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Guest sillyeagle

Engine wear: RPM vs MP

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How does one go about balancing RPM with MP, and how does this relationship affect engine wear? Basically I'm wondering where you draw the line when setting low RPM and high MP for cruise. Is lower RPM and higher MP always better? I imagine only to a certain extent...With my supercharged car for example, there is not much need to spin the engine up like a normally aspirated car, as I can make boat loads of torque at very low RPM. Becuase of this I usually like to make my highway acceleration power with boost rather than RPM. So basically I have the option of making an equal amount of power via a lower gear(higher rpm) and less boost, or a higher gear(lower rpm) and more boost. I my mind it's better for engine wear if i don't downshift and make the needed power via boost, provided I'm not lugging the engine, though I could be wrong and I don't know how that applies to aero engines. Maybe my question cannot be answered generally and I'll have to refer to some specific power/economy charts, but with lots of pilots here who regularly work the RPM/MP relationship I figured there might be some good advice to get my into the right simming habits. Thanks.

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Generally you go with what the poh states but also most Ga pilots go with "squared"...e.g. 23" and 2300 rpm,24" and 2400 rpm. Usually gives close as a rule of thumb to a certain percentage e.g 65%, 55%.GeofaMy blog:http://geofageofa.spaces.live.com/

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The Handbook (POH) for the airplane is the best source. The "square" rule works as a general rule of thumb with many normally aspirated aircraft. The POH lists the possible MP and RPM settings to produce a certain amount of power such as 55%, 65% or 75%. For example, on a standard day in a Saratoga SP at 5000 feet, to produce 65% power (195hp and 13.8GPH) you have a choice of 24.3 inches MP at 2100 RPM23.5" @ 220022.5" @ 2300 or 22.0" @ 2400. A consideration is that above a certain altitude, the throttle can be left forward as the MP will be determined by the pressure altitude of the airplane. In that case the RPM will have to be increased to increase power (or maintain power in a climb).Turbocharged engines run at higher MP since that is the whole point of a turbocharger. Settings such as 29" and 2200 RPM are not uncommon.Personally I feel that the lowest possible RPM which maintains the desired power and is allowed by the POH should be used.1. Engines wear out because parts that rub against each other wear beyond tolerance. A 300 RPM reduction cuts out 18,000 rotations per hour of the engine, while the plane has flown the same distance. Admittedly, the explosions in the cylinders are stronger at lower RPMs, however, that should not have a detrimental effect if the engine is timed and lubricated properly.2. Props tend to be slightly more efficient at lower RPMs.3. Lower RPMs tend to be quieter.A couple of other rules when adjusting power:1. INCREASE power = prop and then throttle (PT as in 'PT boat')2. DECREASE power = throttle and then prop (TP as in 'toilet paper')Happy Flying!Ben

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I have a question related to this subject as wellDo real pilots having these two controls (rpms and mp) usually cruise at 65% power etc? Or can you run the engine all out the entire trip? IE if you really wanted to get somewhere fast could you firewall everything (within tolerances)I'm curious because in FSX, if you reduce your levers per the POH of certain acft that are modeled accurately, it seems to me the plane really slows down. I never pay attention to my green/yellow/red arcs! I normally just fly with firewalled throttles and of course lean mixture for max EGT or whatever.I guess I have poor simming habits lol!

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Until the gas prices went up I was always flying at 65% power-24" and 2400 rpm. In the the Baron that gets around 180-186 knts. TAS and a fuel burn of 28-30 gallons/hr.Now with higher gas prices I have been flying at quite a bit less when possible. If I am just practicing in the local area I will now fly at 20" and 2100 rpm-which now takes the cruise burn down to 19 gallons an hour-and about 140 knts.On recent longer trips, when I had a nice tailwind I went at 23" and 2300 rpm. True airpseed only went down about 10 knts. but a lower fuel burn. Add a 20 knt. tailwind and nothing is lost but some money is saved.Like the commercial airlines I would bet a lot of pilots are now throttling back and taking a little longer to get somewhere to save a few gallons.As far as I know Continental tests the engines but running them full power non stop for many hours past tbo, but I don't know anyone that runs anything full in all the time. It is hard on everything to do so.As mentioned, since you lose 1 inch of power for every thousand feet you go up-at about 5-6000 ft. your throttles will be full in anyway. (Not the props though).GeofaMy blog:http://geofageofa.spaces.live.com/

