Sunday, September 20, 2015

"How high/far/fast will it go?"

I get that/those question(s) a lot, and I can seldom answer it/them.

It's not for lack of wanting to, it's just that the three categories are really all part and parcel with the same question, and it is a question with nearly infinite answer.

It's the definitive "it depends" question.

Most people are interested in the "fast" aspect, so I'll start with that.

The first thing to do is to agree on which units to use. We can choose from Kilometers per Hour (Soviets), Knots (most of the rest of the world), and Miles per Hour (favored by people with really slow airplanes).

I talk in knots.

Knots as a unit is slightly different than the other two in that it does not contain the name of the unit of distance, and the "per hour" is assumed. To make it like the other two, it would be call Nautical Miles per Hour.

As a basis of comparison, 138 mph is equal to 120 knots.  "120" is a large enough number that it needs no embellishment, but assume your airplane is a Searey that only goes 80 knots. You would be much more interested in telling people that it did 92 mph, although that's still somewhat shy of brag-worthy.

The answer to the "how fast" question is still not answerable, though.

Now that we've established a preferred unit of measurement, we have to talk about the two majors divisions of speed. The first, and the one most pertinent to the speed between Point A and Point B on the earth, is the ground speed. This is what most people are asking when they ask how fast it is because they drive cars; the speed of your car is by definition ground speed.

In an airplane, if I was flying along and crossed over Cincinnati and had 120 nautical miles to go get to Columbus, and I was going 120 knots across the ground (ground speed), I would reach Columbus in one hour, just as a car would. The difference is that I'm not guaranteed to achieve any given ground speed because I am subject to the whims of the wind.

The other speed is actually a collection of other speed. We will only talk about a couple of them, but before we do, lets talk about airspeed in general.  Simply put, airspeed is the speed at which the plane is going through the air. Yes, it sounds simple, but... there are a lot of details to it. For now it is important only to understand that moving through the air at 120 knots does not in the least mean that your ground speed is 120 knots. The primary, but not only, factor in that difference is the wind.

This is not obvious to some people, so I use the comparison of a boat on a river. If the downstream current is 10 knots, and the "boat" is nothing more than a floating log, how long it would take for it to go 10 nautical miles (nm) down the river?  Easy: one hours. What was it's speed through the water? Easy: zero knots. It has no propulsion, so how could it go faster through the water?

Now let's assume the boat has a small motor that can push it through the water at 8 knots. It gets the first 10 knots for free and adds 8 more from its motor. Now its water speed is 8 knots and its ground speed (the speed at which it would go past a bridge) is 18 knots.

Reverse the situation, and let's assume a boat is going up stream against the current. It can only go 8 knots through the water, but it's losing 10 knots to the countervailing current.

How fast is it going?

It depends. Its water speed is 8 knots, but its ground speed is actually 2 knots in reverse.

In answer to the usual follow-up question, yes, an airplane can go backwards, for so long as you are using ground speed as the reference.

This is also why pilots aren't fond of headwinds.

"But Dave," you say, "doesn't it all average out? If you have a headwind one way and turn around and come back, doesn't the tail wind make it all better?"


Let's do some simple math. Let's assume we have an airplane that does 120 knots, we're flying to a town 120 nm away, and we have a 30 knot headwind. That makes our ground speed 90 knots, so the trip takes an hour and twenty minutes instead of the one hour it would have taken with no wind at all.

With the tail wind, we get to come back with a 150 knot ground speed, so it only takes 48 minutes to get back.

Our round trip time was two hours and 8 minutes. With no wind at all, it would have only been two hours. The same wind in each direction ended up costing us 8 minutes.

So, we now agree that it pays to minimize our headwinds, right? I hope so, because that's the crux of the rest of this lesson.

So, back to airspeed. The airspeed that is displayed to the pilot via the airspeed indicator is called Indicated Air Speed, or IAS. This one is pretty intuitive in how it works - the air enters a hollow tube (called a Pitot Tube) and the more air there is, the higher the needle goes in the airspeed indicator. And the way to get more air in the tube is to go faster through the air.

Simple, right?

