From: mkfeil@bcrki9.bnr.ca
Newsgroups: sci.aeronautics
Subject: Re: Aeronautical misconceptions
Date: 6 Apr 92 18:43:13 GMT
Organization: Bell-Northern Research

Just a few clarifying notes on my list of myths. As with many myths, there
is an element of truth that causes confusion when discussing the myth. I
will point out the confusion in each case:

In article <1992Apr03.234801.26898@bmerh2.bnr.ca> I wrote:
>                   AERONAUTICAL MYTHS AND MISCONCEPTIONS
>                   -------------------------------------
>
>1. Explanation of Lift
>
>A wing produces lift because the top surface of the wing is longer. The air
>flowing over the top of the wing has to travel faster to meet up with the
>air flowing over the bottom of the wing. Lift is then created by Bernoulli's
>principle which states that faster flowing air exerts less pressure.
 
In this case the confusion is the fact that it is true that air flows faster
over the top of the wing than the bottom, but definitely not for the reason
stated. In fact it flows much faster than length ratio alone would dictate,
especially at higher angles of attack.

>2. The downwind turn.
>
>An airplane flying into the wind will lose altitude when it turns downwind.
>This is because after the turn the direction of the wind will be the same
>as the direction of flight, which in effect decreases the airspeed of the
>airplane, leading to a loss of lift.

Here, the confusion comes in the form of wind gradient, which is a big factor
near the ground. It means that you CAN climb faster by heading into the wind,
for example on takeoff. Unfortunately it also means that you will lose
altitude faster if you are descending into the wind, for example in an
engine-out situation.

>3. The windmilling vs stopped prop
>
>A windmilling prop produces less drag than a stopped prop. Therefore if
>a powered airplane has an engine failure, it is best to let the propeller
>spin freely in the wind.

In this case the confusion is that a SLOWLY turning prop actually will
produce less drag than a stopped prop, but as soon as its speed increases to
the point that the propeller's airfoil is no longer stalled, it will be
producing lift in a direction that is considered drag to the aircraft, and in
a magnitude greater than the drag created while it was stopped.

Also, in rare cases I believe that the windmilling prop may be able to spin
freely enough to have the lift/drag it creates decrease below the drag
created by a stopped prop. I am thinking of indoor free-flight rubber powered
models that have disengaging propellers that can spin very freely (almost
frictionless). These props operate at very low reynolds numbers and
relatively coarse pitches. Many indoor models fly slower than a walking
pace. Another factor with these tiny aircraft weighing only a few grams is
that the propeller is very large compared with the model. The torque effects
of stopping the prop may be undesirable as well (the whole model would want
to roll).

>4. Explanation of dihedral.
>
>Dihedral produces a stabilizing effect in the roll axis since the wing
>that ends up lower after a perturbation will be more level than the
>other one and therefore have more lift (i.e. a greater component of this
>wing's lift will be acting in the upward direction).

The confusion here is that people think of the wings as moving down through
the air instead of forward. If this were true, then like a badminton birdie
or like swept wings, dihedral would help roll stability in the manner
described.  In fact at extremely high angles of attack, I'm sure dihedral
would have a stabilizing effect in roll for the reason stated. However in
normal flight nearly all of the airstream is aligned with the direction of
flight, and it is the yaw-roll coupling effect of dihedral that
predominates.

>5. Longitudinal stability.
>
>Airplanes designed with a tail that does not produce a down-force are
>inherently unstable. You need the help of a computer to fly them.

Here the confusion results if you take a normal, conventional design such as
a Cessna 172 and make the tail lift by simply moving the center of gravity
rearward. You will probably end up with an unstable or less stable design
since you haven't also made sure that the Neutral Point (NP) or aerodynamic
center of the plane is moved back as well to maintain an appropriate "static
margin". Re-design the plane with a larger tail and lifting stabilizer
airfoil, and the story changes.


For more info on any of the above items, email me. I have past articles saved
on most of this stuff.

See ya!
Max
--
Max Feil                 
Bell-Northern Research    Internet: mkfeil@bnr.ca
P.O Box 3511 Station C,
Ottawa, Ontario, Canada.  - What do I know? I'm just a Nerd on the Big Ranch. -
Newsgroups: sci.aeronautics