One of them is the wing loading, that is, the airplane’s weight divided by its wing area. The other is the maximum lift coefficient of the wing, which coincides with its stalling angle of attack.

Extreme aircraft—ones with extremely high thrust-to-weight ratios, for instance, or powered lift—are exempt from this rule, but they form a small minority.

“Lift coefficient” may be a discouraging term for the mathematically challenged, but it’s simply the ratio between the amount of lift a wing produces and the dynamic pressure of the air striking it. The dynamic pressure of moving air is what you feel when the wind blows or when (if you are a dog) you stick your head out of the window of a moving car.

The word “dynamic” is added to distinguish this kind of impact pressure from the ambient, or “static,” atmospheric pressure. At sea level, dynamic pressure in pounds per square foot (psf)  is equal to .0026 times the speed (in mph) squared (use .0034 for knots).

The greatest lifting force a wing can produce per unit of area is the product of its maximum lift coefficient and the available dynamic pressure. Therefore, the lowest speed at which a wing can stay aloft is the speed at which dynamic pressure is equal to the wing loading divided by the maximum lift coefficient.

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Notwithstanding the simplicity of this relationship, people never cease telling tall tales about stalling speeds. I ran across this example in a Wikipedia article about the Antonov An-2, the big radial-engined biplane that was once as ubiquitous in Russia as the Cessna 172 is here:

“According to the operating handbook, the An-2 has no stall speed. A note from the pilot’s handbook reads: ‘If the engine quits in instrument conditions or at night, the pilot should pull the control column full aft and keep the wings level. The leading-edge slats will snap out at about 64 km/h (40 mph) and when the airplane slows to a forward speed of about 40 km/h (25 mph), the airplane will sink at about a parachute descent rate until the aircraft hits the ground.’”

Parenthetically, the reason the An-2 “has no stall speed” is not that it is able to stand still in the air but that its elevator does not have sufficient authority to raise the nose to the point where the wing will stall.

Like the Helio Courier, the SOCATA Rallye, and many other STOL airplanes, the An-2 has automatic leading-edge slats that pop out at low speed to squirt high-velocity air back along the upper surface of the wing and thereby delay the stall. In addition to its leading-edge slats, the An-2 has full-span slotted flaps—the outer segments of the upper wing’s flaps double as ailerons.

There’s nothing magical about this combination of high lift devices. Its properties, along with those of a slew of other combinations of slats, flaps and slots, were pretty thoroughly documented in 1932 in NACA’s Technical Report 427. Its authors, incidentally, included Fred Weick, who would go on to design another airplane whose limited elevator authority made it hard to stall: the Ercoupe.

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Your results may vary, but TR 427 reported that the leading-edge slat allowed the wing to gain another 7 degrees of angle of attack before stalling, and its maximum lift coefficient rose from around 2.0 to 2.25. It’s noteworthy that the increase in stalling angle of attack from 12 to 19 degrees was proportionally much larger than the gain in maximum lift coefficient: The slat delays the stall more than it increases the lift.

The actual maximum lift coefficient of an airplane is always lower than the “section coefficient” obtained from wind tunnel tests. But let us charitably assume that the An-2 really does achieve a maximum lift coefficient of 2.25. What is its minimum speed?

Its wing area is 770 square feet and its gross weight 12,000 pounds, so its wing loading is around 15 psf. Dividing by 2.25, we find that a dynamic pressure of 6.7 psf is needed to keep it aloft. The minimum speed of the An-2 is therefore the airspeed at which the dynamic pressure is 6.7 psf.

That speed is 51 mph.

But let’s generously give our An-2 a single occupant, an hour’s fuel, and no cargo. Its weight is now around 8,000 pounds, and the required dynamic pressure is down to 4.5 psf.

The minimum speed is 42 mph. 

So where does this 25 mph business come from?

• READ MORE: Looking at the Physics of STOL Drag

Setting aside mendacity and venality, the reason for all physically implausible claims about stalling speeds is airspeed indicator error. Pitot-static systems in airplanes are unreliable at low speeds, in part because the instruments are not optimized for accuracy at very low dynamic pressures, but also because pitot tubes go astray when wind hits them at an angle.

