About the Author

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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Monday, December 27, 2010


Well, Santa fell down on the job again this year and forgot to get me my new airplane....

Other than that, there's not much to write this week. But just wait'll you see what I've got lined up for the first third of 2011!

Happy Flying!

Saturday, December 25, 2010

Merry Xmas!

Merry Xmas, Folks! Hopefully Santa brought you some nice airplane stuff. I won't get to do any aerobtics today, but that's ok. I get to hang out around the tree with my ladies and pick up the flying on the 29th when I go back to work.

Merry Flying!

Wednesday, December 22, 2010

Aerobatic Prep

So the students here are all gone home for Xmas, leaving us to conduct proficiency flying and catch up on paperwork. Since I won't actually have any students of my own until after New Years, my paperwork load is pretty slim. That means I get to focus on getting ready for my Aerobatic Instructor flight test—if the weather changes, that is.

The problem, however, is that the weather isn't cooperating. It hasn't been for some time now. I had a 4 week stretch without flying during November/December thanks to the weather. Whenever it did clear up, students had priority. So since I don't have my own students, I got to sit on the ground. I got a couple of flights in a few days ago, and that's about it.

It's amazing how quickly g-tolerance deteriorates when you get out of practice. Ditto for maneuvering tolerance with regard to airsickness. It comes back fast, but the first hard aerobatic flight (or three) after an extended break can be pretty rough.

Unfortunately, the Grob isn't certified for snaprolls, instead they're a prohibited maneuver. It's too bad, since snaprolls are one of my favourite aerobatic maneuvers. The good news is that we can still do hammerheads (my second favorite!), and the other 8 maneuvers required on the ride (Spin, Barrel Roll, Slow Roll, Immelman, Split-S, 1/2 Cuban-8, 1/2 Reverse Cuban-8), not to mention a few others that can be flown just for fun.

Now if only the weather would clear up so I could fine-tune the maneuvers and the teaching...

Happy Flying!

Monday, December 20, 2010

Going in Circles (Lift Part 3)

In 2 earlier posts (here and here), I've talked about the equal-transit-time principle (wrong), Bernoulli' equation (right), and Newton's Second Law (also right). I've also mentioned "circulation" a couple of times, but haven't discussed what that means.

Circulation is a description of the flow velocity around a lifting wing. It is based on the superposition (i.e. - addition) of two flows: the straight freestream flow over the wing, and a rotating flow around the wing. The result of adding these two flows is that the flow over the top of the wing is moving faster and the flow under the bottom of the wing is moving slower. Throw Mr. Bernoulli into the mix now, and we can see that there will be lower pressure on top of the wing and higher pressure below the wing—thus we get lift. A really good bathtub experiment to demonstrate circulation is found HERE

I'm not a big fan of using circulation to explain lift to pilots. It's not that circulation is wrong, it isn't. The problem lies in the fact that in order to really understand circulation, way too much math is required. By "too much math", I mean large amounts of vector calculus. Incidentally, the phrase "too much math" isn't something you'll hear from me very often, so enjoy it while it lasts!

When using circulation to describe lift (for pilots), there is no explanation of where the circulation (i.e. - the rotating flow) comes from—it's just assumed to be there. The circulation has a physical origin, or course, but describing that origin requires a great deal of math, or a significant amount of physical reasoning—both of which are necessary for aircraft designers, but a waste of resources (in this case) for pilots.

Circulation is a component of potential flow theory. Potential flow theory, in turn, is founded upon continuity as one of the key underlying principles. So circulation can be viewed as a more complicated form of continuity. It turns out that if we aren't worried about number crunching and just want to know physical origins, continuity is a quicker, simpler way to meet our objective.

Circulation has the added disadvantage that it doesn't (without LOTS of math) explain the presence of the "adverse pressure gradient" on top of the wing. This adverse gradient is important as the principle source of stalling phenomenon. In turn, understanding stalls is one of the key reasons we need to understand lift.

Using continuity directly doesn't give us a way to calculate lift. That's a problem for the engineering folk. But it does provide pilots with a description of the physical origins of lift—which makes it a good description for pilots.

When reasoning through the lift-production process, you can choose to use either circulation or continuity to explain the flow velocity difference above and below the wing. But regardless of which approach you prefer, you still need Bernoulli to explain the pressure difference—which is really the key to lift.

