Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
It's almost part of the charm, not to mention the ego boost, to be able to deal with it gracefully, safely, and tactfully.) Your description of the process for determining the manufacturer’s recommended TBO agrees with what I have heard in the past, and I can see a considerable market-force incentive for the engine manufacturer to publish conservative cruise rpm limits and other related operating cautions (such as those cited in my earlier post): Responsible engine operation of the early engines leads to extended TBO approval, which leads to more sales.When the C145/O300 engine was first introduced, the mfr estimated that it's ability to produce rated power reliably was approximately 600 hours, and therefore that recommendation was published. However, actual experience in the field (after numerous tear-downs demonstrated acceptable engine wear) later indicated that the engine was capable of a much-extended TBO period, and the mfr then submitted and obtained approval for an increased TBO.
gahorn wrote: . . .
Well, when we're at reasonable weights, say 2100 lbs or less, and just going someplace, she'll clip right along at or better than book while averaging about 7.8 gph. (She can show her white nav-light to a certain green machine up in Dallas without too much trouble.)
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George, that's surprising that you bring up me seeing your white nav-light! Every time it's been me standing on the ground after landing and waiting for a long time, then, there it is, your white nav-light on a S-L-O-W downwind! Thanks, Jim. I think you have reduced my risk of future complaints.Metal Master wrote:The following may not answer anyone’s questions but might inform others that are reading this. It is my cent & half worth
Engine RPM vs. Prop Pitch.
In my seminar discussion on propellers and constant speed propellers I use the comparison to an automobile transmission and gearing. Like this:
When starting a car with a stick transmission particularly when climbing a hill with a heavy load (compared to an airplane taking off at full gross or a float plane). The car is started in first gear. This allows the engine to produce the maximum rpm at the maximum torque curve if the gearing is selected properly. This is analogous with an airplane with a flat pitch propeller or a constant speed propeller selected for maximum RPM.
Once the car is started up the hill load is decreased on the car and we are moving at a steady rate or accelerated to the maximum RPM, the car is shifted to second gear. In the airplane there is a narrower RPM band of operation and we reach that point much more quickly and or we reach a steady state of steep climb clearing a 50 ft obstacle and maintain the higher RPM until we decide to change to cruise climb. After we reduce the rate of climb with a fixed pitch prop the load is reduced on the wings the rate of climb decreases and the load on the prop is decreased or kept the same and or speed increases depending on whether we decrease throttle or maintain maximum RPM. Remember lift, weight, drag, thrust must stay in balance with one another respectively or one must exceed the other to climb or descend.
When we are traveling down the free way in the car we can shift into the highest gear. Load has been reduced, RPM has been reduced, and manifold pressure in a carbureted car goes up because the throttle is closed further but also goes down because the engine is running at lower RPM. In the constant speed propeller equipped airplane somewhat the same condition exists, Pitch the airplane for your cruise speed, the propeller is moved to a steeper pitch (Higher gear) Adjust the throttle for desired manifold setting, RPM prop control for appropriate RPM for the desired power setting and set Mixture for desired % economy power combination. The constant speed propeller maintains the set RPM whether we climb or descend if no other propeller adjustments are made. With a fixed pitch propeller we can’t change pitch in flight. The fixed pitch is selected to give the desired performance characteristics usually selected between climb & cruise. The flatter the pitch the better the climb the higher the RPM at given throttle & fuel mixture setting and the highest HP available. The steeper the pitch of the prop the climb is reduced because at the same throttle fuel mixture setting the RPM is reduced thus making less horse power. When the airplane is then in level cruise flight the propeller is unloaded and RPM increases which allows us to make a higher horsepower in cruise flight and the airplane is moving through the air faster. But in the latter case climb performance suffers because the engine cannot make as much RPM when the prop is heavily loaded in the climb. Such as trying to start a car up hill in third gear it will not go up the hill and you will burn out the clutch trying to make the engine reach the higher RPM. Or you will stall the engine keeping the clutch engaged. In this way the propeller in a way is stalling in the air (it may not actually be stalled but remember it is an airfoil) and it may not be operating at its optimum lifting capability for the situation.
We can reach a point where the throttle is wide open and the fuel mixture is set at the leanest setting and the engine cannot achieve maximum horse power because the RPM can not be increased because the pitch is so steep.
Horsepower cannot be directly indicated. The two most common instruments for determining power output are the (RPM) Tachometer and the (MAP, MP) Manifold pressure gage. Neither of which actually measure horsepower output. Most fixed pitch equipped airplanes do not have a manifold pressure gauge. We depend on the manufactures of engine, airframe & propellers to give us charts to tell us what engine RPM and or Manifold pressure to Give us the best Climb and Cruise performance with a given propeller and what horse power at what RPM. Then we can fly the airplane and determine if what they tell us is true.
I have always thought that any given propeller shop may adjust the prop pitch depending on wear of the propeller in chord and length to a large degree accurately or inaccurately depending on the skill and experience of the technician performing the work. The actual RPM that the prop will turn depends greatly on the Density altitude, condition of the engine more than that of the numbers stamped into the propeller, A worn out engine will not make as much power as a fresh engine with the same propeller. The same is true of magnetos slipping or being timed improperly or worn out plugs or misfiring plug wires.
A good example of what a manufacturer will give you is the Lycoming O-235 L2C engine. The data plate on the engine itself gives three different Horse Power ratings compared to RPM. This information is not in the operator’s manual for the airplane because the airframe manufacturer in the case of the Cessna 152 selected the RPM and horsepower by the propeller installed. And thus the tachometer has specific RPM limits dictated in the Type certificate of the airplane. An STC is available that allows a higher HP out put with a Sensenich propeller at one of the Higher RPMs stated on the Engines data plate.
This is why the RPM gage is not just there to tell you that the engine is still turning and why it is important to know its accuracy or inaccuracy. You can determine base line data for what the best climb rate RPM and cruise rate RPM you should get with a given propeller by studying the manufactures given specifications. If you know what your engine actually does with another given propeller and if you know how far it is off from the expected performance figures. Because I have observed that no two props with the same model and pitch fly the same and once they have been re-profiled and or overhauled the spread becomes even greater. You will only know what actually happens by flying the airplane. I have seen flatter pitch propellers installed by owners wishing to have a higher climb rate only to discover after installation the they are exceeding the maximum RPM limit of the engine at full throttle in the climb let alone at cruise. The effects of this are that the engine is now operating out of its optimum torque band. The prop needs to be re-pitched to put it back at the proper RPM torque curve. The question of whether they are gaining the performance they desired, is to fly the airplane against standard day settings at known weights and see if the performance is that which is desired. Or by the seat of the pants feel “ I am off the ground better and shorter†than I was before while staying at the recommended RPM limits.
I hope I did not confuse the issue,
Jim
