Airplane design and Gerry

Gerry Hasman was my flight instructor more than 30 years ago.  He was a crusty tough dude, and he taught me well.  Vandana and I visited him recently, just to chat.  We talked about the time when the elevator controls froze at 10,000 feet and the two of us had to push really hard to keep the Piper from stalling.  That led to me airing my theory that most airplanes are designed incorrectly.  Gerry politely half listened to me, not seeming to be interested.  Just as I was about to give up, he said “Oh, we know about that, when we reach cruising altitude, we send someone to the back seat and the air speed goes up by 10 knots.”  Finally!  Someone had validated my theory.  We came to it from different angles, Gerry had tons of raw USAF experience and I had a solid high school background in Statics.  RIP Gerry, I think of you often, this note is to tell it to everyone else. 

This note argues that modern aircraft do not fully utilize modern technology and complains about the tail plane in modern aircraft, first the elevators then the rudder.  Elevators are the horizontal fins at the back end of the airplane, and the rudder is the vertical fin.

Aircraft elevators are usually designed to provide negative lift, and that is obviously inefficient.  The Aero engineers know all about it, but it isn't widely publicized.  Here are the vertical forces on an aircraft in flight.  Note the Tail Load pointing downwards.

The problem is that the Center of Gravity is forward of the Center of Lift from the wings.  This requires the tail to provide a downward force.  Yikes.  The tail is counter-acting the wing.  But without that negative lift, the aircraft would pitch nose down.  Besides the loss of lift, the tail causes extra drag and ultimately fuel inefficiency.  Moving the Center of Gravity aft to the following configuration may seem to be more favorable, at least in concept.  The pilot could move all the passengers and cargo to the rear of the aircraft.

Unfortunately, there are several problems with this solution: Ground Handling and Aerodynamic Stability.  There are no lift forces while the aircraft is on the ground.  If the Center of Gravity stays the same, but the upward forces are from the wheels, then clearly these don't balance and the aircraft would tip back to be nose-up while on the ground.

This would be fine for tail dragging aircraft like the Spitfire or the DC3, but not suitable for modern aircraft with nose wheels.  Most pilots prefer nose wheels because they get a better front view while taxiing.  Passengers also prefer boarding an aircraft that is nice and level, and the DC3 Dakota slopes significantly while on the ground.  I've been in a DC3 and remember wondering why they couldn't design a taller tail wheel assembly.

I think, the main reason that Aero engineers live with the inefficiency of the elevator is that it brings Longitudinal Stability to the aircraft while in flight.  The aircraft tends to return to level flight if a gust of wind makes it pitch up or down.  The following diagrams provide a simplified explanation. 

If the plane pitches up, then the angle of attack of the wing increases.  This causes the wing lift to increase, because the airflow around the wing is deflected down a little more.  The Bernoulli lift stays relatively unchanged, as does the aircraft weight.  Simultaneously, the down-force generated by the tail plane is reduced because of how it is angled.  This causes a counter-clockwise turning moment to lift the tail and bring the nose back down.

If the plane pitches down, as shown below, then the wing lift decreases but the down force at the tail plane increases.  This causes the tail to be pushed down and the aircraft returns to level flight.  Note the longer arrow pushing down on the tail.

Things are a bit more complex because of the distance between the forces, i.e. the moment arms, but the diagrams explain the basic concept.  The tail plane is very important because its distant location gives it a large moment-arm.  Small changes to the forces there can cause large changes to the aircraft attitude.  

Most aircraft are designed to be stable in flight, meaning that any minor deviation from level flight causes it to automatically return to level flight.  The Dihedral angle on wings cause the aircraft to cancel out unintended rolls.  It can be argued that a 6 degree tilt on the wings cause a 0.5% loss of lift.  This does not seem like a lot, but it can be a significant loss of efficiency over a 20 year life of an aircraft.  Say 6000 flights at 300Kg per flight adds up to 900 tons of missed cargo and ~$25 million of lost revenue, assuming an average of 1000 miles per flight.  The point is that most aircraft are designed for human pilots that have slow reactions and are prone to chit chatting with comely flight attendants.  We have the computer technologies to incorporate reliable active flight control systems, but they are not used because the public is generally suspicious of computers.  We need some company to build a breakthrough design, like Tesla did, and then the rest of the mee-too herd will stampede after them.

Here is where I start of my case against the elevator portion of the tail plane.  I have explained why the elevator is still necessary after a century of flight, and why I think it is evil (it causes inefficiency, remember).  I am sure I am not the first to say all this, since there are a number of aircraft designs that seem to address this problem, several from Burt Rutan. 

There is a potential solution: the Canard.  This is a small front wing aka the fore plane.  The Center of Gravity is somewhere between the canard and the wing.  The two surfaces provide lift and are nicely balanced by the weight.  One problem is that the canard tends to obstruct the pilot's view.  Several fighters sport a canard, but most use it more for control than for lift.  The Wright Bros built their first aircraft with a canard, though it is hard to know which way that plane flies.  Even a Fathier looks cool with canards.