The bottom line here is that circulation is a valid description of the flow around the wing, and can be used (in conjunction with Bernoulli) to describe the production of lift. However, it is not the optimum approach to use for the training of pilots. For that, we have the simplified approach using continuity—which provides us with a description of physical origins, but no tools to calculate lift.

When we need specific numbers (i.e. - the results of calculation and/or testing), we can go to the flight manual. This document is provided to us by the good folks who designed and/or manufactured the aircraft. In other words, we can let them worry about the hard math so we can focus on physical origins and useful rules of thumb.

Happy Flying!

Thursday, December 16, 2010

The Top 10 Christmas Gifts for a Pilot

Xmas is nearly here (9 more sleeps!). So it's probably past time to start thinking about what to get your favorite pilot for the big day. Nonetheless, if you're like me and prefer to do all of your Xmas shopping on the 24th in one big HURRAH, you might need some pointers. So, with that in mind, The Top 10 Christmas gifts for a Pilot:

Ok, I have to admit a little bias here. I don't really get airplanes that have to stay right-side-up all the time. What's the point of living in three dimensions and then handicapping yourself by cutting one of them off??? Real pilots do it inverted (for further on this, refer to last item on the list). In preparation for doing it inverted, this book will help your pilot develop an awareness and understanding of basic aerobatic maneuvers.

Pilots may sometimes seem superhuman (which, for some reason, non-pilots often refer to as inhuman, go figure), but they still gotta eat! The best place to eat is at or near an airport—especially after flying there. It's a law of nature. This handy guide will help with that.

In case your pilot gets lost, the Spot Satellite Messenger will help you find him/her. Of course, if you'd rather s/he stay lost, you can just ignore the text messages and emails it sends you.

Pilots like shiny things. Watches are shiny.

This particular watch is also useful: it tells time, it tells time (yes, it tells time twice), and it has a handy dandy flight computer build into the bezel—so your pilot can even do airplane stuff when s/he's not in an airplane.

For some reason, photography is a common hobby among pilots. It probably has something to do with having such a great view from one's office. I know at least 2 pilots who have gone pro and started making money off their picture-taking, and a multitude of others who are hard-core photo hobbyists, even on the ground. Your pilot probably already has a camera for taking simple snapshots. You need to get him/her something fancy! I'm a fan of the Canon Digital Rebel, since that's what I have (and for no other reason, really, no expert here).

As one of the reviewers says, "Don't buy these unless [your pilot] can handle copious amounts of awesome.".

And not just any flashlight will do. It has to be a Maglite. These things are bulletproof. Not that we usually carry bullets in airplanes. But we do usually carry pilots in airplanes, and these lights are even pilotproof. Simply amazing.

If you're shopping for me, this is the ticket! Pilots have all kinds of cool gadgets and stuff, and we need a cool bag to carry it all around in. Unlike many other flight bags, this one actually had some thought put into it's design. So it's built around the size, shape, and likely organization of pilot stuff.

Check out the demo video here.

Pilots love showing off, ahem, I mean, having passengers. Ask your pilot to take you up for a flight, then cover the costs of the trip.

And, the number 1 Xmas gift for your pilot:

Need I say more? Need details? Check it out here.

Note that this gift can be conveniently packaged together with the previous one.

Happy Flying!

Tuesday, December 14, 2010


Hey Folks. Heads up on FlightWriter's new address. We were previously located at http://flightwriter.blogspot.com. Now we're at http://www.flightwriter.com. Theoretically, the original addy should still work. And it does in that it will get you to the core content. But it seems that some of the widgets on the site don't work properly from there, and some others work intermittently. I'll try to get this fixed, but alas, my area of expertise is airplanes, not computers. So your best bet is to use the new addy: http://www.flightwriter.com!

All of the behind-the-scenes-computer-magic-infrastructure now seems to be worked out. If you use the http://flightwriter.blogspot.com address, it will automatically forward you to http://www.flightwriter.com.

Happy Flying!

