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RE: Humidity effect on performance?
ORIGINAL: Q500RC Hey Capt! This is a bit off topic but could you PM me please I would like to ask you a few questions on ST90 gas conversions and when I try to PM you it won't let me! Thanks |
RE: Humidity effect on performance?
ORIGINAL: pe reivers ORIGINAL: karolh The difference that altitude and humidity makes is no stranger to me as I fly from two fields, one at sea level and the other at approx. 3,000', and the carbs require leaning at the lower altitude and richening at the higher one. Needless to say that at sea level the engine makes about 300 more rpm. Karol AV8TOR |
RE: Humidity effect on performance?
The prop has less resistance in thinner air. The resistance reduction is air mass related (keeping it simple). Engine power also is air mass related. (again keeping it simple). The balance between the two is not disturbed if mechanical losses are considered constant (smallish error), so rpm stay exactly the same, even though horsepower and propeller thrust change with altitude.
If tuned pipes are used, the tune changes due to change in compression ratio and exhaust gas temperatures. This also is not considered in my previous statement and swept under the rug by stating all else being equal. |
RE: Humidity effect on performance?
Well then why, when I jump into a Cessna 172 for example, which has a fixed pitch prop and a carb with mixture adjustable, is it that at sea level full throttle will produce 2700 rpms, but when I arrive at say 8000 feet, full throttle only produces about 2400 rpms??
And we can take airspeed out of the equation. Because the same basic reduction in rpm is apparent on the ground in a static runup. Another example, but with two strokes. To maintain the same full throttle rpms in my ultralight with a Rotax 503 at 7000 feet base altitude as I was getting at a sea level airport, I had to reduce the pitch of the prop by several degrees? (And this is with proper re-jetting for the altitude change applied.) I am a flight instructor with a couple thousand hours logged. I have never seen an engine that didn't lose rpms with altitude, unless it was turbo charged, and even then it will lose power and rpms above a certain altitude where the tubo can no longer supply sea level manifold pressure or more..... AV8TOR |
RE: Humidity effect on performance?
Well, after flying my Kadet yesterday for the first time in hot humid weather, I can definitely say the temp and humidity play a big role. The plane was just not the same. It lost altitude quickly, the motor wasn't producing as much power, I normally fly at half throttle, but I was almost at full to get at near same performance. My last flight I was doing some T&G and after the last touch, I went to go, and the motor just didnt want to spool up fast enough. By the time it produced enough power the plane was brushing the tops of the corn and took out 1 plant to the root. There were 2" pieces of corn plant scattered around the plane. Thankfully nothing got damaged.
Live and learn. I had tuned my DLE20 yesterday too. It seems to run ok, but after taxiing about at small throttle inputs, when I went to throttle up it died, twice, in the same spot. Just wondering if the humidity and heat played a art in that. |
RE: Humidity effect on performance?
The rule is for static rpm only, when prop efficiency is zero and the propeller acts as a pure air brake.
ORIGINAL: av8tor1977 Well then why, when I jump into a Cessna 172 for example, which has a fixed pitch prop and a carb with mixture adjustable, is it that at sea level full throttle will produce 2700 rpms, but when I arrive at say 8000 feet, full throttle only produces about 2400 rpms?? And we can take airspeed out of the equation. Because the same basic reduction in rpm is apparent on the ground in a static runup. Another example, but with two strokes. To maintain the same full throttle rpms in my ultralight with a Rotax 503 at 7000 feet base altitude as I was getting at a sea level airport, I had to reduce the pitch of the prop by several degrees? (And this is with proper re-jetting for the altitude change applied.) I am a flight instructor with a couple thousand hours logged. I have never seen an engine that didn't lose rpms with altitude, unless it was turbo charged, and even then it will lose power and rpms above a certain altitude where the tubo can no longer supply sea level manifold pressure or more..... AV8TOR |
RE: Humidity effect on performance?
ORIGINAL: acdii Well, after flying my Kadet yesterday for the first time in hot humid weather, I can definitely say the temp and humidity play a big role. The plane was just not the same. It lost altitude quickly, the motor wasn't producing as much power, I normally fly at half throttle, but I was almost at full to get at near same performance. My last flight I was doing some T&G and after the last touch, I went to go, and the motor just didnt want to spool up fast enough. By the time it produced enough power the plane was brushing the tops of the corn and took out 1 plant to the root. There were 2'' pieces of corn plant scattered around the plane. Thankfully nothing got damaged. Live and learn. I had tuned my DLE20 yesterday too. It seems to run ok, but after taxiing about at small throttle inputs, when I went to throttle up it died, twice, in the same spot. Just wondering if the humidity and heat played a art in that. The upper half of throttle generally controls abt 25-33% of power. For this reason I like to usually set throttle throws with a disparity between up and down with numbers like 100% up and 50% down. Doing so, moves the center stick position to less than center of the servo throw to yield more linear power control. |
RE: Humidity effect on performance?
When it is cooler out the throttle has been fine, it only started acting up after the third flight. It didnt quit, just didnt throttle up 100% right away as I was used to. The only time it quits when throttling up is when I first start it for the day. If I let it idle for a minute, then its fine the rest of the day. It used to quit if I idled too long and throttled up, especially when landing, then I turned the low speed in an 1/8th and its been fine ever since.
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