More on checking out on different aircraft (see Type Checkouts article) …
Before you fly a new aircraft, there are a few simple things you can look up, to learn a little bit more about what it’s going to fly like.
One very simple thing to look up (either in the POH/AFM or probably easier, on the internet) is the wing loading of an aircraft. Wing loading tells you how hard the wing is going to have to work, to lift the aircraft. It is calculated very simply: take the weight of the aircraft in pounds, and divide it by the total square feet of wing. This will give you a value expressed in pounds per square foot. I might mention that many high school physics problems can be solved by merely lining up the units, but once again I’m veering horribly off course.
Entering “Piper cub wing loading” into google inevitably led me to the wiki, which told me that the ever-so-docile Piper cub has a maximum takeoff weight of 1220 pounds and a wing area of 178.5 square feet, yielding a wing loading of 1220/178.5 = 6.8 pounds per square foot. That’s not very much weight that each square foot of the Cub’s wing has to support, in order to fly. This results in a marvellously slow stall speed, which tells you that it won’t need much runway to take off or land. However, it’s going to be bounced around pretty badly when the wind is strong, and there’s mechanical turbulence near the ground.
At the other extreme, let’s look at the Lockheed F-104 supersonic fighter jet. The F-104G variant had a max takeoff weight of 29,027 pounds and a wing area of 196.1 square feet (barely more than the Piper Cub). This yields a wing loading of 29027/196.1 = 148 pounds per square foot. Considerably more than the Piper Cub’s 7 pounds per square foot – over 20 times as much, actually. As you might suspect, the F-104 took off and landed at insanely high speeds, requiring the use of a drag chute for a normal landing. However it also gives you a marvellously smooth ride in the bumps down low.
Now let’s look at something in between the two extremes – say the Cessna 421, which is a mid-size light twin-engine piston/prop aircraft. The 421C has a max takeoff weight of 7,450 pounds and a wing area of 215 square feet for a wing loading of around 35 pounds per square foot, which is getting up there for a light aircraft, which means that it will likely want a bit more runway than a light single engine trainer, especially if it doesn’t have vortex generators.
As an exercise, calculate the wing loading for the L39 and F-86 jets, and the MU-2 turboprop. See anything interesting?
So if you know ONE number about an aircraft – it’s wing loading – you instantly have a pretty good idea how fast it’s going to want to fly, how much runway it’s going to want, and what kind of wind you can comfortable fly it in without getting your head bashed about.
There are a couple of other simple characteristics about an aircraft’s wing, which will tell you more about what it’s going to be like to fly.
Wing Aspect Ratio
Another informative value is the aspect ratio of the wing, which is simply the ratio of the wingspan to the chord. A high aspect ratio wing (eg glider) is going to have a nice, slow stall speed due to the reduced induced drag at high angles of attack. At the opposite end of the spectrum is the “Hershey bar” original Piper Cherokee PA-28 rectagular wing, which is not typically known as a short-field performer because of the high induced drag. Don’t slow this aircraft down on final unless you are carefully considering “back side of the power curve” considerations.
You can also learn a lot about a wing by looking at its cross-section. A wing with lots of camber that occurs well forward is going to develop lots of lift and have docile slow speed handling characteristics (see Piper Cub and Aztec) but it’s not going to cruise very fast because of the high drag.
However, a razor-thin wing with maximum camber well aft must be treated with respect. It’s going to cruise blazingly fast (eg P-51, Glasair III) but it’s going to have nasty stall characteristics. Airspeed cannot be ignored during landing, or even takeoff. Also this kind of wing (known as “near laminar flow”) is going to be very intolerant of any kind of contamination on the leading edge, such as bugs or even water!
For more information about different kind of wing cross-sections, look up NACA “four digit” wing airfoils on the wiki.
So before you fly an aircraft, if you calculate and compare its wing loading, then simply look at the aspect ratio and wing cross-section, you will have a pretty good idea how the wing will behave.
Just like you calculate the wing loading of an aircraft, you can also calculate its power loading. It’s also a simple ratio of the weight of the aircraft in pounds, divided by the available horsepower.
Let’s look at the MXS composite monoplane aerobatic aircraft. It has an “aerobatic weight” of 1600 pounds, and an engine ranging up to 380 horsepower. So it has a power loading of 1600 / 380 = 4.2 lbs per horsepower, which is a very impressive number!
A more normal piston prop aircraft, such as the single-engine land Cessna 172R has a maximum weight of 2,450 pounds and 160 horsepower, yielding a power loading of 2450/160 = 15 lbs per hp. This is more typical number for a light aircraft of mediocre performance. A more sprightly SEL aircraft might have a power loading closer to 10 pounds per square foot.
Let’s take a look at the Beech Duchess, which is a common and incredibly boring multi-engine trainer. With a maximum weight of 3900 lbs, and two 180hp engines, with both engines turning we get 3900/(180x2) = 10.8 lbs per hp. So with both engines turning, it should climb better than a C172, and sure enough it does. However, with only ONE engine turning, we get a power loading of 3900/180 = 21.7 lbs/hp which is horrible, and so is it’s 100 fpm (no typo) climb rate with one engine feathered. Over 20 pounds per square foot is lead balloon territory. You’re going to need a lot of runway and flat terrain.
I should mention that the effect of increased power is increased climb rate, and very little effect on cruise speed. Given the exponential relationship between drag and speed, a doubling of your engine power will only yield a 30% increase in cruise speed! And doubling the horsepower will double the amount (and thus) weight of fuel required, which hurts your cruise speed. The lighter you are, the faster you go.
How you make an airplane go fast, is by reducing drag. A good example of this would be the Mooney M20J that I used to fly. It was very heavy for its measly little 200hp four cylinder Lycoming engine, which meant that it did not climb very well (calculate it’s power loading). However, the slippery retractable-gear M20J with all of it’s LoPresti aftermarket mods was very low drag compared to most other light aircraft of it’s era, which meant that it would deliver an honest 160 knots burning 13 gph, which wasn’t bad for a certified aircraft, ‘way back when. Roy LoPresti knew all about drag reduction, and the benefits of it – look him up.
One other thing that must be taken into consideration is what kind of propeller is installed.
A fixed-pitch propeller inevitably reduces performance because it will not allow the engine to reach its maximum rated RPM at slow speeds, which significantly reduces the horsepower produced. It will also perform poorly at cruise, turning too high an RPM, making more noise and burning more fuel than necessary.
Far superior to the fixed-pitch prop is the constant speed prop, which automatically varies the angles of the prop blades in flight to maintain a constant RPM regardless of the speed. Pretty neat, but heavier and much more expensive than an economical fixed-pitch prop.
Another consideration is the number of propeller blades, their chord (they are a wing, after all) and the propeller diameter. Generally more blades helps the climb but hurts the cruise speed, and even a small increase in the prop diameter vastly increases the efficiency of the propeller in producing thrust. As an example, I have flown the classic Pitts S-2B aerobatic biplane with the original 2-blade metal prop, the 190cm MT 3-blade prop, the 203cm MT 3-blade prop, and the legendary 78in Hartzell “claw” 3-blade prop, and the airplane behaves completely differently with each propeller.
The propeller on an airplane is analogous to the combined transmission, wheel and tires on your car. They both apply the power of your engine, either well or badly.
firstname.lastname@example.org Oct 2011