Friday, June 1, 2012

Angle of Attack in a Dive

Well, sooner or later we all have to descend from our beloved airspace. We can get down a variety of ways. We can glide, dive, power off or idle slow flight or descent with power.

Several posts ago I presented a problem on a descent through cloud cover that forced you to make several choices on flight attitudes you could use. 

Some solutions, depending on your training, experience, qualifications of your aircraft, etc. could save your life.

I recall conversations, during a good cup of Joe, talking flying, that a choice may involve putting a plane into a spin. Someone else, with instrument experience thought a normal descent on instruments was his choice.  Another participant in our "hanger talk" thought, since he was intimately aware of his "feel" for his plane, would choose the slow flight method at reduced power.

The dive may be a quick way to penetrate the cloud cover but recovery was questionable if the ceiling was lower than anticipated. You get the idea! 

How does a "dive" work with our discussion of pushing air about to create lift, our possibly, no lift at all?

We don't need  power. Gravity does the job. Obviously, the attitude of the plane is extremely steep. Horizontal forward motion of the plane, separate from the attitude of the very steep nose down attitude of the dive, may be present and, in a certain case, non-existent.

Another way to look at a dive is the similarity to normal cruising flight with a very low angle of attack and increasing speed as you descend. A dive does have its structural limitations.

Since you do have a limited angle of attack, in a vertical dive , the lift generated pushes the aircraft horizontally along the original flight path of the plane. If you thought the dive would take you down to an object directly under the plane ,when you started the dive, it wouldn't be there. It would be well behind you, depending on the altitude you started the dive. Higher the altitude the greater the distance traveled horizontally.

If you really want to perform a military descent to a target directly below you, to make your plane very hard to shoot down, you have to perform a dive where no lift from the wings occurs.

In this dive you actually put the plane on its back past vertical. Look at a war movie where the plane seems to roll over on its back as it descends. Aviators are beginning a "past vertical" dive to a target directly below. The airplane is actually meeting the air at  the "no-lift" angle of attack. Your airplane is not creating lift it has only drag.

In this attitude the airplane can achieve its highest speed and becomes vulnerable to structural failure. It does move forward horizontally.

Civilian planes should not even think about a "past vertical" dive. You have a placard in you plane, colored red, that gives you a "Never Exceed MPH."  

All sorts of things can happen in dives. Turbulence can cause structural failure. Rapid pull out from a dive can cause structural failure. You may experience a "red out" where blood rushes to your head like in a hyperbolic arc flight path. You may put yourself into a "blackout" flight path like a normal pull out from a dive but with significant G-forces to allow the blood to rush out of your head and cause you to lose consciousness.

None of these are good for your health. It is good to know how you can place the plane into the attitude where these can happen. Just don't do them in a civilian plane.

Thursday, May 31, 2012

Miles per Gallon Flying

In the discussion of "Slow Flight"  you could fly forever, so to speak, at a slow speed and a high angle of attack close to a stall. Until you run out of fuel - which is another story.

The planes actual flight path was straight and level and the attitude of the plane's wings was angled upward. This "slow flight" flying will stretch your fuel efficiency if you travel long distances and you want to use the least amount of fuel while doing it.

An airplane will fly the most "miles per gallon" of fuel if you use the "slow flight" technique and it will fly the most minutes per gallon of fuel if flown very slowly , and very nose high. 

The latter technique is most often associated with high altitude flying associated with military missions where flying long distance is the goal and speed of flight is not primary. Think the first bombing raid on Japan ,very early in WWII, from a Aircraft Carrier in the Pacific Arena. It was essentially a suicide mission with the range calculated to reach China after passing over the target areas in Japan.

In straight and level cruising speed attitude the difference between path of flight and the attitude of the aircraft is very slight. The design of all aircraft is focused on efficiency in cruising flight.

If you are trying to get the most miles per gallon in a private plane mixture control may be the solution. 

In the next post I will discuss, in a plane equipped with a six cylinder engine normally aspirated, how the use of a "six cylinder head temperature" gauge and a "six cylinder exhaust gas temperature" gauge can help you find the hottest cylinder burning fuel and lower its temperature by 25 degrees.

Remember, when you lean the mixture the temperature rises and you have to watch out for valve damage.

Wednesday, May 30, 2012

Just What is Relative Wind

Relative wind is always comes at your airplane from the direction toward which your plane is moving. When you fly the most important factor is the direction of the relative wind.

This, put in another way, is the angle from which the air is rushing against the wing. Good pilots have a "feel" for their airplane. You listen to what your plane is saying to you. If it "feels" good you can handle the situation.

Now you can redefine the Angle of Attack as the angle at which the wing meets the Relative Wind.

Wasn't that simple!

Next time the discussion will center on how Angle of Attack and Relative Wind are intertwined in a series of Flight Conditions.

Why Does a Plane Stall

A stall, which you approach in a previous blog post on 'Slow Flying", is not a result of lack of flying speed. In steep turns your stall speed will approach one and a half times the normal stall speed of the airplane in normal straight flight.

It can, in a abrupt pull out from a steep dive, stall. You have to look at the angle of attack. When it is too great, which is greater than approximately eighteen degrees, the plane creates a condition where the guidance over the top of the wing ceases because of turbulence  that contains very little down wash. In a situation where the angle of attack is too great the wing has a big increase in drag and experiences little lift forces.