Monday, December 13, 2010

Wherefore Art Thou Lift?! (Lift Part 2)

In my second-to-last post, I looked at the equal-transit-time concept of lift production (and, for those who didn't read the post, pointed out that it's incorrect). I also mentioned a few things that I hear in theory-of-flight "discussions" that annoy me. The equal-transit-time concept and associated remarks were on that list. Some other important pet peeves included:
  1. Bernoulli is "just a theory" and has never been proven,
  2. If Bernoulli were right, it wouldn't be possible to fly inverted, and
  3. 80% of lift comes from Bernoulli, 20% comes from Newton (or some other set of false numbers).
First things first. "Prove: to test, prove worthy" (from www.etymonline.com. To say that something is "just a theory" and "has never been proven" is a contradiction. If an idea has never been proven (as in tested), it isn't a theory, it's a hypothesis. Theories have been tested, repeatedly, against logical consistency and empirical evidence. To become a theory, a hypothesis must survive this repeated testing and never be "disproved". The "just a theory" nonesense gets used a lot by people who have a personal preference or political agenda that is contradicted by the facts. It shows up all the time in the ongoing religion-v-evolution "debate", but I digress...

Based on this (proper) definition, Bernoulli's Equation indeed qualifies as a 'theory'. It is not a hypothesis. Has Bernoullis' equation been tested against logical consistency? Yes. Has it been tested against empirical evidence? Yes. Has it ever failed a test (i.e. - been disproved)? No. So, is Bernoulli's equation right? Yes, within the limits of the approximations made to get it (no friction and no compressibility). Can it be used to calculate (or explain the physical mechanism of) pressure changes around a wing? Yes, at low Mach numbers (where compressibility is unimportant) and low angles of attack (where friction effects are low and there is no airflow separation).

At higher Mach numbers and in the presence of friction, Bernoulli can still be used if we ("we" being the engineers who do these types of calculations) apply the appropriate corrections. From the standpoint of describing the physical mechanism of lift (i.e. - no number crunching), Bernoulli is useful right up until shock waves start to appear in the high-speed range, and until large amounts of separation result in stalling in the low-speed range.

Beyond empirical testing in the physical world, Bernoulli's equation can be derived from Newton's Second law of Motion. Applying Newton's Law to a fluid element traveling along a streamline in the absence of friction (i.e. - outside the boundary layer) will yield Euler's Equation. Applying this equation further to an incompressible fluid (i.e. - a fluid of constant density) will yield Bernoulli's equation. So the argument that lift comes from Newton, and not Bernoulli is silly, since they are both ultimately the same thing: Newton describes the conservation of momentum, and Bernoulli describes the conservation of energy (as applied to a flowing fluid).

Bernoulli's Equation tells us that as fluid's velocity increases, static pressure decreases. This is ultimately the source of lift on an aircraft wing. The question, then, is, "Where does the velocity change come from?" (Answer: Not equal-transit-time!). Bernoulli's equation says nothing about this. It just states that given a velocity change, there will be a corresponding pressure change.

The velocity change comes from the shape and angle of attack of the wing. The flow of a fluid is governed by the principle of continuity, which, boiled down to it's most fundamental form, is the conservation of mass applied to a flowing fluid. Without getting into details (maybe in a future post), continuity will result in increased velocity over the top of a positively cambered wing at a positive angle of attack.

Ok, so what about downwash and Newton's Third Law? Forces occur in equal and opposite pairs. So if the air exerts a force on the wing, the wing exerts an equal and opposite force on the air. As the wing gets pushed up, the air gets pushed down. How does this all come about? The pressure differnces above and below the wing result in the formation of wingtip vortices. These vortices create an imbalance between the upwash (ahead of the wing) and the downwash (behind the wing) so that there is an excess of downwash. The net effect is for the air in the vicinity of the wing to be deflected downward. But this deflection ultimatly requires the pressure difference above and below the wing—which is calculated/explained by Bernoulli's equation.

This addresses the (non-existent) 80/20 split between Newton and Bernoulli. The physical mechanism of lift can be described in terms of Newton (redirected airflow and action-reaction force pairs) or Bernoulli (velocity changes resulting in pressure changes). Both descriptions are valid, but they don't share the labor. If you know the velocity change and mass flow in the downwash, you could calculate the lift being produced. On the other hand, if you know the velocity changes, and therefore the pressure changes, over the wing, you could calculate the lift being produced. Both approaches would give you the same result, and that result would match measurements of the lift on the wing in question.

What about flying inverted? Well, to quote myself from above, "The velocity change comes from the shape and angle of attack of the wing", and "...will result in increased velocity over the top of a positively cambered wing at a positive angle of attack" [emphasis added in both quotes]. What about at negative angles of attack? The positive camber will induce higher velocities on top of the wing even at low and zero AOA. But if the AOA gets sufficiently negative, the wing will produce negative lift due to increased flow velocity over the bottom of the wing. Positively cambered wings aren't very efficient at producing negative lift, but they can do it—and this is indeed consistent with Bernoulli's equation (or, more correctly, continuity/circulation as a source of velocity change).

