We all want to get the most performance we can out of our airplane. Sometimes that performance is getting somewhere as quickly as possible, sometimes its trying to keep our fuel bum (and therefore our expenses) to a minimum. Other times the performance that matters is short field and climb performance. The performance we can get out of our aircraft depends a lot on how we use our engine. Writing this is somewhat difficult~ as homebuilt aircraft use everything from single cylinder two stroke engines to turbojet engines. I will talk mainly about certified aircraft engines, with some notes for other types as my experience permits. Naturally if there is a conflict between what I say, and what your handbook says, always follow the handbook.

General operating procedures to get the most out of your plane include full throttle/fiill rich for takeoff, reduced throttle and    leaned for low altitude cruise, full open throttle and leaned above 8000 feet. It helps to know how to determine the engine power setting in cruise. The main concern for knowing your power setting is the magical 75% power threshold  that setting the engine should be run full rich to prevent engine damage. Leaning the engine also keeps the plugs cleaner, and reduces fuel bum in the ball park of 10%, so it's a good idea to lean the engine any time you get to cruise conditions. I think there used to be a rule of thumb to only lean the engine above 5000 feet - I saw a Lycoming representative at Oshkosh, and he urged everyone to lean their engine anytime it was operating continuously at or below 75% power - right down to sea level. 

Fixed pitch props make life simple, but at a cost. These props are optimized for one flight condition, and usually you give up some climb rate and/or cruise speed/fuel bum compared to a  constant speed prop. When I owned my Grumman Yankee (Lycoming 0-235, fixed pitch prop), I remember the handbook referencing power settings of 75%, 65%, and so on. I was never really sure how to determine my power setting. I was (am) really cheap, so I usually set the throttle for about 2100 rpm, leaned the engine, and cruised to my destination. Here's how to determine where the 75% power limit is for your airplane. The next time you are flying at 8000 feet, go to full throttle in level flight and look at your indicated (NOT GPS!) airspeed. Any time you are flying in level flight at that indicated airspeed you are using 75% of the rated power of the engine. Once you have this information, you have a level flight indicated airspeed limit for leaning. Note that true airspeed increases with altitude for a given indicated airspeed because the air gets thinner with altitude. Therefore at your 75% power setting, you will bum fuel at the same rate either at sea level or at 8000 feet, but you will be traveling 13% faster at 8000 feet. As you climb above 8000 feet with full throttle, the true airspeed drops off slowly (the indicated airspeed drops rapidly), so I usually try to climb to 8000 to 10000 feet on cross countries. Turbo charged aircraft keep producing 75% power up to a high altitude, typically  20,000 feet or more. True airspeed there is 36% greater than at sea level for the same indicated airspeed - which explains why  Mooneys and Lancair 4s have such great cruise airspeeds.

Other power settings can be determined by your indicated cruise speed. 65% power is at about 94% of your 75% power   speed. So if your 75% power speed is 100 mph indicated you would be using 65% of the engine's power at 94 mph indicated.
Similarly the airspeed for 50% power is about 83% of your 75% power speed.

A word of caution about fixed pitch props. The factory of certified aircraft with fixed pitch props verify full throttle operations are safe for the engine. Too high of a pitch cruise prop can keep the engine from developing full RPM at low speeds, and can potentially damage the engine if the RPMs are too low. For the Lycoming 0-360, operation at full throttle at sea level below 2400 RPM can damage the engine. If you choose your own fixed pitch propeller, make sure your prop RPM at takeoff is not too low - otherwise you need to change the prop.

My BD-4 was the first airplane I had flown with a constant speed prop. Although constant speed props allow you to get  the maximum performance out of the engine, I was really concemed about operating it because you can potentially destroy your engine by improper propeller operation. The key is not to operate the prop too slowly with a lot of power. The  primary gauge to determine engine power with a constant speed prop is the manifold pressure gauge (the tachometer is also used). The gauge measures the air pressure in the intake manifold between the throttle plate and the intake valves, and it measures the pressure in inches of mercury - the same way atmospheric pressure is indicated in the altimeter pressure window. Cruise fuel usage is minimized by operating the propeller as slowly as practical - this allows more engine power to be used to pull (or push) the airplane instead of moving the mechanical parts faster. At 65% power (and the same airspeed) my engine burns 10% less fuel by my setting it at 2200 RPM instead of 2700 RPM. The problem is that if I set the RPM too low, it damages the engine. The rule of thumb is to set the RPM in hundreds no lower than the manifold pressure, so if I set the manifold pressure at 23 inches, I would not set the RPM less than 2300. In fact this is usually conservative, and the RPM can be set up to 200 RPM less than this would indicate. In general I set my cruise with the setting "squared" (23 inches, 2300 RPM) as my engine is smoother at higher RPMs. Again I generally have not been too sure of what my actual engine power output is for a given prop setting. I suspect my 0-360 is similar to most engines with  the following settings:
 

    Manifold      PressRPMPower

                                24          2400             75%
                                23          2300             69%
                                22          2200             63%
                                21          2100             56%
                                20          2000             51%

 Most anyone will tell you that adding a wing to an airplane will not make it fly faster, but many of those same people believe  an extra propeller blade or two will. I think perhaps it goes back to the WWII era. Before the war many fighters had 2 or 3 blades, and during the war the planes got more blades, and flew faster. The reason the planes grew more blades was because the airplanes got more powerful, and more blade area was needed to keep from stalling the props at low airspeed in  order to get the best climb performance possible. The same thing could have been done by making the props blades twice as wide and going from 4 blades to 2, but I suspect that would make each blade really heavy, difficult to manufacture, and difficult to attach to the hub. Jet engines are in a similar situation, using a bunch of tiny blades to either compress the air at the front, or extract power from the exhaust in the back.. Recent technology has allowed jet engine manufactures to produce engines with bigger and few blades to improve efficiency  - and the same thing applies to propellers.

