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Tail Stalls
Category Miscellaneous
Posted by Taylor Grayson Submitted on 1/1/2006 at 7:30 pm
Stalling the horizontal stabilizer can happen in icing conditions where leading edge ice causes the airflow to separate at a less negative AoA than normal. What isn't intuitive is that the recovery procedure is to pull back on the yoke, rather than pushing forward like a stall of the main wing. The reason is that the tailplane is at a larger negative AoA when the main wing is at a smaller AoA (faster), and vice versa. To reduce the negative AoA on the horizontal stabilizer, you must increase the AoA on the main wing. The reason should be clear if you view the orientation of the horizontal stabilizer to the relative wind in each of the two situations, being fast and being slow: Tail AoA A critical piece of information for interpreting the above diagram is that the AoA on an airfoil is normally measured as the angle between the relative wind and the chordline of the unaugmented airfoil, meaning no flaps, and the elevator (and rudder, ailerons) are simply plain flaps. So deflecting the elevator by definition has no effect
Unloaded Turns
Category Miscellaneous
Posted by Taylor Grayson Submitted on 1/1/2006 at 7:30 pm
One interesting idea that sometimes shows up in discussions of turning flight is that of an "unloaded" turn, a turn in which you bank, but avoid the load factor normally incurred by the bank by not simultaneously increasing the AoA by pulling back on the yoke.

The advantage of this technique, so proponents say, is that you get the turn but avoid the increase in stall speed associated with a loaded turn. So there is a free lunch after all.

Well, not really. There are two problems with this technique:

  1. It's primarily the load factor which is responsible for the increased rate of turn in a bank, so you're giving that up, and
  2. Without increasing the AoA, the airspeed of the aircraft will increase while the bank is held until the load factor ends up where it should have been based on your bank angle.

So is there any value at all in the technique? Let's run some numbers, using this diagram and formula as the basis of our calculations:

Figure 1

In the table below, the first three entries in the table are "true
Why Vy Varies with Altitude
Category Miscellaneous
Posted by Taylor Grayson Submitted on 1/1/2006 at 7:30 pm
The greatest rate of climb in an airplane occurs at a velocity where the greatest excess power occurs, as shown by the orange points in the graph below.


Figure 1
At higher altitudes, the less dense air causes the Power Required curve to rise and rotate to the right, as shown below in figure 2. This occurs because
  1. Induced drag increases at every airspeed because the less dense air requires a greater angle of attack to ensure that lift = weight;
  2. Parasite drag decreases at every airspeed because less dense air provides a smaller frictional force at every airspeed.
Since induced drag predominates at low airspeeds, total drag within this speed range increases. At faster airspeeds, where parasite drag predominates, the total drag is reduced.


Figure 2
This shift in the Power Required curve causes the point of minimum drag to increase and shift right, meaning that it will occur at a higher true airspeed.

Increased altitude also changes the Power Available curve, but at least this change is straight-forward. At any given airspeed, there will simply be less power available because of
Circulation
Category Miscellaneous
Posted by Taylor Grayson Submitted on 1/1/2006 at 7:30 pm
How a wing generates lift remains one of the most argument-inducing subjects in all aerodynamics, at least among pilots. What isn't commonly known, though, is that scientists have well understood the process for close to 100 years, and there is no debate within the scientific community. When evaluating any potential explanation for lift, there is one important concept to keep in mind which will help you cut through the smoke: nature only has two ways for a fluid to transmit a force to an object:
  1. pressure
  2. friction
Pressure is merely the sum total of the random motion of the atoms of a fluid. The more atoms that bounce off an object, and the faster they move, the greater the pressure felt by the object. Pressure acts perpendicular to the surface of the object.

Friction is due to the viscosity of the fluid. Yes, air has viscosity, even though it's very low compared to fluids such as molasses, or even water. Friction acts parallel to the surface over which the air flows.

So no matter what you hear about the "true" cause of

Flaps and Slipping
Category Miscellaneous
Posted by Taylor Grayson Submitted on 1/1/2006 at 7:30 pm
The following paragraph is copied from the book "Cessna, Wings for the World" written by William D. Thompson, an Engineering Test Pilot and later Manager of Flight Test and Aerodynamics at the Cessna Aircraft Co.
With the advent of the large slotted flaps in the C-170, C-180, and C-172 we encountered a nose down pitch in forward slips with the wing flaps deflected. In some cases it was severe enough to lift the pilot against his seat belt if he was slow in checking the motion. For this reason a caution note was placed in most of the owner's manuals under "Landings" reading "Slips should be avoided with flap settings greater than 30° due to a downward pitch encountered under certain combinations of airspeed, side-slip angle, and center of gravity loadings". Since wing-low drift correction in cross-wind landings is normally performed with a minimum flap setting (for better rudder control) this limitation did not apply to that maneuver. The cause of the pitching motion is the transition of a strong wing downwash over the tail in straight flight to a lessened downwash angle over part of the horizontal tail
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