What if they are both right? Stay tuned … blog will be online before Nov 30, 2013
How the explanation for lift taught even today is shockingly wrong and incorrect!
Flying is something I enjoy – and although I am a recreational pilot now, I have been a theoretical pilot for far longer – in fact, since my teenage years!
I want to start my Aviation posts with a discussion of one of the biggest fallacies of flight – HOW airplanes fly. Note - The discussion ahead is for propeller-heads (pun fully intended).
Most of us who remember our high school physics, will recall the theory of flight that we were taught – essentially that lift occurs due to Bernoulli’s principle. Recapping in a nutshell, the explanation was that the path for the airstream on the top of the aerofoil (the shape of the wing) is longer than that on the bottom. This means that for the air to meet at the same time at the trailing edge of the wing, (huge assumption – I’ll explain), the air stream needs to be traveling much faster over the wing than below it. According to Bernoulli’s theorem, this naturally results in a pressure differential and causes a force upward, thereby creating lift on the wing.
I have always wondered about this theory. When I was younger, it didn’t make sense intuitively – it wasn’t convincing – but my understanding of physics wasn’t advanced enough to dispute it. As I went through my engineering years in college, I found that my suspicion was indeed correct – and in fact, me and a few friends did experiments to rebutt that theory.
Here are some basic reasons why the popular explanation makes little sense.
First, the premise that air stream has to travel faster over the upper surface of the wing relative to the lower surface air stream is entirely based on the assumption that the two air streams need to meet at the trailing edge. There is no basis for that assumption. There is no reason why the air streams cannot separate – as they do in most airflow situations such as behind a car, at wingtips etc. So that premise is flawed.
Further, calculations show (trust me) that the velocity (as predicted by Bernoulli’s theorem) required by the wing relative to the air, to generate the lift required to take flight is far in excess of the actual rotation (lift-off) speeds of most airplanes. Indeed, the airplane I fly lifts off at 60 mph – and weights approximately 1000 pounds. According to Bernoulli’s formula, I would need to be at a speed in excess of 200 mph to generate 1000 pounds of lift! A 747 would need to take-off at over 280 mph!
Additionally, aerobatic planes fly upside down – as I am sure you’ve noticed. Unless the wings could somehow reverse their shape, the traditional Bernoulli’s principle-based explanation makes no sense – they would simply accelerate rapidly to the ground and crash!
Windmills are aerofoils as well – and yet they turn with the slightest of breeze? The Bernoulli explanation would be that even that small breeze creates a pressure differential sufficient to turn a huge blade. It doesn’t add up.
Finally, look at plain old paper planes – they fly – and their wings are flat – no aerofoil. How do they fly – or float?
SO HOW DO PLANES FLY?
Airplanes fly because of the COANDA effect – the tendency of fluids to stick to surfaces, combined with Newton’s Third Law of Motion – every action has an equal and opposite reaction. Look at the picture below – something quite intuitive and familiar – the stream of water sticks to the side of the bowl. For everyone who has ever tried pouring coffee out of a mug – isn’t it annoying how it runs down the side of the mug, instead of falling straight down vertically? That’s Coanda effect for you.
So here is how it REALLY works! Lift is created when the UPPER surface of the wing pulls air that is flowing over it downwards into a downdraft as a result of the Coanda effect – in effect the air sticks and follows the curved upper surface of the wing downward. Enter Newton’s third law – as the wing “pulls” the air downward into a downdraft, the wind exerts an equal and opposite force upward – and voila! – we have lift! One very important point to note – the UPPER surface of the wing PULLS the air down, as opposed to the lower surface pushing it … pushing the air down does help, but only contributes a small fraction of the total force causing the lift.
This means that the upper surface of the wing is the most critical surface for generating lift. Which explains why all the stuff hangs off the wing bottom (engines, armaments, undercarriage, hardpoints for weaponry, etc.) while the upper surface is clean and left all alone to perform the singular and fundamental job of lifting the plane into the air.
A couple of hours before the planned departure time, the skies started to clear in San Jose (they usually start burning off as the sun picks up). But the Briefer now informed me that KSBP was showing winds from 330 at 25, gusting to 35! That was alarming to me – and it didn’t help that the briefer suggested I postpone the flight given the possibility of very bumpy conditions.
That was it! I decided to can the flight, and called my friends to get ready to drive instead. I was very disappointed, needless to say. Then it occurred to me whether I was being too chicken? I decided to call more experienced pilot friends of mine – and couldn’t get anyone on the phone. I then called my flight club partner, Gerry, and started discussing the situation with him.
Gerry’s answer was very refreshingly straight-forward – he said that in his experience, actual situations were usually less severe than it appeared from the briefing. Further, there was always the possibility to land at Paso Robles (more inland, cross-runways, less winds usually) and have my party pick the rest of us up from there. Ultimately, unless I tried to force the plane to land, the risks weren’t that serious.