Great. Now, what about "Circulation"? It's been mentioned a little bit here, but the details will have to wait till next post! Till Then:

Happy Flying!

Friday, December 10, 2010

Dead Links

I discovered last night that there were a bunch of dead links in some of my earlier posts. The problem was that Blogger inserted extra text into several of the links trying to link back to the blog itself. I think I have them fixed now. So if you had trouble getting to external links previously, go back and try them again. They should work now.

Happy Flying!

Thursday, December 9, 2010

The Straw Man of "Equal Transit Time" (Lift Part 1)

Over the years, I've had the misfortune of hearing far too many arguments about where lift on a wing comes from. The whole debate has become something of a pet peeve of mine, because it's all nonsense. I can't stand listening to people try to present themselves as experts, and supporting the position by making false (and often outright ridiculous!) statements. Does this make me a negative person?

Some of the silliness out there includes things like:
  1. "Equal Transit Time" is wrong, therefore Bernoulli is wrong,
  2. "Equal Transit Time" is right,
  3. Bernoulli is "just a theory" and has never been proven,
  4. If Bernoulli were right, it wouldn't be possible to fly inverted, and
  5. 80% of lift comes from Bernoulli, 20% comes from Newton.
For number 5, the numbers vary, but the wrongness is all the same. Is "wrongness" a word? According to spellchecker it is, but I'm not so sure...

The "Equal Transit Time" of points 1 and 2 is a fallacy that has (fortunately!) fallen into disrepute over the years. But it still gets set up as a straw-man to "disprove" Bernoulli. According to the equal-transit-time principle, the air particles that separate at the leading edge of the wing must meet up again at the trailing edge. So, because of the wing's curvature (the top surface is longer than the bottom surface), the over-top flow must travel faster than the under-bottom flow.

Unfortunately, there is no physical principle that requires air parcels to meet up again once they are separated. In fact, wind tunnel testing (among other methods available) can and has demonstrated conclusively that the equal-transit-time requirement is not met by a lift-producing airfoil—the over-top air travels significantly faster than the under-bottom flow and reaches the trailing edge first (see a useful NASA applet here).

Ok. So equal-transit-time is wrong. So far so good. But here's the problem: Several authors and educators have taken the position that since equal-transit-time is wrong, Bernoulli must also be wrong. The "reasoning" (and I use the term in the loosest possible way) is that equal-transit-time leads into Bernoulli by supplying a source of velocity difference, which is then converted into a pressure difference using Bernoulli (higher velocity over-top means lower pressure over-top). So if the source of the velocity difference is wrong, the pressure conversion must also be wrong. Right? Wrong!

The change in velocity and the change in pressure are two different phenomena that require two separate explanations. There is in fact a velocity difference above and below a wing. And that velocity difference does in fact result in a pressure difference a-la Daniel Bernoulli. It's just that the velocity difference is not explained by the equal-transit-time concept—instead, it must be explained by the principle of continuity, or circulation (which, in turn, is rooted in continuity anyway).

More on Mr. Bernoulli and Sir Isaac in my next post. And for those of you who insist on discussing Circulation, don't fret! I haven't forgotten you!

Happy Flying!

Sunday, December 5, 2010

The Top 10 Considerations for Winter Operations

Well, winter is certainly here. At least it's here on the prairies, and here to stay. We had a -18C day today. I'm new to the prairies, but I'm told it gets quite a bit worse... That would be ok if the wind would just stop filling my driveway with snowdrifts!

So, since winter is here, it's time to have a good look at winter ops. A quick and easy way to do that is with a top 10 list. So, without further ado, the top 10 considerations for Winter Operations (in no particular order):
Engine (and Battery!) Pre-Heat

Engines can be finicky when they're cold. The two main problems are the oil viscosity (although this problem is reduced with modern, multi-viscosity oils) and the reduced vaporization of the fuel—liquid fuel doesn't burn, it must vaporize and mix with air first. These problems are aggravated by reduced battery performance in cold conditions.