The prayerful An-2 pilot holding the yoke all the way back sees the ASI needle trembling around 20 mph, and that is the stalling speed that he reports. The aeronautical engineer knows this is nonsense, but he doesn’t want to spoil the fun and so he stays mum.

Power, to be sure, affects stalling speed. Prop wash over the wings increases their lift, but in a single-engine airplane like the An-2 the propwash affects only a small fraction of the wing area. Tilting the thrust vector upward also helps. If the An-2 is flown at an angle of attack of 19 degrees, a third of its 1,000 hp engine’s thrust acts upward. But this will still not bring the speed down to 20 mph, where the dynamic pressure is only 1 psf.

Illusions about extremely low stall speeds are encouraged by airshow flying, in which the effect of wind can be mistaken for a property of the airplane. Confusion between airspeed and ground speed is endemic to aviation, and the Wikipedia article on the An-2 contains an example: 

“…Pilots of the An-2 have stated that they are capable of flying the aircraft in full control at 48 km/h (30 mph)…This slow stall speed makes it possible for the aircraft to fly backwards relative to the ground: if the aircraft is pointed into a headwind of roughly 56 km/h (35 mph), it will travel backwards at 8 km/h (5 mph) whilst under full control.”

As an occasional editor of Wikipedia articles, I was tempted to delete this silly paragraph entirely. But why deny another reader a chuckle? Perhaps, to more vividly emphasize the remarkable properties of the An-2, I could just emend it to read, “…if the aircraft is pointed into a 150 km/h (92 mph) gale, it will travel backwards at 102 km/h (63 mph)—whilst under full control!”

This column first appeared in the September Issue 950 of the FLYING print edition.

The post Explaining the Fiction of Minimum Speed appeared first on FLYING Magazine.

It was dark when he approached Steamboat Springs. Cleared for the RNAV (GPS)-E approach for Runway 32 at Bob Adams Field (KSBS), he began his descent 20 minutes out, turned eastward at the initial approach fix, HABRO, and then northward at MABKY intersection.

The design of the approach brings you up a valley between high terrain to the east—where a number of peaks rise above 10,000 feet—and 8,250-foot Quarry, aka Emerald Mountain, to the west. The final approach fix (FAF), PEXSA, is aligned with the runway; the 5.4 nm leg from MABKY to PEXSA, however, is oriented at 353 degrees and requires a left turn of 30 degrees onto the 4.6 nm final approach course.

The field elevation at KSBS is 6,882 feet. Category A minimums are nominally 1,300 and 1¼ with a minimum descent altitude of 8,140 feet. The missed approach, begun at the runway threshold, calls for a climbing left turn back to HABRO at 11,300 feet.

The descent profile specifies crossing altitudes of 9,700 feet at the FAF and 8,740 feet at an intermediate fix, WAKOR, 2.4 nm from the FAF. From WAKOR to the threshold is 2.2 nm. Once passing WAKOR, the pilot could step down to the minimum altitude and start looking for the runway.

The Meridian tracked the ground path of the approach with electronic precision. The profile was not so perfect. The airplane crossed the FAF at 9,100 feet, 600 feet below the required altitude. At WAKOR it was 540 feet low and for all practical purposes already at the minimum allowable altitude for the approach.

At WAKOR, rather than continue straight ahead toward the runway, the Meridian began a left turn, similar to the turn required for the missed approach but 2 miles short of the prescribed missed approach point. The ground track of the turn, executed at standard rate, had the same machine-like precision as previous phases of the approach—but not the profile. Rather than immediately climb to 11,300 feet, as the missed approach required, the Meridian continued to descend, reaching 7,850 feet, less than 1,000 feet above the field elevation. It then resumed climbing but not very rapidly. One minute after beginning the left turn at 8,200 feet and on a heading of 164 degrees, it collided with Quarry Mountain. At the time of impact, the landing gear was in the process of being retracted.

When the Meridian arrived in the vicinity of Steamboat, the weather had deteriorated to 1,200 feet overcast and 1 mile visibility—below minimums for the approach. The National Transportation Safety Board limited its finding of probable cause to the statement that the pilot had failed to adhere to the published approach procedure and speculated that he had become aware of the below-minimums conditions only during the approach. Indeed, he would have become aware of the low ceiling by the time he reached WAKOR because he was already practically at the minimum descent altitude there.

He was apparently unprepared for this unexpected development.

