Airspeed (Opinion)
Every airspeed is the end result of thrust overcoming drag. The relative movement of the plane to the earth and the air above is the end result of pilot settings of pitch and power. Airplanes need speed to fly. However, the recording of this speed does NOT use moving air; instead, it uses air pressure. You should know that moving air does not enter the pitot tube. Only air pressure is applied through the pitot tube.

The pitot tube pressure can be indicated in several ways but the most common is a differential pressure indicator that measures the difference between impact pressure and static pressure on different sides of a flexible air chamber. A movable arm is geared to the air chamber so that diaphragm movement is measured on the airspeed dial. The pitot tube measures impact pressure while the static tube measures undisturbed air pressure. A single static port has inherent errors that occur when uncoordinated flight disturbs the air at the static hole. The resulting speed measure, called indicated airspeed (IAS) is uncorrected for the plumbing installation, air density, or instrument imperfections.

Over the years the markings of airspeed indicators have remained much the same. However, the source of the markings have varied. In the 1970 airspeed indicators were usually in miles per hour and in calibrated speeds. Calibrated airspeed is indicated airspeed adjusted for installation and interment error. Generally the airspeed correction tables have indicated airspeeds that show lower and higher than calibrated speeds. This is a built in safety factor in that when you are indicating a slow speed you are not quite as slow as you think you are. When you are fast you are not quite as fast as you think you are. Calibrated airspeed should always be used to calculate the 1.3 Vso approach speed and then converted to indicated airspeed for the actual approach. 1.3 Vso is the speed to use if no maneuvering is required on final. The final authority for any aircraft is the appropriate model and year matching the airplane to the POH.

Calibrated airspeed is different from indicated airspeed in that it makes corrections for installation, density, and instrument errors. The range of difference between indicated and calibrated airspeeds are shown on a chart in the POH.

The calibration of an airspeed system is based on standard conditions of pressure and temperature. As density decreases with altitude the speed of the aircraft must be higher in order to achieve the same instrument reading. While the indicated speed will decrease with altitude due to this decreased impact pressure, the true airspeed will increase. True airspeed is available in the POH for planning purposes. True airspeed can also be calculated in flight using E6B calculations.

If wind is not a deciding factor, it is always better to fly high to obtain the resulting higher true airspeed. The calibration of an airspeed indicator is based on standard pressure of 29.92 inches and 59 degrees F temperature. Indicated and true airspeed are identical. Above sea level your true airspeed will be faster than indicated because it takes more speed in thinner air to register the indicated speed. True airspeeds are slightly faster the cooler the temperature but any increase is negated by the drag of the denser air.

Deviate from the manufacturers V speeds and you will have reduced performance in every flight regime. However, if you trust the performance figures in your POH you are an accident waiting to happen. The book figures are for a new aircraft. A ten year old plane with a mid-time engine may have a 20% performance deficit. Instead of the book we must trust our experience and judgment augmented by local experts. You could develop your own POH book for your aircraft and insert your ‘real’ performance figures. The Piper pitot/static air mast is not good over the full range of speeds and gives variations of static pressure.

Instrument aircraft often have an alternate air valve that allows cabin air to replace the blocked exterior static hole. When being used, the alternate static air causes the altimeter to read high, the airspeed to read high, and the VSI to show a climb for level flight.

True Airspeed
Several things happen as you go higher. The decreasing air density decreases drag. However, your engine power does decrease as well. That doesn't matter so much. A given power will give you about the same indicated airspeed at any altitude. Your fuel consumption also depends on the power you are producing. With a piston engine you will get about twelve horsepower for every gallon per hour of gasoline you burn.

At cruise speed you generally throttle back to something less than maximum takeoff power. Typically you will cruise at sixty-five or seventy-five percent of maximum. Near sea level this will give you a specific indicated airspeed. You will also have a specific fuel burn that corresponds to that amount of power.

