Shooting an Aerodynamic Elephant

Since the reason I spend time blogging is an effort to “give back” by passing on my experiences and knowledge, I’ve focused some articles on the current turmoil in aerospace education concerning the generation of lift. The misinformation and misconceptions are quite widespread; they are now affecting most information sources pilots trust to be correct and that we are tested on. (Yes, I am now speaking to you, FAA, which all the pilot training organizations like AOPA, Gleim, and instructor published pilot books march in locked step with.) Worse, this misinformation is being passed on as fact by educational organizations entrusted with training youth. The bottom line is that we will likely suffer for a decade or more with people not really understanding how a wing works and having completely wrong ideas about it; hopefully, it will result in only having folks busting test scores and looking ignorant and not in an actual accident somewhere. The problem is that is not guaranteed and people usually find a way to make the most improbable things happen. Which is what proper education is supposed to guard against.

Wanting to help tackle the problem, I knew I needed to make sure that what I was writing, thinking, and teaching was correct; so, I began going back to my various aerodynamic texts and refreshing myself on the generation of lift. Some of those are texts I used to get my pilots’ ratings, some of those are texts still tagging along with me after my aerospace engineering degree (“Foundations of Aerodynamics”, Kuethe and Schetzer; “Airplane Aerodynamics”, Dommasch, Sherby, and Connolly; “Theory of Wing Sections”, Abbott and Doenhoff), and others were texts I had come to respect as good references (i.e., “Aerodynamics for Naval Aviators”). As I looked at what the controversies were and how they were spreading, I started re-examining what I knew and looking for the truth. As I integrated all I knew and went through the various arguments, I began realizing there was more to the picture than what my engineering education had taught me, though none of it was wrong. (I couldn’t say the same for the information I was seeing primarily on the Internet and that was creeping into pilot’s educational books, probably because of both an incomplete understanding by their authors and their reliance on Internet sources, which are sometimes difficult if not impossible to independently verify…not that anyone appeared to be going to the trouble.) I was specifically focusing on the shortcomings of the Bernoulli explanation (though I knew its basic heart of using pressure distributions to explain lift was correct) and how Newton’s Third Law was being misapplied..and, later, how the Coanda effect was being incorrectly drawn into the explanation of basic lift. (We NEVER discussed Coanda during any engineering class I took and my literature search only turned it up in a NASA paper on high-lift devices…for very good reasons!) I became convinced that a lot of the problem was centered around not considering the wing and air as a “system” and that the application of Newton’s Third Law really had more to do with interactions in the pressure field than it did with any kind of direct mechanism where “the wing pushes down on the air” (a.k.a AOPA).

Then, along came Doug McLean tackling the problem with his book: “Understanding Aerodynamics: Arguing from the Real Physics” (Wiley).

I can’t say enough good things about this book. It has become my main resource for enhancing my own understanding of this subject AND tackling the issue on the aerospace education front. It is a very technical read; but for most folks interested in understanding what’s really going on, if you do nothing but read Chapter 7 (“Lift and Airfoils in 2D Subsonic Speeds”), then you can come to understand where the “truth” lies.

I expect to be working on an explanation of the generation of lift that relies heavily on what he had to say; but all I want to address today is this. I strongly disagree with him on one thing: he believes it’s easier for most folks to understand lift using Newton’s Third Law. Most folks experience Newton’s Third Law lies only with some kind of reaction engine (i.e., jet and rocket engines), and so they erroneously think that the wing lift occurs in exactly the same way (leading to little jets of air pushing out of the wing). But this is NOT where Newton’s Third Law comes into play with a fixed wing. This sentence from McLean’s book captures it well: quote: “The pressure differences exert the lift force on the airfoil, while the downward turning of the flow and the changes in flow speed sustain the pressure differences”. This pressure difference is how the fluid pushes back on the wing; in other words, lift is a reaction to the downward turning of the flow that with other mechanisms generate the pressure forces that lift the wing. Lift is NOT just a reaction to downwash or any direct jet of air shot off the wing by air deflection.

BANG! I just shot one “aerodynamic elephant”. It’s not the only one alive, but it’s the only one I’m going to spend a bullet on right now. I just don’t want to hunt all afternoon. (You can wait for more from me but, frankly, buy McLean’s book and you can read along as he beats me to it and shoots them all.)

The Best Trip Home That Didn’t Happen (Part 3)

About ten days later, the airplane was fixed again. The mechanic had gone back in, rechecked the valves, and discovered that neither the intake or the exhaust valves were getting a complete seal. He had reworked them, done several ground runs over thirty minutes long, and had seen no issues. To be absolutely sure they had a fix, they decided to test fly the airplane and contacted Scott, a Light Sport pilot and instructor who had worked for them in the past and whom I personally knew, to fly the flight. Once I was sure Scott would be covered by my insurance, I okayed the flight, contingent upon him calling me and making sure we set up good test conditions. He did, and I shared with him everything I knew and had seen. A day or so later, both he and the mechanic manned up the CT and flew for a one hour and twelve-minute flight, performing multiple climbs up to 7500 feet. They were convinced there were no issues, so they talked Scott into flying the CTSW to Houston if I would run him over to Covey Trails, a pretty little airpark northwest of Pearland and Sugarland and on the west side of Houston. I agreed. It was a small price to pay not to have to go through the hassle of getting the CTSW back from Denton. We scheduled the flight for Wednesday morning, July 19th. Scott hoped to launch about 7 am and get to Pearland about 9 in a two-hour flight. It was theoretically possible to do it if you cut straight through the Class B with either no headwinds or winds in your favor, but I was skeptical it would actually happen that fast.

