Monday, October 13, 2008

A Dozen Aeronautical Myths

A Dozen Aeronautical Myths

One often overhears comments about the behavior of model aircraft at the field, and while the remarks are about models, they would also apply to full size machines.
Some of these expressed ideas reflect common myths, so I dug into my memory from 40 plus years ago for the explanations of why they are wrong, and present them for discussion.

Myth 1:
A model will tend to weather cock into wind during flight.

Assuming a steady (non gust) wind, the aircraft can do no such thing, short of being anchored to the ground in some way. A fuller explanation follows below at myth 4.

Myth 2:
A model will stall if it flies too slow.

This one can be correct, but not necessarily so. It is not speed that causes a stall, but separation of the airflow from the wing. This means an aircraft can stall at high speed, e.g. a snap roll, or not stall at zero speed, e.g. the top of a hammer head (sometimes called a stall turn despite no separation burble). Think of stalling as an angle of attack (around 16 degrees), rather than speed.

(By the way, a 60 degree level turn increases your stall speed by 41%). It is the higher angle of attack in a turn, that can cause a stall, leading into a low speed snap roll

If you lose power, you are unlikely to stall if you get the nose down below level flight. Warning signs to get the nose lower come from having a lot of up elevator applied. It means you are approaching 16 degrees.

Myth 3:
The more stable an aircraft, the better it is at aerobatics.

Quite the opposite. A stable aircraft wants to keep doing what it is designed to do, normally regain level flight if disturbed from it. You don't need this stability fighting you if you are trying to make the aircraft follow your commands. A good trainer should be stable, but few are these days.
Neutral stability best serves the aerobatic pilot since the machine keeps doing what was last commanded to, with no deviation.
Unstable aircraft are usually beyond human control, as they increase any deviation input given to them. Too far back a C.G. can make a simple up command turn into an unwanted loop for example. Modern fighter jets are unstable in order to attain rapid response, but need a computer to fly them.

Myth 4:
Turns down wind are more dangerous than turns upwind.

In some ways this is true, but not for the usually given reasons. In a turn downwind, a gust will tend to roll you on your back since the high wing usually presents more under wing area to the gust, than the low wing.

Secondly, your increased ground speed downwind makes a prang take place at higher ground speed than turning up wind.

Thirdly, the increase in ground speed gives the illusion of a higher airspeed, tricking one into slowing the airspeed, perhaps to the stall.

That said, the aircraft has no way of keeping track of it’s relationship to the earth below it. This is an important point to remember and this enlarges on the explanation on Myth 1. Assuming a non changing wind, once the machine is airborne, the ground relationships cease. It is in a river of air, and the motion of the river over the earth, doesn't affect it's flight characteristics.

One way to get this clear, is to imagine you are in a free floating balloon watching a model circle around you. The balloon may be doing 100 kph over ground, but you won't fell a breath of wind. Neither will the model circling you, other than it’s own airspeed.

Only when a machine reemerges from the "river" of air, and touches the shore do we need to worry about the earthly relationship.

Another example: Imagine you are on a train moving at speed. As you walk in the direction of travel, your speed over the ground is increased by the speed of your walk, and conversely, if you walk to the back of the train, your speed over the ground is reduced. The speed of your walk in the train, is not affected by the speed of the train. If you bump into someone, your momentum is relative to the train, and it doesn’t matter which way you walk.

Just as you are in the moving train, the aircraft is in the moving air mass. In the example above, the aircraft speed is equivalent to your walk speed, and the air mass (wind speed) is like the train moving over the ground.

Consider a 180 degree turn in still air. It involves reversing ground speed from say + 100 North to - 100 South, a relative speed change of 200 within the time of turn.

Now imagine you are flying into a headwind of 100 going North,. Your ground speed is now zero. You now again do a 180 degree turn, in the same time frame. Your final ground speed is 200. Once again, a relative speed change of 200 within the time of turn.

Notice, the aircraft undergoes the same accelerations within the time of the turn. It make no difference because the wind is blowing. As you turn down wind, it may help you to visualise the wind is helping carry the aircraft down wind and accelerate it over the ground.

Myth 5:
The model can make a tighter turn if it slows down.

Look at our pylon champ for an answer to that one. It again comes down to angle of attack and it is the stalling angle of your machine that determines it's minimum radius turn. You can turn at the minimum radius at more than one speed, but the faster the speed, the more bank is required which in turn means you increase angle of attack. If one stalls at these speeds, a snap roll usually results. The increased angle of attack required in a level turn will slow you down if you don’t add power.

Myth 6:
A high wing gives pendulum stability.

This is misleading, because a pendulum is fixed to a support, whereas the aircraft is not fixed in any way. What happens is that as the aircraft banks, It sideslips towards the low wing, and it is the retarding effect of this relative airflow on the top wing that rights the plane (see myth 7).

Myth 7:
Dihedral works because the horizontal lift component of the lower wing is greater than the other.

Yes, partly but more is involved. Imagine you can slide the model along a wire through it's C.G., and it's not hard to see that while the above effect will slow a rotation, it won't stop it, and certainly won't bring the model upright. Once again, it is the sideslip that increases the lift on the lower wing and levels it. As the wing drops, the model slides in that direction, causing a greater relative angle of attack and lift on the lower wing. The opposite wing has a lesser angle to the relative airflow.

Myth 8:
"Dual servo rate should be low for strong winds and high for light winds". (After hearing this, I assume the proponent thinks that strong winds give more airflow over the controls and less control deflection is thus required.)

If one realizes that an airplanes "wind" is due to it's motion, and not the wind speed (when airborne, remember it is in a river of air), then different rates of throw are not involved with wind speed although they are with airplane speed through the air (relative wind).

Myth 9: Big vertical stabilizer (fin) means directional stability.

If we remember that an aircraft will sideslip towards the lower wing in a bank, the relative air stream creates side forces either side of the C.G. These can either yaw the model into the slip or out of it. Too large fin acts like a weather cock during the slip, and tightens the turn. The end result is the nose dropping and a spiral dive. The perfect size fin will balance the area ahead of the C.G., so that the yaw is appropriate for the sideslip involved. Too small a fin will yaw a machine out of the turn.
It is the rudder effect that turns (yaws) the airplane (else it would crab in a straight line but wouldn't turn), despite the fact you may only use aileron input. The bank causes the machine to sideslip, which brings in the rudder yaw effect.

Myth 10:
A model will glide farther if it is light.

Assuming no wind, the angle of glide relates to the lift and drag of the machine. Without drag you would glide horizontal indefinitely and without lift, you have a vertical descent (don’t we know). The actual glide angle is thus a ratio of these two (Lift to Drag), and not related to weight. Wind has an effect on the angle over the ground, but not through the "river of" air. The heavy model will reach the ground sooner, since it glides at a higher speed, but both will glide the same distance in still air.

The minimum sink speed of a model is flown slower, near the maximum angle of attack (not the faster best distance speed), since one is more concerned with lift than speed. Here a light model will glide a longer period than a heavy one.

Myth 11:
A model will gain the most height in a given time, if we climb it at the best angle of climb.

There is a difference between angle and rate of climb. The first compares altitude to forward distance and is affected by wind, the second compares altitude to time and is not influenced by wind. Best rate of climb is done faster than best angle of climb.

Myth 12:
A headwind will slow a model more than an airliner.

If we go back to the "river of air" concept, one will find it easier to grasp the picture that everything in the "river" is carried along at the same speed. Hence a ten knot headwind will take 10 knots off the model and airliner speed, equally.

Now let the arguments begin.


No comments: