Atp Question

[givpicachanc]

page 205 Q)9068 Gliem: under normal conditions, which combination of MAP and RPM produce the most severe wear and fatigue to high performance engines:

1) high RPM Low MAP
2)low RPM High MAP
3)High RPM High MAP

I would think the answer is 2, but it says its 1. enlighten me folks

 

[avbug]

Answers two and three are acceptable operating conditions, with adequate manifold pressure to ensure that the engine is driving the propeller, and not the propeller driving the engine.

The greatest wear will always occur at high RPM settings; more wear will occur because the engine is moving more; more revoloutions in a given time period mean greater wear. You can narrow the answers to #1 or #3 on that basis alone.

The next issue is that of manifold pressure. All things being equal, a high performance engine operates best at a manifold pressure closely approximating the RPM value, or higher. Except in cases of excess pressure (overboost), a higher manifold pressure will do an engine no harm. Temperatures are kept up in the operating range, positive torque is maintained, backlash values and clearances kept more constant than a low manifold pressure setting. Thermal changes are less likely to be as drastic when maintained at higher manifold pressure settings. The engine is operating closer to peak efficiency at higher power settings, with torque and horsepower output being proportional to RPM, and increasing with increases in manifold pressure.

Low manifold pressure, except where necessary, will only increase engine fatigue thermally, and increase wear by reverse backlash and stress. Operating an engine extensively at very low manifold pressures, below that necessary to drive the engine in particular, can lead to excessive fatigue stresses. Additional hazards may be lack of adequate heating leading to thermal stress due to dissimiiar expansion throughout the engine, and the attendant mechanical wear.

At reduced temperatures, oil viscosities change, and may not provide adequate protection at higher RPM's. Fuel mixture may be enrichened by reducing manifold pressure while operating at excessive RPM for the power/speed combination. This can lead to icing issues, and fouling issues. Additionally, for carbureted engines, should ice develop, inadequate engine operating temperature may not provide enough energy for carburetor heat when required, or may simply place carb air temperature in a more ideal range.

Your reasoning is logical, as it would seem at first blush that high manifold pressure might increase wear. However, just the opposite is true. Similiar reasoning might assume that an aircraft which has experienced relativley low flight hours over a long period of time might be better off than an airplane with many flight hours over the same time period. However, this isnt' true, either. The less the airplane is flown the harder it is on the airplane Operating at low power manifold pressure settings while maintaining a high RPM is similiar in that it's harmful, or potentially harmful in comparison to more closely matching manifold pressure and RPM.

 

[Traumahawk]

Like I'm a 2 year old.....

Avbug.........

"Except in cases of excess pressure (overboost)..."

"Temperatures are kept up in the operating range, positive torque is maintained, backlash values and clearances kept more constant than a low manifold pressure setting. Thermal changes are less likely to be as drastic....."

"Low manifold pressure, except where necessary, will only increase engine fatigue thermally, and increase wear by reverse backlash and stress........Additional hazards may be lack of adequate heating leading to thermal stress due to dissimiiar expansion throughout the engine, and the attendant mechanical wear."

I follow 95%...but just for the record...
Can you explain these 3 things to me like I'm an autistic 15 year old student pilot?

Thanks....T-hawk

 

[avbug]

"I follow 95%...but just for the record...
Can you explain these 3 things to me like I'm an autistic 15 year old student pilot? "

Probably not. I'm not a very good instructor.

Several things may apply to your question, however. Your engine is made up of different metals. These metals expand and contract at different rates. The ideal thing is to keep your engine operating as close to a uniform temperature as possible. Certain parts of the engines will be warmer than others all the time, but one needs to keep temperature changes, and the differences in temperature to a minimum. When one part expands faster than another (an aluminum part next to steel, for example), the result will be increased wear between the two parts.

In the case of high RPM, as we discussed before, you're seeing the scenario for the highest degree of mechanical wear. With reduced power, the cool parts of the engine will stay cooler than normal, with a greater difference in temperature between the various parts of the engine (greater disparity between cylinder head and barrel, for example). Wear is increased.

Your engine has a certain amount of play, or backlash in it. Wear and stress should always occur in one direction in the engine. In flight and in normal operation, this happens as the engine drives the propeller. At very low manifold pressure settings, or at higher airspeeds with reduced power, the relative wind may end up driving the propeller. This amounts to a negative torque situation. Turboprops automatically compensate for this, but the governing systems on recip engines don't.

As the tension or backlash is taken up by negative or zero torque, mag timing changes. The stresses which have been applied in one direction to all rotating and reciprocating components in the engine (crank, cam, lifters, rods, connecting rods, etc) reverse. This increases wear drastically, and induces undesirable loads on components which have become directionally stressed. This is especially the case with geared engines, where a great deal of wear and eventually damage can occur by operating at reduced power.

Mag timing can change by several degrees, changing the time between firing and top dead center in the engine. This affects the way the engine runs, and can lead to early or advanced timing. This means the cylinder fires sooner than it should, which can lead to excess resistance in the cylinder as the piston moves to the top of it's compression stroke. The result can be a hammering effect on pistons, increased valve temperatures, and the potential for damage similiar to detonation. Detonation is not likely due to reduced power, but it is also possible.

The crankshaft experiences it's greatest stress in torsion, or twisting. This is the torque applied that turns the propeller and provides the necessary force to drive it. At reduced power settings, torque is cut back or non-existant. This produces a condition that changes crankshaft harmonics, leading to unusual stresses on engine mounts, accessories, cylinders, connecting rods, and the crank itself, as well as increased wear at each crankshaft bearing journal. Additional stresses are imparted to the propeller, which are then reflected back as prop harmonics to the engine and associated components.

Many engine-propeller combinations have restrictions about crosswinds or tailwinds on the ground during certain RPM ranges. The same stresses that are prevented by these limitations on the ground may come and go repeatedly during low MP/high RPM operations. Likewise the changes in torque produce an alternation in torsion (twisting) that has the fatigue effect of bending a paperclip back and forth until it eventually breaks. The crank fails only extremely rarely, but all stress on metal is cumulative. The properties of the metal change, and metal remembers. Stresses build up, and remain with the part; they never go away. Add enough stresses over time, and eventually the part may fail, or exhibit excessive wear. (Look at the engines and propellers on aerobatic airplanes, and see how long they last in comparison to the same engine on non-aerobatic airplanes).

Overboost means too much manifold pressure. The two most immediate results of engine damage from overboost are detonation (leading to excessive temps, melted pistons, burned valves, and broken connecting rods), and lifted cylinder heads or failed cylinders. When I stated "except in cases of excess pressure" I was referring to sources of damage resulting from low manifold pressure, the opposite of overboost. The damage caused by overboost is readily apparent and easy to pin down.

This probably didn't answer your question entirely, but it's late, my mind has shut down for the evening, and I've got a long three days coming up. Good luck!

 

[Traumahawk]

Pretty much did the job...thanks.

---T-hawk

 

[givpicachanc]

Thanks a lot avbug