To improve the start-up reliability of an engine in cold weather, pre-heat the engine before starting. The easiest way to pre-heat is in a warm hangar, but an engine heater can also be used. An engine heater is simply a supply of warm air (warmed electrically or by combustion) pumped into the engine cowling. Heater effectiveness can be improved by using a cowl cover—which is essentially a form-fitted blanket that wraps around the engine cowl to help trap heat. Having a cowl cover provides the extra advantage that during short turnarounds, the cowl cover can help the engine hold the heat from the previous flight, thus eliminating the need for an engine heater.

Engine Warm-Up on Start

After the engine is running, it still needs time to come up to full operating temperatures. As long as there are strong temperature gradients in the engine, there will be unnecessary mechanical stresses. Running the engine at high power settings prematurely can lead to damage and a need for extra maintenance and/or early overhaul. Hopefully, you have a cylinder head temperature gauge, as this is the most important temperature indication for winter ops. But if you don't you'll need to use oil temperature as a proxy. This is less accurate and less reliable, but sometimes a necessary evil.

There is some good information on engine pre-heat, start-up, and warm-up HERE.

Shock Cooling

Shock cooling of piston engines during prolonged descents is always a concern. But it becomes much more of a concern during winter ops thanks to the increased temperature difference between the engine and the cooling air. Having a winterization kit helps (by reducing the cooling airflow) if that's available, as does closing the cowl flaps if you have them. But in any case, you should carry partial-power during all descents. If that won't get you down fast enough, use flaps, gear, spoilers, or a finer propeller setting—or even better, fine-tune your descent planning and start down sooner.

It's also a good idea to warm the engine by spending a few seconds at cruise power (or full power for lower-powered engines such as those on most trainers) every couple of minutes. This can be referenced to altitude instead of time (for example, every 500 or 1,000 feet) for the sake of keeping track.

Warming the engine has the added benefit of confirming that the engine is still actually running. At low power settings in a descent, an engine failure may be difficult to recognize. Further, as the engine cools, the quality of combustion in the cylinders can be reduced—sometimes resulting in an outright engine failure. Identifying a failure sooner means you have more time (and altitude!) to deal with it.

Note that there's a pretty good argument against the very existence of shock cooling HERE, but until more data is in regarding the problem, I'd stick with conventional wisdom. Worst-case-scenario then is that you're erring on the side of caution—not a bad course of action in an airplane!

Critical Surface Contamination

Surface contamination can occur year-round in flight under the right conditions. But only during winter operations will you see frost and icing on the ground. Inspecting for and eliminating this contamination is a vital part of your winter pre-flight inspection.

Surface contamination has two key and detrimental effects—it increases drag and decreases lift. The ultimate effect of both of these is to degrade performance—sometimes to the point where retaining control or remaining airborne become impossible. Surface contamination will result in reduced cruise speed, reduced climb rates, reduced glide range, increased stall speed, increased takeoff distance, and the potential for control malfunctions—including things like aileron reversal and tail stall

Dealing with in-flight icing is a topic unto itself, and is briefly touched upon below. But dealing with ground icing is easy—get rid of it. Any adhering ice or snow (including frost) should be removed from your aircraft before departure. You can do this during pre-flight with brushes and scrapers or in a warm hangar, or you can get your aircraft de-iced with de-ice fluids before takeoff. The choice between these two approaches will depend on the available facilities and your available budget. Either way, plan ahead and make sure your schedule accounts for the time required and that you have the required equipment.

Further information regarding ground icing can be found in NASA's excellent online course right here.

Runway Contamination

Contaminants on the runway (water, slush, snow, ice) can hit you with a good one-two punch. They degrade acceleration (on take-off) and deceleration (on landing) plus, they degrade directional control. This can result in performance degradations leading to runway over-runs, or loss of control resulting in going off the edge of the runway. In extreme cases of landing on thick snow, the result can even be flipping over and stopping very short after the landing gear digs in.

Avoiding runway contamination completely isn't always an option. So the key is to be aware of contamination present and to be familiar with your aircraft's limitations (and your own!). When considering these limitations, don't forget to account for the crosswind component and the gustiness of the winds.

At airports with ground facilities, such as control towers or flight service stations, advanced notice of surface contamination is often available in the form of Runway Surface Condition (RSC) reports. At uncontrolled aerodromes with no ground stations available, you may be able to get reports from other pilots who have taken off or landed there recently. In the absence of these reports (or if you doubt their reliability), a precautionary inspection of the surface is called for. If you're taking off, this can be done by taxiing (or in extreme cases, walking) the length of the runway. If you're landing, conducting a low-level inspection would be prudent—as would including fuel for a possible diversion in your flight planning

Airborne Icing

Airborne icing can occur year-round, so it really isn't a winter-specific problem. However, it is more common during the winter. It's also far more likely in the winter to occur outside of cloud—for example, in precipitation or mist—making it a much more significant concern for VFR pilots during winter than during summer.

