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David-aviator

Well Known Member
What is the criteria to get 75% power at WOT?

I've always just climbed to 8500' MSL and let her rip. Obviously, that is not very scientific.

In the past my machine has come in at 203 mph doing just that, rpm up around 2750-2800 with the Catto, fuel flow 11-12 gph, leaned out to peak or just ROP. But it was a cool day.

Last week returning from OSH at 8500' it came in at about 197 mph, max rpm 2700, fuel flow peaked at about 10.5 gph. If fuel flow was increased by going richer, nothing changed with speed except more gas was being burned. The difference from the 203 mph day was DA over 11.000' due to 36F above standard. (temp at 8500' was 68F)

Later after a descent to 4500' approaching destination, DA was about 8000' and with WOT throttle, not much changed in performance except rpm was up to 2800 and fuel flow over 12. (Man it was hot, OAT was around 90 and 100 when I landed.)

Could it be there was more total drag at 4500' MSL even though DA was at 8000'?

There's a lot I don't understand about this stuff. I'm just wondering if it is possible to simulate 75% power, the benchmark for performance data, when it is as warm as it has been this summer.
 
THere are dozens of threads on % power calculations.
You can gets a lyco power calculator here.
 
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Kahuna said:
THere are dozens of threads on % power calculations.
You can gets the lyco power charts here.
Click to expand...

User Input Data
Altitude (ft): 8500
rpm input: 2700
MP (in HG): 22.8
temperature units (F or C): F
temperature (deg F): 68

Calculated Data hp (% power)
power at actual temperature= 160.2 80.1%
power at standard temperature - 20 deg F= 170.0 85.0%
power at standard temperature= 166.5 83.3%
power at standard temperature + 20 deg F= 163.2 81.6%

That chart concludes power was from 80-85%. I don't believe it.
 
Might help

Figure 3-21 on pg. 3-36 in the 60297-12 Lycoming Operators manual might help. Russ
 
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AK4x4 said:
Figure 3-21 on pg. 3-36 in the 60297-12 Lycoming Operators manual might help. Russ
Click to expand...

Thanks. The chart you reference is for 200 HP.

Using a chart for 180 HP engine, it looks like it was 70% power at 8500' with a plus 36F factor that day. The performance number of 197 mph with the RV-7A fits the scenario. I understand all that.

The question is how do you find an altitude at a given temperature to come with 75% power? I suppose one could go into the chart with the 75% HP number and work backward to arrive at the correct pressure altitude.

This isn't all that important except I was trying to come up with a speed number for HEvans (that may be misspelled), he did some performance testing recently.
 
Lycoming Power Chart Perusing

Lycoming has a note on all power charts in the O360 Operator's Manual "Approximately 1% correction for each 10F variation from Ts." Ts being standard temperature.

So if the temp is 100F instead of 60F, power loss is 4% due to non-standard temperature.

Hmmm, I'd swear take off performance at 100F vrs 60F reflects a lot more power loss than 4%. Some GA aircraft have an increase in take off distance up 25% under such circumstances.

At 8500' a plus 36F reduces WOT power from 75 to 70% (rough pencil calculation from the power chart). Ok that makes sense, speed is off from 203 to 197, a drop of 3%. This seems reasonable.

But during take off the penalty is much more severe. The 4% power loss reduces performance into the double digits.

Interesting. I wonder if the "Approximately 1% correction for each 10F variation from Ts." is valid across the board. The power loss may be much more than 4% when not in flight.

Another marker, Vans shows performance numbers for their aircraft with different HP engines. Granted, some of these numbers are estimates but the general presentation is if power is reduced 12%, like 180 to 160, take off performance is affected 12%.

That's what leads me to believe the Lycoming statement about power gain or loss due to non standard temperature needs to be qualified. It does not appear to be true during take off.
 
Our propeller and wing also lose effiency with increase in temp. High humidity can take another 10% of our engines power. Interesting subject.
 
On the way home from Oshkosh, I was flying at 10.5K. The Dynon told me density altitude was about 13400. I was at wide open throttle and LOP at 7.4GPH (ECI IO-360, 9.2:1 compression) The Dynon also told me I was making 57% power at 2400 rpm, still making 160kts GS with probably 8-10 kts worth of headwind component.
 
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