Well, no. There are a few complexities that complicate the situation, but the most prevalent one is that the air pressure varies from day to day, and decreases as you go higher into the sky. The thinner air at higher altitudes affects the airspeed indicator and causes it to display a lower value that what's actually happening. There is a mathematical formula for correcting to what it called the True Air Speed, or TAS, but it involves the multiplication of the square root of a division - Common Core math or no, this is not something that you can typically do in your head. If you can, you should quit your day job and make a living winning bar bets.

Fortunately, my Dynon Skyview is good at math and does it for me. Before that, I just assumed that I would get an extra 2 knots for every 1000' higher I flew. If I would get all the way up to 10,000', I would be getting an extra twenty knots for free! But.. winds can be much stronger the higher you get, so you may given up the extra TAS by flying higher. Or, you may get a mighty boost from a strong tailwind.

So why not fly high when you have a strong tailwind and lower when it would be a lower headwind?

Good question, and for the most part, I try to do that. But....

It takes time and gas to climb. I can climb at airspeeds ranging from 70 - 90 IAS.  My level IAS usually falls into a range of 107 - 113 knots, depending on the current air pressure and temperature and the current weight of the plane. If it's a short trip, it doesn't make sense to spend all of that time climbing.

In the summer, another reason to fly higher is that the air is smoother. But the higher you go, the more you have to deal with clouds. I have to stay out of those. Sometimes it's so bumpy down low that you accept a headwind in exchange for cooler, smoother air.  Other times, you have no choice and you have to slog through hot, humid, turbulent air. And you haven't experienced turbulence until you've flown in a 1,000 lb. airplane.  It can be brutal.

So, at long last, to see how all of this ties together with "How high/far/fast will it go?" we'll consider a couple of trips. I've been bad about keeping up with the blog, so this will be a "two birds" kind of thing.

A couple of weeks ago I made my most common trip, which is from Bolton Field (KTZR) to Darke County (KVES) airport. I got a late start due to morning fog, which was not entirely unexpected since we had been stuck in the same high pressure weather for long enough for the air to become humid and stagnant. I waited just long enough for the weather to reach four miles visibility, which is legal for VFR flight, but is referred to as "Marginal VFR." Back in the day, before I had sophisticated GPS-based nav equipment, that would have given me pause. It no longer does, but I still give it awhile to stabilize lest it regress to something far less forgiving.

The Skyview with ADSB is really good for this kind of thing. As long as the dot at the airport stayas blue (or turns green), I'm still good. Turning red would cause me to make an immediate turn back home.

By the time I was ready to head back, the humid morning had given way to a hot, humid day with indications of a lot of turbulence in the air. The biggest indicator is those tall puffy white clouds. The only smooth air to be found is up above them, but as I am legally limited to 10,000' and the plane can't go a whole lot higher than that anyway, I usually can't get over them.

I tried anyway. At 9,500', I still wasn't over them and knew that I would have to go down under them and endure the hot, bumpy ride rather than the smooth, 50 degree air up high. Which was fine - I seldom fly that high, and even though I know it's no different than flying lower (you can drown in 10' of water as easily as 1,000'), it still makes me tense,

As a bonus for going down into the muck, I caught a glimpse of what I think is the track my brother races at. It's called Shady Bowl:

A couple of days ago, I flew out to Wheeling, West Virginia for lunch. It was another marginal VFR, which again was no surprise given that the weather really hadn't changed much.

Again, the blue dot was key.

It wasn't hot yet, though, so I was able to find smooth air at 5,500' and just kick back to let the autopilot do all the work.

Actually, I had a little work to do myself. I'd never been to Wheeling, so I had to spend some time getting familiar with the layout of the airport. Once I landed, the tower was going to give me taxi directions and it pays to be familiar enough with the runways and taxiways to be able to follow those directions. They carry the force of law, you know.

I figured the wind was favoring runway 21 and that I would easily be stopped by the intersection with the other runway, but I didn't know if they would have me turn right and go down to the end of the other runway, or have me continue on down runway 21 until I got to taxiway A1, where I would then follow taxiway A back to the other runway. My bet was on the first option, assuming I could land and slow down before getting to the intersection of the two runways.

My opinion is that the visibility was far better than four miles.