Unless you are in an aircraft equipped for icing conditions and you have received training in operating that equipment and dealing with in-flight ice, the best way to deal with the ice is to avoid it. This means checking forecasts and reports thoroughly prior to departure. It means getting weather updates while en-route. And it means developing an in-depth understanding of the weather and the conditions that may lead to icing.

If you find yourself in icing conditions (without the equipment or training to deal with it), deal with the problem like any other emergency—and yes, this is an emergency—keep flying the plane, and assess before acting rashly. It's often a good idea to undo the last thing you did. If you were in cruise when you entered ice, turn around. If you were climbing, descend. If you were descending, climb (if you still can).

In any case, your objective needs to be met in two steps: first, stop any further ice accumulation; second, get rid of the ice you have. The first step means getting away from liquid water. The second means finding some warmer and drier air to get the ice to sublimate off. Both of these objectives can often (but not always) be met by changing altitudes. As you're dealing with the problem, talk to ATS and/or other pilots to get more information on where the icing conditions are. Also, pass the word on so other pilots can avoid the conditions you've found yourself in.

Further information regarding in-flight icing can be found in NASA's excellent online course right here.

Cold Weather Survival

If you're lucky, you get to do your pre-flight inspection in a warm hangar, and you have an airplane with a good heater. That makes winter flying much more comfortable and enjoyable. However, it can lull you into a false sense of security. If your heater fails (or if you have to shut it off due to CO poisoning, discussed below), or worse, if your engine fails and you have to spend a night in the woods, you'll be very sad if you only dressed for a warm hangar and a good heater. If you're lucky this time, you'll only lose your fingers and toes to frostbite. If you're not so lucky, Search and Rescue will become Search and Recovery. Even barring an emergency, un-forecast weather can force you to make an unplanned landing off-field or on a small middle-of-nowhere aerodrome with no facilities.

Keeping all of this in mind, you should always be dressed for the walk home. Sure, you can (and should) store some extra stuff (including blankets and fire starter) in the back. But imagine crashing and being pinned in your seat unable to reach that stuff in the back—or narrowly escaping from burning wreckage only to watch your blankets go up in smoke. Won't you be glad you turned the heat down and wore some extra clothing while flying?

Cold weather survival is something that deserves attention during winter ops. I recently heard a statistic from some military folks: Average Search and Rescue time is 48 hours. So if you're on the ground and nowhere near civilization, you need a way to keep warm.

Spatial Disorientation due to Whiteout

Whiteout occurs when an overcast sky visually merges with a featureless, snow-covered ground—eliminating the horizon from a pilot's visual reference. This condition can lead to spatial disorientation and a subsequent loss of control, even in clear weather with good visibility. The problem can be compounded by empty field myopia, which sets in in the absence of features to focus on. Whiteout can also be aggravated by reductions in visibility, especially by falling snow, even when light.

Increased instrument reference is a must in whiteout conditions. If you find yourself disoriented, try to change directions (with instrument reference) to find ground features that will help you regain orientation. Hills, mountains, heavily forested areas, bodies of water that haven't frozen yet, large built-up areas, etc. can help break up the snow cover of the ground, and thus reduce or eliminate the whiteout.

Carbon Monoxide Poisoning

In the winter, we tend to seal up the cabin as best we can, and fly with the heat on. There's a good reason for this: It's COLD! The downside to this quest for warmth is that it increases the risk of CO poisoning. Heat is supplied either by a heat shroud over the exhaust manifold/pipe, or by a combustion heater. Either way, there's a risk of combustion products leaking into the cabin.

The best way to deal with this risk is to be equipped with a CO detector. The detector you use should be sensitive enough that it will sound the alarm before any negative physiological effects kick in. And what do you do when (if) the alarm goes off? 1) Turn off the heat, 2) Open the vents and windows, 3) Land. Yes, it's going to get cold. So dress for the walk home.

The Pre-Flight Inspection

Although we've already looked at the engine and battery pre-heat, as well as critical surface contamination—both of which are key elements of the pre-flight inspection—it's worth revisiting the pre-flight from a broader, human factors, perspective. It's cold out! Human nature being what it is, cold is a strong motivator to hurry it up and take shortcuts. It also creates a significant distraction—we focus on staying warm instead of inspecting the aircraft. In a word, this is dangerous.

