This Post popped back up in our forums and is so good I thought I'd share it with everyone. We are very lucky to have guys like Tweet in our squad who have a wealth of information from "Real World". As his name suggests Tweet used to be a flight examiner in the USAF flying the Cessna T-37's. Affectionately known as the "Tweet"
Bits & Pieces of Aerodynamics for LOMAC Pilots
By:
“TweetPuke”
169th Panthers
I promised to cobble together some information about a variety of aeronautical subjects for the members of the 169th Panthers. Many of you asked for more information about navigation and the instruments that make it possible in LOMAC. Cobra and I are working on that project but it is fairly involved if we are to do it correctly. In the mean time I thought I’d try to squash some of the misconceptions and inaccuracies I’ve heard while flying with you. This information will also serve as a basis for the navigation module. I hope you enjoy this one.
Airspeeds
This might seem like too simple of a first topic but I often hear LOMAC jocks being confused by their inability to keep up with a team mate despite having coordinated their speed. I always point to the fact that the two of them are at different altitudes as the first hint to the cause. Lets look at why this is.
There are basically five airspeeds we could be concerned about: indicated, calibrated, equivalent, true, and ground. The first four are directly related to characteristics of the aircraft and atmospheric conditions of altitude and temperature while the last is a vector sum of the air mass the aircraft is traveling in, wind, and the true airspeed relative to the ground. This might not seem very easy to understand but it is quite simple.
Indicated airspeed is what an aircraft uses to fly. An airfoil will fly at virtually the same indicated airspeed regardless of altitude, temperature, or air density. It has to have a certain amount of air flowing over it to generate sufficient lift to equal the weight it carries. Only then will it fly. The instrument used to measure indicated airspeed simply uses a diaphragm to measure how much force the moving air is exerting. So, if you can determine how much force the air exerts you can figure out how fast the air is moving and, conversely, how fast the instrument/airfoil is moving.
Almost. There are a couple of problems based on the atmosphere and the nature of the gas we call air. Our atmosphere is not homogeneous. It varies in density based primarily on altitude and temperature. The further you go away from the surface of the earth the thinner air becomes. Half of the atmosphere is below 10,000 feet. The edge of the atmosphere is generally considered to be something over 200,000 feet. We experience around 14.2 pounds/ square inch at sea level, half of that at 10,000 feet, and 0 psi at 200,000 feet. The temperature also varies with altitude losing 2 degrees C/ 1000 feet of altitude up to about 35,000 feet where the temperature becomes relatively constant at -56 C until near the edge of the atmosphere. The overall effect of this is that the density of the air changes with altitude and temperature which, in turn, affects the number of molecules of air that impact the airspeed indicators diaphragm affecting our ability to accurately measure how fast it is traveling.
The other problem is that air is a gas which acts like a fluid, a compressible one. That is, at certain speeds the local density of the air changes because the measuring device is moving faster than the molecules can get out of its way. This causes them to stack up and create a false reading.
Damn! That sounds complicated. It would be but pilots have a habit of condensing out the messy details to leave only what they need to know to do the job. Here it is: ICE-T.
The indicated airspeed is the measured speed of the air at the instrument, a pitot tube. This speed is notoriously inaccurate due to instrument errors caused by position, mechanical faults, damage, etc. In older first and second generation jets the calibrated airspeed could be 50-100 knots different than that shown on the instrument. Imagine how confusing that would be to you as a pilot. Indicated airspeed corrected for instrument error is called calibrated airspeed. The advent of the modern air data computer, and better engineered instruments, all but eliminated this consideration as well as determined equivalent airspeed automatically. It is calibrated airspeed corrected for compressibility. Remember the jamming together of the air molecules that occurs at higher speed that I mentioned earlier? That is what this is. Finally, true airspeed is equivalent airspeed corrected for temperature.
This all boils down to needing to know two types of airspeeds: indicated and true. Indicated (really calibrated in modern aircraft) is needed by the pilot to give him an idea about how to fly the aircraft. A good pilot knows how much IAS he needs to avoid falling out of the sky, how fast he can go before parts start to fall/burn off the airframe, and what he can do with the amount he has at any instant. He uses true airspeed primarily to help him navigate.
So how do you use TAS to navigate? Well, if you know the direction of the wind, its speed, and your heading you can determine your track and speed across the ground. It’s a simple vector sum that modern avionics keep track of for the pilot. Have you ever noticed the velocity vector in the F-15 or A-10 HUD being off center? That’s the result of wind or you standing on a rudder. You’ll also notice your TAS and GS (ground speed) in the bottom of the radar display in the Eagle. This can be very helpful.
The practical uses of ground speed are many. The one I use most is to figure out how long it will take me to get somewhere. We navigate over the ground so our speed over the ground is most significant. Timing is always important in military operations. (It will be very important in some upcoming scenarios I’m putting together for your enjoyment and consternation.) Let’s say I have to be at a point 32 nm ahead in 4 minutes. What does my ground speed need to be? That’s 8 miles a minute which is also .80 mach and 480 knots. How about 36 nm in 4 minutes? 540 knots and 9 nm/min.
How do I do this? Shove the throttles into full blower until I see 540 on the left side of the HUD? Nope, think again. It might work if you are at sea level with no wind but it won’t work in all cases. The best way is to call up the radar and accelerate to a speed that causes a 540GS to appear in the lower part of the radar display. Note the indicated shown in the HUD and fly that while remaining level. You could also watch the number in the radar display if you need to climb or descend much.
Continued..