I did, in fact, get cleared to land on runway 21. Typical of West Virginia airports, this one is situated on the top of a chopped off hill. Another hallmark of these airports is that the runway is wavy - they never seem to get them completely flat. Finally, the abrupt ending of the ground at the approach end of the runway is scary - I seem to always land long on these because of my irrational aversion to be plastered  against the side of a cliff.

I'm weird that way.

I ended up landing well short of the runway intersection. In fact, I was approaching it at essentially walking speed when the tower called, "If able, turn right at runway 34, right at taxiway Delta, Delta to the ramp."

Just what I expected. Pre-planning helps. The interesting dichotomy of the autopilot is that it makes be both a better pilot (tons of information at my finger tips and time to read it) and a worse pilot (I don't hand fly nearly as much as I used to).

The terminal building was worth the trip in and of itself:

An original Wright Brothers bicycle! I've never seen one before.

Kyle arrived about twenty minutes after I did.

He didn't land on the numbers, but still managed to get slowed down in time for the intersection turnoff:

We used the airport's courtesy car which, as with many of these free loaner cars, was a bit quirky. The 'Service Engine Soon' light never turned off, but it was periodically joined by the red 'Door Open' warning light, despite the four firmly closed and latched doors. It would also emit a mytersious BEEP now and then, lock the doors, and run one cycle of the wiper blades.

Worst of all, it had a very weird tendency to enter parking lots from the wrong direction.

We went to a place called Primanti Bros.  Apparently it's "a thing" out east.

I had the "Stupidly Big" burger (my name, not theirs) which was comprised of a 1/2 lb. of ground beef, bacon, pastrami, provolone cheese, a fried egg (the purpose of which was to ensure that everything slid out of the buns every time I tried to take a bite), red onions, banana peppers, and enough lettuce to feed a rabbit for a year.

I took the lettuce off.

I drink my whisky neat, too.

The scenic tour of downtown Wheeling was, well, brief:

Looking back as I departed, you can see how the runways look in real life as compared to the diagran I studied on the way in. I had landed on the lighter colored of the two runways.

The weather had developed to the degree that I again wanted to get up by this time, but I had to settle for 6,500'. Any higher and I would have backed my way into Pittsburgh.  My indicated airspeed was probably around 107 knots, but the extra altitude gave me another dozen to make a respectable 119 knots, but the headwind was costing me 25 knots.  The net result was a ground speed of only 95 knots

Well, not 95 knots.

It was actually 109.324 mph!  Doesn't that sound better?

Today was the first day after a front blew through and cleared out all of the stagnant air we had been dealing with. It seemed an auspicious time to make another trip to KVES.

To get a day any clearer than this, it would have to be a very cold day in February.

Smooth air + autopilot = selfies!

Another indication that the weather is changing. Only 46 degrees at 5,500'!

Resulting in the first time using the cabin heat since, well, a long time ago.

There they are, the only clouds for a hundred square miles!

With the wind out of the east, I had to circle around to the other side of the airport.

We ran into this guy. He found this old plane in a barn, literally close to being crushed as the barn slowly collapsed. It's based on a plans-built plane called a Fly Baby, but it's unique in that it has a closed cockpit and it's stressed to be aerobatic.

 It was not only bumpy down low on the way back, it was actually very bumpy. It was enough to want me to get up out of it, even if only for fifteen or twenty minutes. At 5,500', I was within 500-600' of the clouds, and it was still quite bumpy.

I found a nice long clear area that was only about 20 degrees off my desired course, so I climbed up to 7,500' - with the cooler temps and thick air of a high pressure area, the extra 2,000' only took four or five minutes, and it made a world of difference.

Here you can see all of the reasons what it is hard to answer how high/far/fast it will do. There are so many variables involved that the answer changes by the second.

Back at the hangar, I discovered that I now have another hangar frog. I don't think I've ever seen one join me this late in the year.

I also discovered that my hangar beer is older than a hamster lives.  Three years old!

Know what? It does NOT improve with age.

1 comment:

Hugo said...


95 knots sounds better as "175.94 kilometers per hour". See how much faster you get home when you switch to metric?

Great write-up and photos, as usual.

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