A stall is the immediate failure of the air to take the downward curve of the upper surface of the wing. Turbulent flow creates a break in the smooth flow of air that is needed to push air downward to create lift.

The design of a wing is very important to make the flow of air over the top of a wing as smooth as possible. When the flow becomes turbulent, a stall happens.

Slow Flight - Is it Possible

With an Instructor aboard, if you are a student pilot in training, you will learn a valuable lesson about flying. 

After reaching a safe altitude for demonstrating aircraft flight characteristics, according to your Instructor's directions, trim your airplane for straight and level cruising speed. Once this is achieved cut back on your RPMs. Now do whatever is necessary to maintain your altitude. 

What I remember, it took a lot of back pressure on the stick to maintain altitude and the plane flew slower. The discovery that it could continue to fly, albeit slower, without stalling, was invaluable.

Think about it. The nose of the airplane was well above the horizon but it was maintaing a constant altitude at a slower speed. Most folks would think the airplane should be climbing - but it wasn't. The flight path of the airplane remains level.

Your lesson, slow flight is a great way to teach you about angle of attack. Angle of attack is defined, another way , as the difference between where the airplane points and where it goes - in his case straight and level flight.

Your flight instructor may warn you that, yes, you are close to a stall but the airplane has not stalled. As long as you maintain the same back pressure and power settings the plane will fly as long as you want it to.

This is "slow flight" training. The CAP uses this technique to carefully survey a site where a person lost in the woods can signal you and be found. It is a "search and rescue" technique.

With experience you will find many airspeeds, power settings and back pressures that allow slow flight to occur without the airplane stalling.

If you fly powerful single engine aircraft, like the Beechcraft Bonanza, they can maintain slow flight. Be careful though, they can heat up quickly that won't allow you to maintain slow flight for long periods. You can help the engine cool through the use of cowl flaps.

Newton's Law is very important in slow flight. The increase in angle of attack is necessary, at slower speeds, to force enough air mass downward to result in an equal but opposite force upward that exceeds the weight force of the plane. It keeps flying. The wing is "planing" through the air.

If you water ski you understand this well. When the towing boat first applies power the angle your skis make with the surface of the water is the angle of attack. It is high (angle of attack) at first, to reach a point where the downward push of water causes an upward force that begins to slowly force your skis and you toward the surface of the water. This occurs as the towing boat gains more and more speed. Finally you are skimming the surface of the water and the angle of attack in much lower. simple isn't it.

Monday, May 28, 2012

Angle of Attack Perhaps

The angle of attack is the angle at which the wing meets the air. It remains, as stated in this blog, for heavier than air flight the wing keeps the airplane up by pushing the air down.

This is like a Technical Writing course I took at The University of Michigan years ago. The professor asked us to describe the game of baseball to an Englishman.

We all wrote extensive, complicated things like bases, balls, outs, etc. After a hour of this he asked one student how he would explain baseball to an Englishman. His answer was cut off quickly like all of our answers were.

"Baseball, in its essence, is one man throwing a ball at another man who tries to hit it with a piece of wood, everything else is appendiced, " said the Professor.

Technical writing is the art of leaving the reader no other choice for the reader. 

The definition of Angle of Attack stands alone. Hence Newtons Law of Action and Reaction. In exerting a downward force upon the air the wing receives an upward counter-force equal and opposite to that force.

Use a Physics Handbook to figure out that all the air molecules in a cubic yard of air weigh two pounds at sea level. As the wings plane across the surface of the air they move mighty big amounts of air down and receive mighty big reactions of force back.

 (According to the CRC Handbook of Chemistry and Physics, the density of dry air at 20 degrees C at 760 mm of mercury (one atmosphere of pressure) is 1.204 milligrams per cubic centimeter. 
1 cubic foot = 28,316.8467 cubic centimeters.
So, dry air weighs 34,093.48 mg per cu.ft.
Which is about 1.2 ounces per cu.ft.  A cubic yard of air contains (3x3x3 or 27 cubic feet. 27 x 1.2 ounces per cubic foot or 32.4 ounces of air in one cubic yard of air. 16 ounces in one pound means a cubic yard of air weighs approximately 2 pounds.)

Why do you think an airplane is called an airplane?  It's wings are nothing more than a air deflector. This is the essence to flying and piloting a plane.

The angle by which the inclined plane (wings) meets the air is the all important Angle of Attack.

I will pick up on how the angle of attack is essential to every pilot in later blogs.

With you always in safe flight,


A lie gets halfway around the world before the truth has a chance to get its pants on.

A symptom that is fatal, under certain conditions, is a blackout. If you are not acrobatically trained or in just average physical condition a sudden recovery from a stall may induce a blackout.

Briefly, the blood rushes down to your body and extremities leaving your brain without sufficient blood. Net effect, you lose consciousness.

If the blackout endures for more than a few seconds you may lose control. Air Force research developed, many years ago, a pressurization suit that inflated gradually as the G-forces on your body increased as a fighter performed a tight turn or pull out from a dive. 

This suit prevented the blood from rushing out of brain that produces the blackout. This is not a suit that is inexpensive and is certainly not required in private airplanes in normal use.

Obviously, you try and avoid such a sudden situation, but weather may force your hand. Beware of predictions of extreme turbulence or embedded thunderstorm activity.

Those weather extremes can produce a blackout. A blackout in turbulent weather may cause a plane to break up if, upon awakening,  you respond too quickly.