Scott launched only a few minutes later than he had hoped to; at about eight a.m., I received a text with a picture of the instrument panel and a comment: “Ugh! It’ll be a little while….”. I could see his groundspeed was 103 knots, his estimated time of arrival was one hour and fifty-six minutes, he was cruising level at 5500 feet, and he was just clearing the south side of the Dallas Class B. He didn’t feel or see he had any issues. Another text a few minutes later showed about the engine gauges: 5200 RPM, CHT at 180 degrees F, Oil Temp at 200 degrees, and Oil Pressure at 50 psi. Those were all normal readings.

I texted him back saying: “Dude, this is LIGHT SPORT!”

A little before nine-thirty in the morning, I left my house and drove out to the airfield where I parked my car at my hangar and walked over to the FBO. Inside, a radio was feeding in traffic calls; I heard him call five miles west and request the active runway: someone else called they were using runway 14 and Scott latched on. Even though I’ve seen plenty of photos of what the CT looks like in flight, I am always curious to see it for myself, so I walked outside and watched him cross the field directly over my head. Banking left, he swung into the downwind and expertly flew the base and final legs, making a gentle landing on one four before turning off on taxiway Bravo and heading for the FBO. I took a couple of cell phone shots of him coming toward me as I looked for any traces of oil in case the repair had not gone as thought. As he spun the airplane around a few feet away, I saw that the bottom quarter of the whole left side was covered in oil. Apparently, the oil had been leaking for a while because it had saturated a piece of white “speed tape” used to cover a gap between the rear fuselage and the lower tail cone and its front end was flopping loose. After Scott shut the airplane down and was unstrapping, I stepped up to his window and said: “Dude, you’ve got oil all down the left side of the airplane!”

“WHAT….?!” He stammered, as he then climbed out. We both took out our cell phones and snapped pictures. Once I got evidence of oil saturating the fuselage, I looked in the oil door to see if I could see where oil was hitting the inner cowling (I could and tried to take a cell phone picture of it that didn’t turn out.) and then popped open the door to the left baggage compartment to find, much to my surprise, the compartment was coated with oil. I momentarily paniced as I realized that the canvas satchel containing the aircraft logs were there and its top had been left unzipped open, calming down as I examined it and saw that the oil hadn’t gotten in. There was a roll of “speed tape” in the compartment that wasn’t exactly in good shape, and I pulled it aside to dry it off and see if it was salvageable. I wiped down what oil I could off the side of the compartment and the oil that had run down the baggage compartment door and collected in a little pool on the bottom of a rim. I had a spare liter of oil in that compartment, so I pulled it out to use on the next step.

Returning to the cowling, I opened the oil door and checked the engine’s oil level on the oil dipstick. Scott grunted as we both saw the stick was completely dry. I added 200 ml and checked again. Still dry. 400 ml more. Still dry. The rest of the liter. That topped it off! I asked Scott where the oil level was when he took off; he answered it was in the middle of the cutout section of the stick, which defines the Min to Max quantity. He thought that meant he took off half a liter low; but a check of the difference between Min and max is .43 liters, so a spot in the middle would mean he took off only .22 liters low. That meant the airplane had shed .78 liters of oil, 26 % of its total oil capacity.

With the oil level temporarily topped off, I started my wounded airplane up and taxied her back to my hangar. Once we had her buttoned up, Scott and I piled into my 2014 Mustang convertible and headed for Covey Trails. Unfortunately, our twenty-minute flight turned into about a two hour road trip for Scott and a four hour road trip for me, though Covey trails was one of the prettiest airparks I had ever seen. Too bad I didn’t get to do a grass field landing there. (Caveat: To anyone who’s thinking about dropping in unannounced, better bring $100 cash with you. That’s their “landing fee” unless one of the residents at the place vouches for you.)

Round 4 Begins

The shop where I had the work done and I conversed fro about a week about how this was now going to get resolved. They first said were coming down on Friday, July 21 but then pushed back to Tuesday the 25th because they were “waiting for parts”. Since no one had been down to examine the airplane and do a preliminary failure analysis, I had questions about what parts they were bringing but didn’t get an answer. I hoped that meant they were bringing everything they needed to completely rebuild the head. While there was every reason to think that the third oil leak was associated with a root cause we hadn’t mitigated or hadn’t identified, there was no guarantee of it. It could be that the continued operation of the engine with a fault had triggered another failure mode. Additionally, while the shop had been stepping up to get me transportation most of the time and take care of the issue, I wasn’t feeling there was any sense of urgency about getting my aircraft back up in the air, especially after I got a note from the mechanic that he was going on vacation on the 27th and started mentioning August 7th as the next date for us to pursue anything. I hadn’t fussed at them much until I got that, but I did then and let them know I was going to pursue “alternative remedies” soon. I wasn’t kidding; I had selected an aviation attorney to go have a conversation with, though going down that road was the vehicle of last resort. I was also talking to a local Rotax certified mechanic to see if he could “get her done”, even if I had to pay him out of my pocket, assuming he would come to Pearland to work.

Thankfully, the business owner for the shop that had done the work stepped in. I got a call from the head mechanic apologizing for the inconvenience (not the first time he had done that) and telling me the owner was flying him and the mechanic down on Wednesday, July 26th to get the airplane up in the air. I was told that no matter what the problem was (and even if something else had gone awry), they would return the aircraft the service. That was a huge relief to me. I personally liked the mechanic and still had confidence in him (though I wasn’t convinced he didn’t have some blind spots—who doesn’t?), so I was happy to hear they appeared to be REALLY stepping up to the bar! It’s when things are going to hell in a handbasket you really see the character of a person or a company; and this one was looking like one I wanted to continue to do business with (though I would reserve final judgement until we actually got a resolution).

They came, they went, and the airplane leaked again…!!! (Part 4 follows)