Can A Balloon Stall?
Balloon Aerodynamics and the Importance of Transient Aerodynamic Lift
By Adam Magee, FAASTeam Lead Representative
Lift, thrust, weight, drag: these four primary forces acting on an airplane during flight are drilled into the heads of aspiring aviators. As learners progress, they build upon this knowledge to understand stall dynamics of an airplane. As a balloon pilot, I’m often asked by those who fly fixed-wing: “Can a balloon stall?” Before we dive into that topic, let’s begin with an undertaught topic, even in the lighter-than-air world: the aerodynamics of ballooning.
Lighter Than Air Dynamics
Balloon aerodynamics is often explained as hot air rises and cold air sinks. While that is true, this only brushes the surface of what is truly happening. Balloon flights are based on the laws of aerostatics and the buoyancy or displacement principle commonly referred to as Archimedes’ principle. In hot air balloons, the warmer air will experience decreasing density (molecules of air are getting further from each other or pushing neighboring molecules away as temperature increases). Heat is added to a hot air balloon envelope through the operation of burners. This leads to the density of hot air decreasing by pushing air out of the mouth opening of the balloon and increasing the gross or effective lift.
There are three main factors that impact balloon performance: ambient temperature, altitude, and weight (payload). For the balloon to reach a state of positive buoyancy, the air inside the balloon must be heated above the ambient air temperature plus a temperature differential to lift the weight. What this means is that on a cooler day, the balloon would fly cooler than on a warmer day, given the same altitude and weight. Pilots often easily understand the relationship between ambient temperature and weight on the envelope temperature and balloon flight. There is a maximum gross weight for a balloon, so while the temperature might be cool enough to allow for the max payload to be exceeded as far as performance, the manufacturer’s limitations on gross weight are still in force.
Pilots often struggle to understand the relationship between altitude and balloon performance. Altitude, and in its more complex form, density altitude, play a significant role in balloon performance. For a balloon to fly at a higher altitude, the density of the air inside the envelope must remain lower than that of the ambient atmosphere air outside the balloon. This is accomplished by maintaining a higher internal temperature through continuous heating, compensating for the natural decrease in ambient temperature and density with altitude.
The Truth About Lift
Displacement or aerostatic lift (aka “true” lift) is the main force that creates upward lift in balloons. There’s also lift, which balloon pilots commonly call “false” lift, and that is where our discussion on stall dynamics begins. Luckily, there are people much smarter than me, like fellow FAA Safety Team Representative Nihad Daidzic, a skilled aviator, professor at Minnesota State University, Mankato, and published author of works on mathematical models of balloon flight. His research and lengthy personal discussions on balloon flight aerodynamics have shaped the science and base of the safety knowledge I share.
Like airplanes, balloons also experience aerodynamic forces due to slip flow. Transient aerodynamic lift only exists during transient slip flow; this is what generates lift for an airplane and a decrease of lift that creates a stall. When a balloon transitions wind layers, it is affected by transient aerodynamic lift and is generating lift. Therefore, when the transient aerodynamic lift ceases (like an airplane stall), the balloon pilot is unaware that prior lift was transient aerodynamic lift and not lift due to Archimedes’ principle. The pilot likely stopped adding heat, which is problematic when the lift ceases, resulting in an even more accelerated descent rate. So, can a balloon stall? Yeah, in a way it can! Even a nonpowered, non-winged aircraft can lose lift due to the exact same fluid dynamics.
While balloon pilots call this phenomenon “false lift or false heavy,” we should probably change the nomenclature to what the science really is; it represents creation of transient aerodynamic lift in an upward or downward direction as the balloon descends through the shear layer into a different airmass — slower, faster, or just different direction. Normally, winds decay closer to the ground due to friction in the Earth’s boundary layer. Slip flow develops as the balloon descends through the shear layer, and the lower part of the envelope now acts as a lifting surface, developing transient aerodynamic lift downward. Unbeknownst to the balloon pilot, this can cause a balloon to suddenly steepen its descent path, accelerate its sink rate, and even start moving backward into an obstacle on the surface due to recirculating flow past the obstacle. It feels as if the balloon was “sucked” back by the obstacle. That often happens on approaches to landings into cavity-type fields or approaches over step-down obstacles, such as a line of trees, buildings, etc., in the presence of significant horizontal winds over the obstacles.
