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airernie

Fixed vs Constant Speed Prop

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I've decided that it is finally time to step up from the 172 to the 182Q. Having never flown, in real or virutally a constant speed prop I'm a bit confused. Does anyone have a chart of the suggested manifold pressure/rpm settings for the various stages of flight? I realize that it may not be 100% applicable to the Carenado 182Q, but close enough is okay to start.

 

Also, a link to a really good tutorial on the relationship between manifold pressure and RPM and when to change which would be helpful. I've found a few on the Internet, but nothing that has cleared it up for me yet.

 

 

Thanks,

Ernie


Ernest Pergrem

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It's pretty simple actually, but sometimes seems hard to explain...

 

You take off with the throttle and prop all the way forward, which at sea level will get you 2700 RPM and about 28" manifold pressure (MAP). At 1000' AGL or so, reduce to climb power by throttling back to 25" MAP and then reduce the prop to 2500 RPM (25 and 25, if you will), and continue climbing. If you were going to level off at 1000', you could reduce throttle to 24" MAP, then set for 2400 RPM (24 and 24, see!), which is a very proper cruise setting at lower altitudes. The general rule is to always reduce the throttle first and the prop second, so the MAP number stays below the RPM. This is not mandatory, but is a good operating practice that is considered easier on the motor.

 

As altitude increases, the maximum available MAP will fall, so that by 7000' feet or so, full throttle will get you only about 22" MAP. Most people seldom reduce RPM below 2400 at cruise; there is no reason to if you're trying to go fast. If economy is more important, experiment with lower RPMs.

 

As you descend, you might need to reduce throttle, as the full throttle MAP will be increasing. Remember to increase power by leading with higher RPM, then increase throttle.

 

Finally, when you reduce throttle for landing, the prop will eventually fall off the governor and RPM will drop. At that time, push the prop control fully forward, so you'll have max RPM available if you have to go around.

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I found the following page helpful as a qualitative description: http://pilotbrian.blogspot.com/2009/05/stepping-up-to-c182.html. There are also copies of the Cessna 182Q POH online and they include cruise performance tables and worked examples, though I haven't quite wrapped my brain around this level of detail. :huh:. Carenado includes similar MP/RPM tables with the 182Q.


Barry Friedman

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Fixed pitch means that there is a direct link between engine power and RPM, which is why a fixed pitch prop airplane does not have manifold pressure gauges (at least not normally).

 

In a constant speed prop airplane, you have both RPM and manifold pressure gauges. Generally, the MP gauge is the most direct method of measuring engine power output. The RPM gauge becomes a dedicate prop-related measurement giving you feedback on how fast the prop is rotating. Manifold pressure is just the air pressure of the air traveling through the intake manifold before it feeds to the pistons for the compression and combustion cycles. In non-turbocharged airplanes, manifold pressure cannot ever be more than outside air pressure (except for a half inch on takeoff) but can be reduced by "throttling back" on the throttle lever (which closes off the intake manifold butterfly valve). On takeoff, you can sometimes get a "ram air" effect from your forward velocity with the thick air at sea level, and sometimes exceed outside air pressure by no more than a half inch, but that is dependent upon your airplane. Usually the resistance given by the air filter on the manifold air intake will reduce the air pressure about an inch below outside air pressure even if the intake butterfly valve is fully opened (balls to the wall with the throttle).

 

A real world complex airplane's Pilot's Operating Handbook (POH) will always feature some form of power tables (or graphs) where you are given a matching set of MP and RPM settings which the engineers have determined correspond to a percentage of full power. Normally, there are three percentages considered useful for what is called "cruise power." Those three percentages are: 100%, 75%, and 65% of full horsepower. Also, often the terms, "Economy cruise," "Best cruise," and "Full Power cruise" are used in lieu of the outright percentage of full power values. If your airplane is not equipped with a turbocharger, then at around 6,000 feet, full power will be impossible to achieve because the air is too thin. As you increase the altitude, the max power value listed in the power table will further reduce until at around 12,000 feet you might only see the 65% power settings shown because it's the highest power you can achieve.

 

For a constant speed prop, you use the prop control lever to set a desired RPM and then the hydraulic prop governor uses the balancing forces of flyweights working against oil pressure on a piston in the governor to maintain that desired RPM setting. On a fixed pitch prop airplane, changes in aerodynamics (such as air pressure and airspeed) will often cause the RPM to rise of reduce without any change made in the throttle's power setting -- sometimes rather significant fluctuations in RPM that require you to throttle back to avoid exceeding RPM red line.

 

But, on a constant speed prop, within reason, the hydraulic prop governor makes subtle automatic adjustments in prop pitch to counter those changes in aerodynamic forces and therefore the RPM you set is tightly maintained even when you increase or decrease airspeeds or outside air pressures change. This is just one of the advantages to a constant speed prop airplane.