The canard comes with its own set of aerodynamic problems.  They can disturb the air flow over the main wings behind them and cause a loss of lift.  The canards need to be large enough to balance the lift generated by the main wing, or more correctly, the moment-arm needs to be large enough.  The aircraft will also lack the passive stability that the old designs have.  A pilot can find operating canards to be much more complex, but that is what computer controls are for. 

Next, let's rant on Rudders, the huge big vertical appendage that stops planes from fitting into hangars.  Many think that rudders are needed to make planes turn left or right while in flight, but that is not really correct.  Aircraft turn by using their wing ailerons to bank into the direction of the turn.  This causes the wing's lift vector to tilt and that drags the aircraft into a turn.  That is well known and there are lots of diagrams on the Internet showing how the forces add up.  But the rudder's contribution to turning isn't that well described.  The figures below try to explain. 

In the figure below, the aircraft has banked to the right and incurs a 'centripetal' force to the right, as shown by the black arrow.  It turns along the dotted curve.  But there is nothing causing the aircraft to spin around its vertical axis and change its heading.  So the aircraft will wind up flying slightly sideways, as shown on the left.  This is called skidding.  It definitely undignified and also inefficient.  The rudder force (the red arrow in the diagram on the right) pushes the tail outwards and helps align the nose back along the path of flight.

The rudder is also needed to allow a 2 engine aircraft to fly with only 1 operational engine.  Engines fail sometimes.  The rudder is useful counter the yaw created by asymmetrical propulsion when only the left engine is working.

But there is an inherent inefficiency in the way a typical rudder is laid out.  It works against the ailerons and tries to roll the aircraft in the wrong direction.  The following is an axial view from behind as an aircraft is turning to the right.  The ailerons on the wings are causing the aircraft to roll clockwise (blue), while the rudder is forcing an anti-clockwise roll (in red).

Ships and boats don't have this problem, because the rudder there protrudes downwards and causes the boat to tip over in the direction of the turn.  This is useful since it stops drunken sailors from being flung into the sea by the centrifugal force when a ship turns.  Unfortunately, such a design will not work for aircraft as the rudder would noisily scrape on the runway on takeoff and landing.

Now we have discussed what the rudder does and what its problems are.  There are several ways to delete the rudder. 

1. One way is Asymmetric Thrust, as was used effectively on UA-232 near Iowa City.  Assuming that there are engines on each wing, the engine computers can adjust their thrust to yaw the aircraft as needed.  In the figure below, the left engine is generating more thrust and will cause a yaw to the right.  However, jet turbines can't change speed rapidly and this technique cannot be used to provide small and quick adjustments. 

2. Thrust Vectoring Vanes can be used as mini rudders in the jet flow behind the engine.  These vanes would get pretty hot, but this technology has been used for many years in fighter jets like the F-22.  Mounting the vanes at a 45 degree angle could even provide 3D thrust vectoring !!  Vanes would respond rapidly, but the magnitude of directional thrust would be small.

3. It should be possible to mount engines on wing pylons that allow the engines to swivel horizontally.  This would allow rapid and fine changes to the thrust vector.  Perhaps a +/- 5 degree swivel would be enough for normal flight regimes.  However, an engine may need to be swiveled out to an extreme degree to deal with situations where the other engine is non-functional.  This idea hasn't been tried yet, except possibly on the V-22 Osprey.  Something like the semi's fifth wheel may work.

4. Lastly, most commercial aircraft have an APU sitting in the tail.  These drive a generator and do not produce any thrust, and they are usually not active in flight.  I wonder if it would be possible to design an APU that produces vectored thrust in flight and also generates electric power while on the ground.  Why not? That is what engineers are there for.  The rightmost diagram below shows the APU thrust in red at the tail.

In closing, the rudder and the elevators provide a huge amount of drag at the tail.  This helps keep the plane flying straight, but is a bit like dragging an open braking parachute behind the plane.  There is this interesting report on what happened to a B-52 when it lost its rudder and the pilots deployed the tail landing wheel to add drag and improve yaw stability.

The point of this note is that commercial aircraft today are very reliant on electronics to operate.  There is engine management, fuel management, fly by wire, navigation and even the pilot's coffee is brewed by a computer.  So why not make the aircraft flight also be inherently unstable and reliant on active electronic controls?  I suspect there will be at least a 20% savings in fuel economy, perhaps more.  It would be expensive to design and build a new aircraft, but it would pay for itself as it operates, and will make the green guys happy by reducing carbon issues. 

Finally, if the humans in ATC can be replaced with computers, the gas savings would be much larger.  However, we would need Ronnie to do this.

All in fun...

Dec 2017