What is a stall?
The dreaded stall... To hear people talk about it, you'd think
it was some sort of mythical beast that flies around, pulling
aircraft out of the sky with one swipe of its mighty paw. As you
might have already surmised from my slightly sarcastic tone, this
simply isn't the case. :-)
In reality, a stall is nothing more than the ultralight trying to
do "the right thing". (Not some politician's ethical
views about what is right; your ultralight follows the laws
of physics.) ;-) A stall is nothing to be afraid of; it is
a perfectly natural occurance, and there is nothing "wrong"
with an ultralight when it stalls.
First of all, a stall has nothing whatsoever to do
with the engine. When an ultralight is said to have "stalled",
it is the wing which has stalled, not the engine!
To understand why an ultralight stalls, we first have to understand
how it flies.
Without getting into a whole bunch of gobbelty-gook
about the theory and physics of flight, the basic reason why an ultralight
flies is because of the shape of its wing.
(Which a whole bunch of people just love to argue about. As
strange as it may seem, it doesn't appear as if we really fully know for sure
why an airfoil flies, even after all of these years of flight.)
Air molecules rushing over the curved, smooth surface on the top of the wing
create an area of low pressure above the wing. (As compared to the
relative high pressure under the wing.) This is because air which speeds
up will create an area of low pressure. The shape of the top of the wing
causes the molecules to speed up. (A similar thing happens in a carburetor;
the air that is being forced into it is sped up, which creates a low pressure
area, which sucks fuel into the air stream.) Because of this, the
aircraft is more or less "sucked" upwards. There are
those who say that the air is actually being pushed downward (by the
bottom of the wing),
or a combination of a multitude of factors, but whatever the
actual reason the wing flies, the smooth flow of air over the
top surface of the wing is an essential part of flight. This
is also the reason why frost or icing will cause an ultralight
to stop flying. The air can't flow smoothly over the top of
the wing (the ice or frost is rough), so the low pressure system
doesn't develop to the extent that it needs to, and the wing can't
generate enough lift to overcome the weight of the aircraft. The reason the
ultralight flies of course, is because it happens to be attached
to the rising wing. (Note: If you think you've got a better or more
accurate explanation of why a wing flies, I'd be interested in
Ok, now that we know we need a smooth airflow over the wing, what
is a stall? Basically, it is an interruption of that smooth airflow.
As you increase the angle of attack*, the air has to travel over a
larger and larger "hill" before running down the top of
the wing, and the high pressure air under the wing can more easily
move around the back of the wing toward the low pressure air on top
of the wing, thus weakening that low pressure area. The air on
top of the wing begins to roll and burble. Eventually, the low
pressure system is completely destroyed, and there is no longer
anything holding the wing up at that exaggerated angle. The
wing drops, and as it does, it picks up speed, and
the relative wind flows smoothly over the top of the wing again.
As it does, the high pressure area increases, and the ultralight
is flying once more.
Below is a depiction of a dynamic stall (a stall caused by a
rapid maneuver). However, the basic principles should remain the same
for most any stall, I should think. In panel 1, the wing is in level
flight, and the
airflow is smooth, and attached to the wing. As a result of
the viscosity of the air, the individual fluid particles spin.
The red color indicates particles that are spinning clockwise;
the blue color indicates particles that are spinning counter-clockwise.
As the angle of attack increases in panels 2 through 5, the airflow
at the trailing edge begins to seperate from the wing. Lift is
increasing. Panel 6 shows the clockwise spinning particles interacting
with the counter-clockwise spinning particles at the trailing edge.
Panel 7 shows a small blue bubble appearing on the upper surface of
the wing (near the leading edge). In panel 8, the interaction of the
counter-spinning particles has formed a vortex, which grows rapidly
in panels 9 through 12, until it is driven away from the upper surface
by the blue counter-rotating particles, and a stall occurs.
As the ultralight approaches a stall, it will usually give you some
warning signs. Everything may get very quiet, and you may hear a sound
similar to mice running through your wings as the air burbles over
the surface, and your wing will probably be at a high angle in relation to
the horizon. (Usually... Keep in mind that it is the angle of attack which
is important, not the angle of the wing with the horizon, but in straight and
level flight, the horizon and the relative wind are generally parallel.
You can actually stall your wing in any attitude, at any speed.)
The proper recovery for a stall is to let the stick forward and add full power.
If you do not let the stick forward, you may
stall the ultralight all the way to the ground. (If you can keep it
from spinning*.) If you are approaching a stall near
the ground, it can be difficult to let that stick forward, but it
is nescessary to prevent the stall. If you have enough altitude,
stalls are prefectly harmless, and even a lot of fun! Practicing
stalls at altitude will alert you to the warning signs as your
ultralight tells you it is about to stall, and it also gives you
an opportunity to perfect your recovery techniques. You'll
want to practice trying to lose as little altitude as possible, something
that becomes increasingly important for stalls that occur near the ground.
If you don't have any stall experience, it would be a very good idea
to spend some money and a little time with an AFI, BFI, or CFI and have
them "show you the ropes" before going off and trying it on
your own. (If you aren't careful, a stall can develop into a spin*.)
- * - Explanation of terms:
- Angle of attack:
- Is the angle created by the chord* of the wing and
the relative wind*.
- Is the immaginary line that would be created if
you drew a line from the leading edge of the wing
to the trailing edge.
- Relative wind:
- Is the wind created by the onrushing air. For example,
if you were riding a bicycle at 10mph on a calm day, the
relative wind would be coming straight at you at 10mph.
Ride directly against a 10mph wind at 10mph, and the relative
wind would be coming straight at you at 20mph. Ride with a
10mph wind at 10mph, and there is no relative wind.
- A spin occurs when you are fully stalled, but one wing is
more stalled than the other, so that wing drops faster than the
"less stalled " one. The nose points almost straight
down, and the
ground rapidly rotates around you in a most disconcerting
manner as your ultralight rushes toward it, similar to a
spiral dive. (To determine wether you are in a spin or a
spiral dive, look at your airspeed indicator. In a spin,
you will be stalled, and hence, not gaining speed. In a
spiral dive, you will be rapidly gaining speed.)
Some people do spins for pure enjoyment, but
don't mess with them unless you know the proper recovery
technique and your aircraft is rated for spins. Spins are
generally not recommended in ultralights. A well-behaved
aircraft will bring itself out of a spin if you just release
the controls, but there are some that will not. You cannot
spin an aircraft without stalling it first. If you want to
do spins, find a CFI to work with you. Do not
try to spin unless you have a full understanding
of what is going on, and you know the proper recovery
technique!! Your first few spins should be done under
the direction and guidance of a CFI.
The history of this article:
I found some great pictures of a stall which were created by researchers
at the University of Cincinnati, which I thought would
look neat as an animated .gif. I scanned 'em in, and struggled with them
for a long time before I was moderately happy with the results. (They could
be better, but at least they're not horrible.) ;-) I decided to write
an informative article about stalls around 'em, and this is the result.
I'm no expert, and this is my first attempt at such an article, so
if you find anything missing, misleading, or just pain wrong,
please let me know and I'll correct it ASAP. --Jon
Many thanks to Walter Grooms (CFI), for correcting my spin explanation.
Many thanks to Mark E. Carruth (CFI), for correcting my stall recovery technique.
Back to the Ultralight home page
Jon N. Steiger / firstname.lastname@example.org / SUNY College at Fredonia