 

The other advantage of the constant speed prop airplane is perhaps the biggest one. It allows you to change the pitch angle of the prop blades to maintain optimal thrust. You see, as you increase forward airspeed, you have to flatten the angle of the prop blades to overcome what is called "relative angle of attack." You see, as the air flow in the direction of travel increases, the relative angle of the prop blades to the airflow changes since there is an increased horizontal velocity of the air. So, a fixed pitch prop can be carved (or molded) into three different blade angle shapes. These are called: speed props, climb props, or cruise props. The speed props produce max thrust at max airspeed. The climb props produce max thrust at slower speeds (normally Vy climb speed). The cruise prop produces max thrust at the normal cruising speed of the airplane.

 

But, the constant speed prop gives you the best of the three worlds! For takeoff and climbs, you set full RPM which rotates the blades to their finest angle relative the air flow. Since you takeoff and climb at Vy (which is much slower than cruise or max speeds) you need to rotate the blades to have the most fine angle. But, as you increase speed, you need to flatten out the blades, which is accomplished when you pull the prop control levers aft in the cockpit to reduce the RPM setting. By rotating the blades' angles to maintain this ideal relative angle of attack, you can reduce fuel consumption by letting the optimal blade angle allow you to throttle back the manifold pressure, which reduces fuel consumption but allowing you to maintain an airspeed that with a fixed pitch prop ideal for slower climb speeds would have required a higher level of fuel consumption to achieve.

 

In short, it is often true that with a given manifold pressure setting, a lower RPM will achieve a more ideal ratio of fuel consumption to speed, but not always. Again, you have to reference those power tables. The power table will give you a listing of altitudes you cruise at with various temperature ranges. If you want to cruise at 75% of power at say 3,000 feet, on a warm summer day, you consult the power table and look up the listings for 3,000 feet and then the column for standard day temperature plus ten degrees and then finally the entry for 75% power. As an example, it would likely show a manifold pressure setting of 24 inches mercury with an RPM of 2400.

 

But, you always pay a price for anything you get in aviation. With constant speed props you have to pay attention and not put too much aerodynamic stress on the engine's crankshaft. You see, just as with a paddle on a canoe, when you flatten the blades of the prop, you increase the amount of air resistance on the blades. This causes more force on the blades and this force is translated directly onto the crankshaft which spins the prop. Therefore, you never fly with the manifold pressure full out with the RPM dialed way back. Yes, it can give you an outstanding fuel economy, but it can also cause physical destruction to your engine -- nasty little things like fractured crankshafts, exploded camshafts, or even thrown pistons through the engine cowling! These things will grab your undivided attention should they happen!

 

Hope this helps!

 

Ken

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By the way, something I teach my students is a brief history lesson. I ask my student pilots if they have ever wondered why the fastest airplanes in the world during the Roaring Twenties were floatplanes -- you know like the ones that won the Sneider Trophy. They often answer yes and voice the same thing I used to wonder: why would the fastest airplanes of that old era feature obviously draggy big water floats?

 

Well, the answer lies in the heart of the technology inherent in the constant speed prop and that concept of relative angle of attack that I discussed in my previous post.

 

Next time you see some vintage movie clips of that era, especially of the Supermarine racing plane that won the Schneider Trophy for the third consecutive time for Great Britain, notice just how flat to the air the blades are on that prop. That is a prop very much carved with the "speed prop" concept. Those blades are about as close to a 90 degree angle to the direction of travel that I have ever seen!

 

That prop did not produce optimal thrust until the plane got near maximum enroute airspeed. But, because of that flat blade angle, the prop did a horrible job of producing thrust for takeoff, since the airplane naturally goes a lot slower at takeoff and landing than for full speed in level flight.

 

So, because of that, this speed plane needed a very long distance for takeoff, and in that era of the twenties, the world was fresh out of 20,000 foot long concrete runways! But, there were a lot of calm lakes, streams, and estuaries! On these, you could afford the luxury of taking 15,000 feet to reach takeoff velocity! So, until the technology to change blade angles inflight was developed, despite the drag of the floats, it was the best way to make a prop plane go real fast!

 

The first technology to solve this known issue of relative angle of attack was engineering a direct pilot control on the blade angles. The pilot would set the desired blade angle and it would stay there. This was called the variable pitch propeller and it was a very nice development, but the pilot had to be very careful in managing the blade angles to avoid overspeeding the prop or overstressing the engine.

 

The next development was using some means of automatic tweaking of the blade angles to maintain a desired RPM setting -- being called the constant speed prop. The first method was electrical control which worked, but had a fairly high degree of problems with runaway props. This was the problem Howard Hughes encountered as one of the counter-rotating props on his high technology propsed spy airplane had the props suddenly reverse and produce copious amounts of drag vice thrust. A proficient test pilot would have realized what happened and immediately shut down the engine (as this is the only solution and has to be done fast to hope to avoid a crash landing). Alas, Howard delayed doing this until he was out of altitude and airspeed and the rest was history.

 

The first models of the Boeing YB-17 featured electically controlled props. But, it was soon discovered that oil in cylinder working against a piston, countering flyweights, was vastly more reliable and provided much superior levels of refined control of RPM. And to this day, the hydraulic prop governor is still the technology in use for constant speed prop airplanes!

 

Ken

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