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Engine management is important in piston aircraft for better fuel economy and long engine life. All pilots have to think about these factors and a good pilot follows the tables in the POH. 55% power is considered economy cruise and 75% power is considered high speed cruise. As avgas prices go up, I suspect the 55% power settings is becoming more popular. Yes, the airplane will be much slower and the flight longer, but will consume less fuel getting from point A to B.Although engines are tested by the manufacturers at full throttle for long periods of time, most engines have limitations imposed by the manufacturer of max takeoff power for no more than five minutes. Fuel efficiency goes out the window at max power plus the engine will wear unnecessarily.The third control for proper engine management is the mixture control. The engine must be leaned with altitude to maintain power. Leaning too much for too long can cause damage to the engine because of excessive heat or detonation. 1. In simple aircraft, it is common to pull the mixture back until the engine starts to sound rough and then push it back in slightly.2. Normally aspirated engines can best be leaned with an exhaust gas temp gauge (EGT). As the mixture is slowly pulled back, the EGT will peak and then go back down. Different manufacturers have different recommendations but generally about 50 degrees lean of peak gives best economy and about 50 degrees rich of peak gives best power.3. On turbocharged engines, the turbine inlet temperature will have a maximum which must not be exceeded. This temperature is directly controlled by the mixture.Ben

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In relation to my last post, I read an article recently that said that aircraft rental companies are thinking of going back to the tachometer for charging custo9mers. The tach counts the revolutions of the engine. The traditional Hobbs meter just counts the time which the engine is running. Since fuel is included int he price of a rental, renters tend to run the aircraft at very high power settings, which causes operating costs to skyrocket between fuel consumption and excessive engine wear. Going to the tach would cause pilots to use more economical settings.Ben

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This is a good tid bit to read.. its in 3 partshttp://www.taturbo.com/future.htmlIt goes back in time to the Wright turbo-compound engines that use water methanol for increased MP in conjunction with the superchargers but the proven principle of economy and wear and tear in lean vs. rich adjustments is the same todayI am rebuilding the P5M2 Marlin for FSX in which I am incorporating engine mismanagement in MP/RPM for pilot errors which incorporate that water methanol system.Todays fuels will not allow those Wright 3700HP SC engines to run the same MP they did back in the 50's... if you tried to run them at their FAA rated MP on takeoff you would blow the engines on todays fuel

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Hello Nick:It might be worth mentioning technique as well concerning care of an engine.1. Always reduce MP first2. Then reduce RPM3. Always advance RPM first4. Then advance MP.As an example for many typical planes taking off somewhere near sea level.A. Firewall everything (Throttle, Prop, Mixture) on the takeoff roll. The readings might be something like: MP=26", RPM=2,700B1. After lift off and a climb is established, reduce MP to 24"B2 Then reduce RPM to 2,400C1. Upon reaching cruise altitude, reduce MP to 22.5" (or whatever the manual recommends for your desired power. If you are at a reasonably high altitude, full throttle may give you this setting, or less.C2. Reduce RPM to 2,250D1 For whatever reason, if you need to add power, Increase RPMD2 Then increase MP.Respectfully:RTH

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>B1. After lift off and a climb is established, reduce MP to>24">B2 Then reduce RPM to 2,400>Actually, there is even some good arguments out there, for keeping the throttle at full MP, while reducing the RPM only, in climb. It helps cool the engine, without the engine working as hard. At first look, it seems as if you're shifting a hard working engine into a higher gear while climbing up hill, as with an autombile. And you'd think..........no way! However, it's all explained in the article, and works within the engine manufactures guidelines. Now, if I could just find the article. It's in the AvWeb online news.L.Adamson

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Wow Ben you nailed it. I didn't expect to get such a dead on, all encompassing answer. Thank you very much! Thanks to everybody else too!

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>>D1 For whatever reason, if you need to add power, Increase>RPM>D2 Then increase MP.>I think the idea is to get your piston speed up prior to increasing the power of the explosion, to prevent undue engine stress. If the RPM is to low for the given power the engines will lug. Likewise you don't want to force the engine to slow under enormous pressure, so it's best to decrease your explosive power first, then force the engine into a lower RPM. While I've never read this as fact, this i what logic tells me. lol

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