As your altitude increases your true airspeed becomes greater than your indicated airspeed, increasing with altitude. You do have to advance the throttle to maintain your cruise power output. However, as you advance the throttle you fuel consumption doesn't go up because you are leaning the mixture and still producing the same absolute amount of power, even if it takes more throttle to do it. At around eight thousand feet, you require full throttle to get seventy-five percent power in a normally aspirated engine. You are still burning the same amount of fuel that you were at that power setting down lower and still seeing the same indicated airspeed. However your true airspeed is higher, so you are both going faster and getting better "gas mileage".

Once you can no longer maintain "cruise" power you will start to slow down, but when you do your fuel consumption goes down as well, so you still get good "gas mileage" and you airspeed doesn't decrease very much.

Any airplane is the most efficient at covering the ground at the airspeed where the lift/drag ratio is greatest. That is pretty much a constant indicated airspeed for you weight. The airplane will get the greatest range at the altitude where you can just maintain your "best glide" speed ( optimum L/D ratio ) at full throttle. For most normally aspirated single engine aircraft that is somewhere between ten and eleven thousand feet above sea level on a standard day.

If you look at the POH for a Cessna 182 you will see that the airspeed for best range is quite low and the altitude is about 10,500 msl. That is not necessarily the fastest ground speed, but it does give you the most distance covered. It is your best "gas mileage" flight condition.

Highest cruise speed is at the altitude where full throttle yields your selected cruise power output. Generally a bit lower, around 8000 msl.
Highflyer

Pitch vs. Power
For years old timers and the FAA have been arguing what controls airspeed and altitude. 'Stick and Rudder' pilots believe that elevator controls airspeed. The FAA demands that the elevator controls altitude with attitude controlling airspeed. The conflict has became one of theory against reality. Will an FAR soon legislate the laws of physics?

The altitude and airspeed performance is not independent of either attitude or power. The pilot is the controlling factor. What he does in the cockpit with the elevator set attitude and what he does with the throttle sets the power. One control or the other is a factor in all flight and only in some situations does one dominate the other. How well you know your airplane’s flight characteristics is as essential for all flight regimes not just landings. Every aircraft has idiosyncrasies your checkout must expose you to those. Otherwise, you become a test pilot.

In cruise flight and constant speed approaches the elevator dominates attitude and altitude while power sets airspeed. The ability of the elevator to exercise this control exists when power is both variable and available. It is only when power is not available or considered as a locked constant that the elevator can control airspeed. The FAA expects you to use the vertical speed indicator (VSI) (elevators) to maintain pitch and the throttle to keep airspeed. This works for flying the instrument landing system (ILS). If the throttle is set as a constant 1500 rpm then the trimmed elevator can control airspeed. The FAA accepts this idea that elevator controls airspeed when power is constant. The trimmed elevator gives greater control over the glide path than power.

Power is a relatively coarse adjustment to the glide path. Massive reductions or increases in power produce illusions of change, especially, if airspeed changes occur simultaneously. Only by keeping the airspeed constant can the illusions of change due to power application be overcome. Mistakes, and corrections, in being high and low on approach will be an important part of your landing training. Reduction of power in increments of 100 rpm can allow slight and smoother changes in the approach path. Since the effects of power are so variable, due to inertia, most pilots chose to set power at predetermined levels and use the elevator to set the trim to give the attitude most likely to meet the airspeed sought. Why? Because it works.

We use pitch as a form of energy control. We can covert kinetic into potential and potential back to kinetic. With speed we can use the elevator control to create a climb at a cost of airspeed. With altitude we can use the elevator control to forgo altitude in exchange for airspeed.

In reality the pilot controls the situation by using a combination of elevator and power which through this combination determines airspeed and altitude. At any given moment the pilot will make a control decision between altitude or airspeed and use power or elevator, in combination, to meet the needs of that decision. Where ever the FAA does not have a recommendation as to procedure "...a coordinated combination of both pitch and power adjustments is usually required". The Flight Training handbook is being rewritten as of 1995. Hang on.

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