Fixed-wing pilots often think about emergency landings off field behind trees and the loss of lift generated on approach to landing. Downward acting transient aerodynamic lift is created by the lower part of the balloon envelope due to relative motion as the upper parts of the envelope are still in a different speed airmass dragging the lower envelope parts and the basket and creating slip flow. Transitioning wind layers may cause the simultaneous generation of upward and downward transient aerodynamic lift. Downward transient aerodynamic lift disappears as the balloon descends and settles into the new airmass, but that may be too late for the landing balloon as the flight path was already steepened, and the balloon accelerated downward significantly.
For balloon pilots, the easiest way to tell if you are flying in a transient aerodynamic lift situation is to feel the wind at your face. This telltale sign signals that slip flow aerodynamics are at play, but the wind on the face doesn’t always occur.
Importance of Controlled Descents to Land
When transitioning wind layers upon approach to landing, it is imperative that a balloon pilot maintain a controlled descent rate. Too sharp of a descent rate could cause dishing of the envelope — when a side of the lower portion of the balloon is caved in — and an increased descent rate. The main issue is that if the pilot is in a steep descent upon encountering transient aerodynamic lift, the balloon’s descent will slow, possibly even level out or ascend. If the pilot reacts to the aerodynamic lift by venting more heat, once the aerodynamic lift diminishes, the balloon will be even cooler and further accelerate the descent. The same problem occurs if the pilot simply doesn’t operate the burner and allows the balloon to cool. The cooler the envelope is from equilibrium temperature, the greater the risk of encountering a highly accelerated descent once the aerodynamic lift diminishes.
As the balloon descends through the layers and encounters transient aerodynamic lift, the pilot must remain in control of the balloon so that one standard burn can initiate an ascent, and a vent (or allowing to cool) can get you through the layer. To counter the transient aerodynamic lift, short, frequent burns can be used to maintain the balloon temperature slightly below equilibrium. The pilot wants to transition the wind layers as smoothly and as controlled as possible, which relies on keeping the balloon near equilibrium temperature. This technique also mitigates the challenges faced with dishing of the envelope if that also occurs during the descent to land.
Balloon pilots are aware of the need for larger take-off spots when transient aerodynamic lift could be present with high winds aloft and light winds near the surface, but I hope to make pilots better aware of this phenomenon upon descent to land. In these situations, larger landing sites are best. Often, the wind layer impacting transient aerodynamic lift on landings is slightly above obstacle height. Therefore, landing sites with obstacles on approach need to have enough depth so that the pilot can transition the wind layer after crossing over the obstacle. Landing sites that are too small will require the pilot to transition the layer while over obstacles, increasing the risk of collision.
It is imperative that a balloon pilot’s pre-flight planning involve careful analysis of the wind’s speed and direction to ensure a launch site that will take them to suitable and appropriate landing areas.
Importance of Weather Awareness
Accident narratives show a common theme in pre-flight pilot decision-making. When faced with a temperature inversion and strong winds aloft, pilots fail to recognize, assess, and manage the risk associated with upward or downward transient aerodynamic lift.
Using vertical wind profiles, the pilot can also prepare for weather conditions that will create aerodynamic lift situations both on launch and descent to landing. The forecasted conditions should be incorporated into the pre-flight risk assessment and go/no-go decision.
An awesome forecasting tool built by balloonist Alex Norrie for weather awareness and risk management is the ballooning weather tool at bit.ly/balloonwx. If an inversion is forecasted, the cells are highlighted yellow. Orange highlights indicate the temperature and dew point are close, signaling fog or clouds. Red highlights indicate wind shear. The presence of red highlights, red highlights close to the surface, or too many red highlights can indicate increased transient aerodynamic forces present for balloon flights.
A Graceful Landing
Transient aerodynamic lift awareness and a lack of risk management are key contributors to hot air balloon accidents. Utilizing the weather awareness resources we’ve noted here is a huge step toward improving how we teach and train balloon aerodynamics. Unfortunately, transient aerodynamic lift encounters are quickly becoming more common. To stay safe, we need to be aware of this weather situation, know how to handle it, and exercise better risk management.
I wish you “gentle breezes and soft landings,” with, of course, more knowledge of transient aerodynamic lift!
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Adam Magee is a commercial hot air balloon pilot and flight instructor, designated pilot examiner, and FAASTeam Lead Representative. He was named the 2024 National FAA Flight Instructor of the Year and the 2021 National FAASTeam Representative of the Year. He is co-founder/president of The Balloon Training Academy, a 501(c)(3) nonprofit organization, and an industry member of the FAASTeam, as well as serving as a member of the board of directors and treasurer of the National Association of Flight Instructors (NAFI).
