% Power Estimate @ 5000 & 5500 RPM...

[Piper J3]

This is interesting…. I asked ChatGPT to estimate % power at two different cruise settings normally used on RV-12...

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Estimate % power for 5000 RPM - Vans Aircraft RV-12 with Rotax 912ULS engine, 3-blade E-prop optimized for cruise, 73F and 2800' Density Altitude while flying at 2000 MSL. Running 93E10 Mogas and burning 4.5 GPH

Given your specific conditions, we can make a fairly good estimate using both the Rotax 912 ULS brake-specific fuel consumption (BSFC) and typical RV-12 performance.

Conditions

• Aircraft: Van's RV-12
• Engine: Rotax 912 ULS (100 hp)
• Propeller: 3-blade E-Prop, cruise optimized
• RPM: 5000
• OAT: 73°F (23°C)
• Density Altitude: 2,800 ft
• Pressure Altitude: 2,000 ft MSL
• Fuel: 93 E10 mogas
• Fuel Flow: 4.5 GPH

Estimate from fuel flow

The Rotax 912 ULS typically has a BSFC of about 0.42–0.45 lb/hp/hr in cruise.

Fuel weight:

• 4.5 gal/hr × 6.0 lb/gal ≈ 27 lb/hr

Power produced:

At BSFC = 0.43:

• 27 ÷ 0.43 ≈ 63 hp

At BSFC = 0.45:

• 27 ÷ 0.45 ≈ 60 hp

So the engine is likely producing 60–63 hp.

Percent Power

The 912 ULS is rated at 100 hp at sea level.

That gives:

• 60–63% power

Since you're at 2,800 ft density altitude, the engine cannot make the full 100 hp because it's naturally aspirated. Maximum available power there is roughly 91–92 hp.

Relative to the available power at that altitude:

• 60–63 hp ÷ 91–92 hp ≈ 65–69% of available power

Cross-check with RPM

For a cruise-pitched E-Prop on an RV-12:

• 4800 RPM → ~55–58%
• 5000 RPM → ~60–65%
• 5200 RPM → ~68–72%
• 5400 RPM → ~75–80%

Your measured 4.5 GPH agrees very well with the 60–63% estimate.

Best estimate

5000 RPM under your conditions corresponds to approximately:

• 61–63% engine power (relative to the engine's 100 hp rating)
• 65–69% of the maximum power actually available at a 2,800 ft density altitude

The midpoint estimate is about 62% power, producing roughly 62 hp, which matches your reported fuel burn almost exactly.

If you also know your true airspeed at 5000 RPM (or your GPS groundspeed on a calm day), I can refine the estimate further by correlating power, propeller efficiency, and aircraft drag for your specific E-Prop.

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Estimate % power for 5500 RPM - Vans Aircraft RV-12 with Rotax 912ULS engine, 3-blade E-prop optimized for cruise, 73F and 2800' Density Altitude while flying at 2000 MSL. Running 93E10 Mogas and burning 5.0 GPH

Given your configuration, we can make a fairly good estimate.

Conditions

• Aircraft: Van's RV-12
• Engine: Rotax 912 ULS (100 HP)
• Prop: 3-blade E-Prop pitched for cruise
• RPM: 5500
• Density Altitude: 2,800 ft
• OAT: 73°F
• Pressure Altitude: ~2,000 ft MSL
• Fuel: 93 E10 Mogas
• Fuel Flow: 5.0 GPH

Estimate from fuel flow

The Rotax 912 ULS has a Brake Specific Fuel Consumption (BSFC) near 0.42–0.45 lb/hp/hr at cruise.

Using 5.0 GPH:

• Gasoline weighs about 6.0 lb/gal
• Fuel burn = 30 lb/hr

Estimated horsepower:

• At BSFC = 0.42:
  • 30 ÷ 0.42 = 71 HP
• At BSFC = 0.44:
  • 30 ÷ 0.44 = 68 HP

So the engine is producing roughly 68–71 HP.

Relative to the 100 HP engine:

• 68–71% power

Check against RPM

A cruise-pitched E-Prop on an RV-12 typically produces:

• 5000 RPM → ~55–60%
• 5200 RPM → ~60–65%
• 5400 RPM → ~65–70%
• 5500 RPM → ~68–72%
• 5600 RPM → ~72–75%

Your fuel flow agrees almost perfectly with this.

Estimated power

≈70% power (±2%)

A reasonable estimate is:

5500 RPM = approximately 70% power

Interesting observation

A stock RV-12 with the Sensenich prop often burns around 4.7–4.9 GPH at this power. Your 5.0 GPH is consistent with:

• an E-Prop pitched for cruise,
• slightly richer mixture at this operating point,
• or simply normal engine-to-engine variation.

Nothing appears unusual.

If you also tell me your true airspeed (or GPS groundspeed in calm air) at this setting, I can estimate your propeller efficiency and determine whether your E-Prop is pitched slightly coarse, about ideal, or slightly fine.

 

[seagull]

When I have asked Chat the same HP related questions manifold pressure was always part of the equation.

 

[N8DAV8R]

If you really want to crunch these numbers, take your EFIS logs and dump them into GPT to analyze.

I'm kind of an AI skeptic. The conclusions you get are probably in the 'close enough' bucket, but I have my doubts about its ability to track nuances as it scrapes the web and various documents to draw conclusions. I think it can do a much better job if you feed it specific reliable data to work from. Your EFIS logs are an abundance of useful real world data.

This was part of my strategy for the Hayward Air Rally this year. I fed it almost a year worth of EFIS logs (G3X and IS motor in my case). We then had a long discussion about the power settings and performance that result in the least variation, along with strategies and settings to make adjustments. Adjustments were not necessary and we flew a 252 mile course that turned out to be accurate to about 15 seconds and 1.5oz fuel. As rookies, we won the whole contest. I believe that AI analysis of logs and strategy contributed to the win.