The Meridian was equipped with a lot of fancy avionics that recorded every detail of the approach, and the accident docket includes extensive graphic depictions of those records. (These are not included in the published report.) What is striking about them is the contrast between the undeviating steadiness of headings and the large random fluctuations in airspeed, vertical speed, and altitude, which are evidently being controlled by the pilot. During the last two and a half minutes of the flight, the Meridian’s airspeed fluctuated between 89 and 110 knots and its pitch attitude between minus-5 and plus-10 degrees. Approaching WAKOR, its vertical speed was zero. Crossing WAKOR and beginning the left turn, the vertical speed first dipped to 1,500 fpm down, then, 10 seconds later, corrected to 1,300 fpm up. Ten seconds after that, it slumped again to zero before shooting back up to 1,500 fpm, holding that rate momentarily and then dropping again. The impact occurred a few seconds later.

The pilot’s logbook, which recorded 580 hours total time with 43 hours of simulated instruments and 45 hours of actual, contained four instances of this same GPS approach in the month preceding the accident. In some of those log entries, no actual instrument time was recorded, and at least two of them ended with a low approach but no landing. In some, if not all, of those approaches, the pilot was evidently practicing in VMC. Plots of two of those approaches, one a month earlier and the other a week earlier, display the same precision in ground track as the one that led to the accident, so it appears that he was relying on his autopilot for horizontal navigation.

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Being based at KSBS and having repeatedly flown the approach in good weather, the pilot would have been aware that the terrain below him never rose above 7,000 feet. He might therefore have believed, consciously or unconsciously, that as long as he didn’t get much below 8,000 feet, he wasn’t going to collide with anything. That idea could have factored into his starting the missed approach 2 miles short of the runway. Or perhaps he simply forgot about Quarry Mountain. Or, possibly, he made the decision to miss at WAKOR and began the turn without even reflecting that an important element of any missed approach is the location at which it starts.

His unsteady control of airspeed and pitch attitude, and his failure to retract the landing gear until a full minute after beginning the miss, suggest a pilot unaccustomed to balked approaches and now struggling with a novel situation. Anticipating VFR conditions, he had not filed an alternate and would now have to make a new plan and execute it in the air.

The difference between simulated instrument flying and the real thing—compounded, in this case, by darkness—is difficult for novice instrument pilots to imagine. It is not just a matter of the complexity of the required actions. It is the effect that anxiety, uncertainty, or surprise may have on your own capabilities. What looks like a dry script on a piece of paper can become a gripping drama—comedy or tragedy—when the human protagonist steps onto the stage.

This column first appeared in the September 2023/Issue 941 of FLYING’s print edition.

The post Dissecting a Tragedy in the Third Dimension appeared first on FLYING Magazine.

Our flying lives tend to revolve around missions. How far are you traveling on average? How many people and how much cargo are you planning to take? What are the length, surface, and conditions of the runways at your expected destinations? And of course, how quickly do you want to get there?

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When I began seriously shopping for an airplane a little more than a year ago, these questions seemed to cover all of the important details. Ideally, I wanted an aircraft with good short-field performance and a cruising speed above 160 knots. It would have to carry my family of four, our two 50-pound dogs and a modest duffle bag for each of us.

I narrowed my sights to a Cessna 210 or a Beechcraft A36 Bonanza. They had enough power and capacity to replace the family car on our next summer vacation trip. Soon, though, numerous other factors came into play–from affordability and reliability to reality checks about how we would use the aircraft. For starters, how often will our two sons, 19 and 15, travel with us on major excursions?

Soon we were considering other things, like cabin comfort, the advantages of shopping locally, and how the machine would present on the ramp. The last detail might sound vain, but we all want an airplane that looks good.

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After adding these elements to our search, we quickly found “Annie,” our 1992 Commander 114B. She was for sale at an airport close to home, had a roomy cabin that my wife loves, and we both think she looks great. But we still quietly wondered how she would perform.

Commanders are somewhat rare, so any time we arrive at an airport, we receive compliments and field questions about our airplane. Commanders also have a reputation as slowpokes among high-performance four-seat retractables. But most people don’t know that and just assume she’s fast because she looks that way. Believe it or not, we recently parked at an FBO where the manager was certain Annie was a turboprop. I had to assure him of her piston status.

