

Another technique is simple aileron-rudder mixing.  This is usually achieved by some sort of mechanical mixing system.  While this is very efficient, it is also very linear, and as we learned in the demonstration we practiced in part 1, adverse yaw is not linear but varies with airspeed (among other things).  Perhaps a subset of this idea is the Yaw Damper.  This system is employed on larger transport category aircraft to overcome things like adverse yaw and dutch roll (dutch roll is another aerodynamic problem for some aircraft).  Yaw dampers are usually comprised of some kind of rudder actuator coupled to some sort of flight control system that senses unwanted yaw and automatically counteracts it.

Yet another way to counteract adverse yaw is the Frise Aileron.  This is an aileron design that utilizes a hinge point above the centerline of the aileron allowing a part of the up-going aileron to deflect downward into the airflow over the bottom of the down-going wing (remember, the up-going aileron makes the wing go down).  This adds profile drag to the down-going wing and pulls the airplane into the turn about the yaw axis.  While Frise ailerons can be effective, and are used on modern full scale airplanes, they also cause drag and sometimes buffeting.  They are a compromise, just like every other thing designers do to counteract adverse yaw.

Computer Mixing to Counteract Adverse Yaw

Modern RC computer radios are affordable and most of us now own one.  With these transmitters we can use mixing to counteract adverse yaw in a similar way that full scale designers use the techniques we’ve described.

Differential ailerons are easy to add via most computer radios.  Consult your manual, but usually there is a specific menu where the function is applied, and often it can be coupled to a switch or flight mode.

Aileron-Rudder mixes are also available in most computer radios and can also be implemented on a switch or condition or flight mode.  In addition, the gyro stabilizers mentioned in Part 1 act very similar to yaw dampers and can do much of the yaw correction automatically.

If you are interested in designing your own RC plane or are confident in your building skills, there is no reason you cannot implement some of the same techniques used by full scale designers to counteract adverse yaw.  In fact, I noted on a build I did a few years ago (the Mountain Models Tyro 150), that the ailerons were top-hinged…with the resultant aerodynamic differential designed right in.  Pretty cool.

You Still Need to Fly The Rudder

Here is the bad news:  None of the techniques described above completely eliminate adverse yaw.  In fact some of them introduce other unwanted aerodynamic problems.  For instance, aileron-rudder mixing is great for general flying, but the coupling gets in the way for aerobatics.  The same is true for differential…it’s great for big sky, IMAC style aerobatics, but in the 3D realm that loss of aileron effectiveness on the down-going aileron can be felt at times.  This is why it’s good to put these features on a switch.  RC Groundschool is all for adopting the latest techniques and technology, but the rc pilot should never become dependant on them.  Learn to fly the rudder.  It will pay huge dividends.  What are those dividends?  We’ll discuss that next week.

Adverse Yaw Part 1: Using the Rudder

Modern RC trainers typically include full 4-channel control.  Technology has advanced to the point that the old 3-channel trainer (Rudder, Elevator, and Throttle) has mostly fallen by the wayside in favor of the inexpensive, ready-to-fly, “full house” trainer.  A few of these modern trainers also incorporate stabilization systems with multiple flight modes and bail-out functionality.  The FireBird Delta-Ray, by Horizon Hobby,  is one such airplane.  My 5 year old son is now flying flying this particular model with very little intervention from me (I only take the controls for landing).

With the advent of these artificially stabilized trainers, it’s even more critical to educate the new RC pilot on basic aerodynamic concepts as he will not necessary see the evidence of them during his initial education.  This isn’t a bad thing, as these new models make it easier than ever to enter the hobby and find success.  Still, the compensation for ‘bad habits’ often associated with more advanced aircraft provided by these sophisticated trainers has a minor drawback and itself must be compensated for by focused education.  At RC Ground School we seek to provide this education, and with this first post on our shiny, new website, we will start with the first part of a multi-part discussion of one of these aerodynamic bad habits:  Adverse Yaw.

First, here is a great definition from our friends at Wikipedia:

Adverse yaw is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll. It is caused by the difference in profile drag between the upward and downward deflected ailerons, the difference in lift and thus induced drag between left and right wings, as well as an opposite rotation of each wing’s lift vector about the pitch axis due to the rolling trajectory of the aircraft. The effect can be greatly minimized with ailerons or other mechanisms deliberately designed to create more drag when deflected upward than downward and/or mechanisms which automatically apply some amount of coordinated rudder. As the major causes of adverse yaw vary with lift, any fixed-ratio mechanism will fail to fully solve the problem across all flight conditions and thus any manually operated aircraft will require some amount of rudder input from the pilot in order to maintain coordinated flight.

Some may ask what is “yaw”.  Yaw is movement about the vertical axis of the airplane.  Yaw is normally induced by the rudder, but is any movement about that axis. This is a good segue into how we counteract adverse yaw.  There are multiple ways to do this on an RC aircraft, but the primary flight control used in most cases is the rudder.  In Part 2 we will talk about other airframe characteristics used to counteract adverse yaw, but for now let’s focus on the Rudder.

I know many RC pilots utilize the rudder very little.  Most of us are guilty of thinking of that left stick as throttle and ground-steering.  However, the rudder is a flight control and thus, something an RC pilot should seek to master.

Whenever the pilot initiates a turn with aileron, there will be adverse yaw, just because the pilot can’t see it from the ground does not mean it’s not there.  One way to accentuate the effect is to turn off any stabilization system (or fly an airplane not equipped with such a system), slow the airplane down (have plenty of altitude), and apply full aileron one way and then the other multiple times.  You will see the nose “swing out” of the bank, and this is adverse yaw.

Next, do the same maneuver, but this time compensate for the swing with rudder (you will be applying aileron and rudder at the same time).  Try to keep the fuselage level with the horizon, keeping the rolling motion axial.  This is good exercise for the left thumb (and finger if you fly pinch as I do).  The slower the airplane flies, the more rudder will be required to keep the nose from swinging out of the bank.

Another way to see adverse yaw is to fly a plane with a long wing, like a Cub, at about half to three-quarter throttle (cruise).  Fly in a circle, and watch the tail as you bank.  Does it “drop”?  Does the tail look like it’s “falling out” of the turn?  This is adverse yaw.  Use the rudder to “lift” the tail and keep the fuselage parallel to the horizon (for a level turn, if you are climbing the fuselage needs to be parallel to the path of flight).

These exercises will help you loosen up that left thumb.  Next time we will discuss airframe design choices and radio programming that can help compensate for adverse yaw.