So I reverted back to flying. We took off runway 30L at San Jose around 2:30PM with 14 knots headwind. The flight enroute was quite smooth barring some turbulence over the hills near Salinas and crossing from Paso Robles to San Luis Obispo. I had VFR flight following – and at one point, Norcal was nice enough to veer me away from a restricted MOA enroute.
Coming up on Oceano County Airport was very scenic. We flew across the bay, made a wide U-turn to aim for Runway 29. I chose to come in with 10 mph extra speed to compensate for gusts. Also, I had no flaps. The headwind must have been close to 25 knots, and I ended up coming in too high. I went around, and came back this time with 10-degree flaps, and with only 5 mph extra speed. Landing was fine, except I bounced twice slightly.
The return flight was much more interesting – the briefer called for moderate-to-high turbulence coming off the coast. In fact, one of the pilots I met at Pismo Beach Airport chose to defer flight to later in the day to let the winds possibly drop off. I decided to go for the flight because, postponing the flight meant a much higher risk of clouds rolling (there was a coastal high-pressure system coming in later) The winds were definitely brisk on take-off, but straight-down the runway. We were airborne, and hit plenty of turbulence over the hilly terrain. The valley (Paso Robles to Salinas) was smooth, but the section from Salinas to San Jose was very bumpy. We came into San Jose after 94 minutes from take-off to a smooth landing.
Overall a thoroughly enjoyable experience. Incidentally, I have a Garmin 496, with full Nexrad weather – and I can’t recommend it enough. It’s nice to be able to see real-time winds and doppler-radar information in-flight, not to mention the obvious benefit of all other information it provides such as ground-speed, track and ETA.
Overall, a fun and enjoyable experience.
I learned flying in a taildragger – a 7ECA Citabria with a 115HP Lycoming engine. Back in the mid-90s, I used to learn flying in Morristown, NJ with an independent flight instructor who owned a Cherokee (PA-24). After a few rough sessions, I remember commenting to him that the Cherokee was a hard plane to fly – mostly with the low wing, increased ground effect etc (which I know now, but not back then). I remember him responding by saying that if I was really committed to being a skilled and safe pilot, that I should learn in a taildragger.
And I did.
Fortunately, most taildraggers today are high-wing (unlike the war birds of the past). Taildraggers are fun to fly – and quite unforgiving – particularly the Citabria. It robust, simple and hardy plane, and it certified for aerobatics (Citabria is essentially “Airbatic” read backwards). Most pilots end up learning on Cessnas. Citabrias are very different – they are stick and rudder (which, in my humble opinion, is a heck of a lot more intuitive and natural) – fabric over frame, and with very minimal and basic instrumentation.
Learning to fly on a taildragger is akin to learning to drive on a stick shift manual – and that’s the right way to do it. Moreover, flying the Citabria has taught me a lot – they are very unforgiving of pilot error, and hence they force pilots to learn good piloting skills.
Fundamentally, all taildraggers have their Center of Gravity behind the front wheels – which means that unless the plane is straight down the runway when landing, the plane will tend to weather-vane. This one time I was landing at Fresno International Airport with a 12 kt cross-wind. The plane wasn’t stabilizing with a wing-low approach, so I chose instead to do a landing while crabbing into the wind. I was just a fraction too late straightening the plane fully prior to touch-down, and the next thing I realized was that my plane’s tail was coming around behind me. It was very scary – I was quick enough to recover – full opposite rudder and full power, and I was back on track for a recovery take-off. I went around, and came back in – this time successfully. When I was taxing off the runway, I noticed how vigorously my legs were shaking.
The other advantage (or disadvantage) is that being a light and powerful plane, Citabrias WANT to fly! Landing then is not easy. This plane taught me another very important lesson – even the slighted excess speed at landing can really complicate landings. Powered landings in a Citabria are really hard (unless there is a strong headwind) – which is not the case in most other airplanes that are designed for power-on landings.
If you haven’t flown Citabrias or other taildraggers, trust me, they are fun! Incidentally, Steve Fossett loved the beefier cousin of the Citabrias – the Super Decathalon. His tragic accident is a mystery to most – but speculation is that he may have suffered a medical emergency. He was simply too skilled and experienced a pilot for any other plausible explanation for that fatal accident.
On a final note, I would highly recommend all interested pilots and want-to-be-pilots to check out Aerodynamic Aviation (www.aerodynamicaviation.com). That is the leading school and flight club in the San Francisco bay area for taildraggers – and the lead pilot (and owner) there – Zdravko Podolski – is a very experienced and super-nice guy to train with.
Would love to hear if you have any fun taildragger stories …