Aerodynamics Simplified: Scary Stalls

by FliteTest | August 8, 2018 | (4) Posted in Just Fun

Stalls can be pretty scary. How do you avoid them? Well, first you have to know all about how they work. 

Stalling occurs when an aircraft's wings reach a point where their lift can no longer support the aircraft's weight. You'll probably have experienced this numerous times before; you're pulling back whilst decreasing your throttle until your whole plane is at an aggressive angle of attack - then the stall bites. Sometimes it can be quite a 'mushy' experience (as Josh Bixler would say) where the plane simply sinks a little. Other times, it can be abrupt and really throw your aircraft over to one side or the other. Here's a Flite Test guide to the aerodynamics of stalling. 

Flow separation occurring on an airfoil at a high angle of attack.


Cause and Effect

On almost every flight you will experience a stall. That is to say, every flight should end with a stall. On landing, whilst bleeding off the lift from your wings on an approach, you'll eventually stall your plane as it touches down. If you've not stalled your wing by this point, you might bounce back up as your plane will still want to fly! 

An FT Edge coming in for a landing


The alternative to a fast landing, where your plane isn't ready (aerodynamically speaking) to come down yet, is falling out of the sky prematurely. This could happen while you're still several feet above the runway. Let's talk through how you would get to this unfortunate situation: imagine that you're bringing your plane in for a landing. On your final approach, you turn to align with the runway and slow down by increasing your angle of attack and decreasing your airspeed. If you're not familiar with how it feels at its slowest 'safe' speed, you might end up exceeding your plane's limits and stalling too high up above your landing strip. If you're flying a scale model warbird, for example, this could be a catastrophic 'tip stall' where your wing drops suddenly. Ouch. 


Avoiding Them!

The nature of the stall, and the speed at which you stall at, is dependent on a number of factors. First and foremost, weight is a serious contributor. The lighter an aircraft, the slower it can fly without stalling. Think about the extremes - a light rubberband powered model cruises at around 3mph. This is quite a difference to some EDF models that cruise at around 70mph. 

This aerobatic aircraft can fly at a high alpha without a problem thanks to its power and lightweight airframe. 


If you want to avoid stalls at all costs, most of our Flite Test DIY airplanes can be built with low wing loadings by installing lightweight electronics. This means that they are more 'floaty' than a lot of heavier RC aircraft. 


Special Measures

Many aircraft include special design features to help with their stalling characteristics. Fences, for instance, are verticle protruding surfaces that prevent air from spilling over the end of the wing. These are quite like winglets, in a way, that you would see on an airliner. 

This Su-20, like many other fighter jets, uses several wing fences.


Some models also have slats on the front of the wing. Like flaps, these can help reduce the minimum speed that a model can fly at. TheFT Simple Storch is a Flite Test model that can include slats. Airbrakes are also used to slow aircraft down. 


That just about wraps up this brief guide to stalls. Make sure that you know the stalling characteristics of each model you fly. Don't be afraid to take your aircraft a few mistakes high and repeatedly put them into stalling situations. Experiment with angles of attack and airspeeds. Doing this, and learning more about how your aircraft behaves, will only result in you being a more informed and prepared pilot.


More in this series:

Lift

Acting Forces

Wing Chords

How Flaps Work


Article by James Whomsley

Editor of FliteTest.com

james@flitetest.com

www.youtube.com/projectairaviation

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Aerodynamics Simplified: Scary Stalls