There are 2 reasons I know of to go from 2 blades to 3. First is vibration. A 3 bladed prop has more inertia and different    properties than 2 blades, and I believe in some aircraft a 3 bladed prop is smoother. The other reason is to allow a smaller   diameter prop for better ground clearance. I talked with someone who had gone from 2 to 3 blades, with a few inches  less diameter - he said the plane was smoother, but  gave up some climb rate. That's exactly what I would expect. At slow speeds propeller efficiency is directly determined by diameter. If you could put a 20 foot diameter prop on a Cessna 172 - you could produce enough thrust to climb straight up - its called a R22 helicopter. So larger diameter props are always better - as long as the tips of the prop remain subsonic. If you have seen a P-51 on the ground you will notice it has about a 12 foot diameter prop. To keep its tips subsonic, it turns only a fraction of the engine's speed, and actually turns a lot slower than a Cessna 172 prop.

So generally FEWER blades are better. There have actually  been some I bladed props built - but if you are adding a heavy    counter weight to one side - why not make it shaped like the other blade.

The airspeed to fly for maximum range is within a few miles per hour of the airspeed for maximum climb rate. Most pilots don't    want to fly this slowly, but its useful to know this in case you are running low on fuel and want to give yourself the best chance of making it to an airfield. So what's the cost of cruising faster than optimum? Generally your range (and therefore fuel mileage) is reduced about 25% by cruising at 75% power - but you will get there about 30% to 35% faster. By the way, range is independent of altitude, so for long trips look for the altitude with the best tail wind. What about headwinds? Maximum range is the best climb speed, plus 1/3 the headwind. So if your best climb speed is 90 mph, and your headwind component is 15 mph, cruise airspeed for maximum range would be 95 mph. For tailwinds you would fly your best climb speed but subtract 1/3 the tailwind component speed.

Its also useful to know the same basic headwind rule applies for best glide angle. If you are in a forced landing situation, and   need to try to glide into a headwind, glide at your best glide speed (this is once again about the airspeed for maximum climb rate at full power) plus about 1/2 the wind speed. I have flown gliders and hang gliders, and it seems unnatural to push the stick forward to glide filrther, but it DOES work.

Certified aircraft handbooks show the airspeed for maximum  rate of climb, and also the airspeed to fly in case of engine failure   (best glide speed). These speeds should be very close for aircraft with constant speed props. For fixed pitch props the best climb speed should be 5 to 15 mph faster than best glide  speed as faster speeds allow the engine to rotate faster, generating more power. The best way for you to determine the best climb speed for your airplane is to measure the time it takes you to climb one or two thousand feet (this is much more accurate than trying to get a reading from the rate of climb gauge). Start about 500 feet below a designated altitude on a  calm day near sunrise or sunset (for smooth air). Apply full power, stabilize the climb by concentrating on holding airspeed, and start your stopwatch as you pass through the  designated altitude. Stop the time as you pass through the tar get altitude. Now fly back down to the original altitude an( repeat, but change your airspeed 5 or 10 mph. Repeat this procedure until you are confident you know your best climb speed. Knowing your best climb speed not only lets you get to your cruise altitude as fast as possible (thus reducing the amount of fuel you burn, and getting to your destination quicker) but gives you the best shot at clearing trees / powerlines on short fields. Also, the best way to clear obstacles is to let the airplane accelerate while rolling on the ground to best climb speed (do not exceed tire speed limit) instead of pulling the airplane into the air early and trying to accelerate in ground effect. Naturally if you are flying from a runway with long grass or lots of standing water, its best to get in the air early, and try to keep as low as possible to let ground effect help.

Ground effect is the change to the flow field around an airplane due to flying close to the ground - usually becoming noticeable below 1/2 the  airplane's wing span. Contrary to popular belief, ground effect does not increase the maximum lift of the wing - so stall speed in ground effect is the same as at attitude. In fact some high lift aircraft actually have a slight reduction of their maximum lift in ground effect. So why can an airplane get stuck momentarily in ground effect after takeoff, and why do they float in ground effect as they land? The reason is because being close to the ground reduces the induced drag of the wing. This allows you take off very slowly, but climb just a few feet and, if you are slow enough, the drag increases enough to keep you from climbing further. Voila, you are stuck. This is why it's important to not takeoff too slowly if you are near gross weight or at a high density altitude.

Because ground effect reduces just induced drag, it is more noticeable the slower you fly. On a high speed fly by, no matter how low, ground effect will have essentially no effect. At stall speed with the wheels inches off the ground it can potentially reduce your drag by  1/2.

Another effect of ground effect is on pitch stability and trim. Close to the ground the downwash of the wing over the tail is  reduced,  resulting in greater pitch stability and a nose down pitching moment.  The pitch stability change is usually a small effect, but it is possible  for an airplane to be stable in ground effect, but to have reduced stability as soon as it flies to altitude. The nose down pitch change in  ground effect may not be noticed by many pilots as flight near the ground usually involves rotating the airplane which masks the pitch  change. The forward CG limit on many aircraft is set by the ability for the airplane to rotate up to the stall angle in ground effect which is  more difficult than stalling out of ground effect because of the nose  down pitching moment.

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