A friend who owns a beautiful Piper PA-24, one of my favorite models and one we considered buying, often mentions that Annie’s tall, substantial trailing-link landing gear is more attractive than his Piper’s short struts. I have to admit that hearing another pilot covet my aircraft is gratifying.

• READ MORE: Finding Your Ideal Aircraft: Bringing the New Baby Home

But looks get you only so far. Performance is still an important consideration, just not as significant as I first thought. During a recent solo flight, I conducted speed trials to see just how fast Annie would go. The overcast kept us under 3,000 feet, where I figured I could expect 150 knots true with a decent job of trimming.

Flying along a ridge that separates New Jersey from Pennsylvania, I flew numerous circuits, northbound and southbound, experimenting with power settings and trim. Cruising at 145 knots with power at 24 squared, I figured 25 squared would make the difference at my low altitude, which is not included in the published performance charts.

I increased the rpm to 2,500 and manifold pressure to 25 inches. Airspeed began to creep upward but eventually stopped at 149 knots. I made more trim tweaks, getting Annie to fly essentially hands-off, but the speed did not budge. Then I remembered the single, large cowl flap was open. That had to be worth a knot or two. I closed it, and 150 knots appeared on the Garmin G5.

I was overjoyed at this triumph, and knowing that a few thousand feet higher, we should be able to reach 155. I had also nearly forgotten about the 160-knot requirement I earlier had in mind. And like most pilots I know, I soon reduced power to get back to a fuel flow between 12 and 13 gallons per hour instead of the 14-plus I was burning at the higher speed. What’s the rush?

During more than 20 hours flying Annie so far, I have come to value her smooth handling, comfort, and forgiving flight characteristics (and great looks) far more than the raw speed I once craved. In my mind, cruising at 150 has gone from pretty slow to fast enough.

The post Overcoming the Speed Obsession appeared first on FLYING Magazine.

A: The laws of physics are strictly enforced: When there is sufficient airflow over the wings, an aircraft will take off—whether or not there is sufficient runway remaining depends on the pilot’s planning for the performance of the aircraft using FAA-approved sources, specifically the performance charts in the aircraft’s pilot’s operating handbook (POH) or airplane flight manual (AFM).

With airplanes, the pilot needs to know not only liftoff speed and proper climbout speed, but also the ground roll distance and the distance required to clear a 50-foot obstacle at the end of the runway. This information is found on the aircraft’s performance charts created by the aircraft manufacturer.

The recommended liftoff speed—along with the runway distance required for specific conditions, such as field elevation or density altitude, aircraft weight, wind, and runway surface—is listed on performance charts. The charts show the distance required for the takeoff roll and the distance needed to clear that 50-foot obstacle. 

The charts also show the speed the pilot should use—either Vx for best angle of climb to clear obstacles at the end of the runway, or Vy, the best rate of climb to gain the most distance in the shortest amount of time. 

• READ MORE: Everything About V Speeds Explained

They also have notes on how the pilot should adjust their calculations to adjust for the aircraft’s weight, wind, air temperature, takeoff surface, and field elevation. Note the fine print on aircraft configuration: it may indicate the pilot should deploy partial flaps or use a particular power setting.

The liftoff speed (Vlof) for the airplane is published in the POH/AFM under the V (for velocity) speeds. If the airplane is operating within weight limitations, when it reaches this airspeed, the airplane should be able to take off.

On days when a higher density altitude is present—because of high temperature, high humidity, and/or high field elevation—the aircraft engine performs as though it is operating at a higher altitude, so it takes longer to accelerate to reach the indicated Vlof, which means more runway is used.

The length of the runway can be found on the VFR sectional and on signs at the airport.

It is up to the pilot to determine the length of the runway and if, by referring to the metrics in the POH or AFM, the aircraft will have sufficient distance.

The surface of the runway also makes a difference—expect a longer takeoff roll on grass runways because they lack the traction of hard surfaced runways.

 A pilot should determine their needed distance for takeoff before every flight.

Pro-tip: The metrics in the POH and AFM are based on a new airplane with a test pilot at the controls. It’s a good idea to give yourself a little extra distance of about 500 feet to be safe.

The post Do I Have Enough Airspeed for Takeoff? appeared first on FLYING Magazine.