There are two key steps to addressing this problem. The first is to be aware that there is in fact a problem and to make a conscious effort to focus on the task at hand (i.e. – the pre-flight inspection). The second step is to dress warm. If you're dressed for the weather, the weather will be less of an impediment for you. This goes back to cold weather survival, too. If you're carrying winter gear for an emergency, why not use it for normal operations as well?

An even better solution, when the option is available, is to complete your pre-flight inspection inside a heated hangar. This keeps you warm—helping ensure a quality inspection—and it keeps the engine warm until it has to go outside.

Wednesday, December 1, 2010

The Myth of Centrifugal "Force"

As a follow-up to a previous post on Equilibrium, I thought I'd talk a little bit about a common non-equilibrium maneuver—the steady turn. Much of what is said here can also be applied to pitching maneuvers—in fact, any maneuver that involves a curved flight path and the associated accelerations.

Our interest in turns, for today, stems from the often-debated "Centrifugal Force". Centrifugal Force is "often-debated" because there are those who claim that it doesn't exist, while other insist that it must because you can "feel" it in a turn. From the title of this post, you can probably guess which camp I fall into. I would argue that centrifugal force does not exist. However, in fairness tho those who disagree, it really does depend on your perspective (I'll clarify that shortly). And we'll see that centrifugal force is still a useful concept when considering objects that are moving in a circle.

Centrifugal force fits into a category of forces that physicists call "pseudo-forces", and engineers call "inertial forces". these forces exist—in that they can be both calculated and measured—only if you are using an accelerating object as your reference (a "non-inertial reference frame" in technical terms). Normally, we measure position, velocity, and acceleration from a position of equilibrium. In this case, the position and velocity of an object might vary depending on our reference, but the acceleration will always be the same.

This changes if we measure or observe from an accelerating reference. Measuring the acceleration of an object from an accelerating reference means that we will get a different value than someone else measuring the same acceleration from a different reference (either in equilibrium or accelerating differently). Inertial forces, including centrifugal force, ultimately stem from this discrepancy—recall (from Newton's Second Law of Motion) that acceleration is closely related to forces.

Let's consider what happens in a car going around a turn (we'll use a car here instead of an airplane in order to eliminate the small complication that comes from considering bank angle). When you're sitting in a moving car and the car is taken around a turn, you feel yourself being "pulled" to the outside of the turn. That sensation is very real. The problem is that your frame of reference—the car—is not in equilibrium. The physical sensation of centrifugal force is in fact the sensation of the car pushing you into the turn. An observer outside, sitting on the curb and in equilibrium, sees the friction on the tires pushing the car into the turn, and sees the car seat (and associated hardware) pushing you into the turn. No forces are present directed to the outside of the turn.

The same principle applies in an airplane, except that we turn in a banked attitude, so the "force" that we feel is directed straight down through the floor of the aircraft, not to the side (assuming the turn is coordinated—a topic for another day). Ditto for wings-level pitching maneuvers—such as entering or exiting climbs or descents, or aerobatic maneuvers such as loops.

In practice, the importance of Centrifugal "Force" is in it's relationship to load factor. Load factor is a measure of the aircraft's "normal" acceleration—where "normal" means perpendicular to the flight path. There's a limit to how much load factor we can take. At low speeds, we eventually reach the stall limit. At high speeds, we eventually reach the structural limit. At any speed we might reach our own physiological limit (grey-out – black-out – G-LOC), which can vary significantly from person to person. Load factor is controlled by controlling lift. At any given airspeed, lift—and therefore load factor—is controlled by Angle of Attack.

Is Centrifugal Force real or isn't it? Well, when you're sitting in an aircraft pulling 6 g's it certainly feels real! The structural loads are real, the physiological effects are real. So at the very least, the concept is useful. Perceiving the aircraft as if it were in equilibrium with a new "gravity" force may be easier on the brain than keeping track of all the physics of flight—especially while simultaneously trying to stay oriented and maintain control!

So the bottom line here is that Centrifugal Force is not truly a force, but it's still a useful concept. It simplifies our picture of the world and helps reduce the brain power dedicated to things other than flying the airplane. But if you want to hangar fly and argue the nitty-gritty of flight theory, it really doesn't exist!

Happy Flying!