# Power loss at Altitude Question?

#### ventana7

##### Gold Member
Does anyone know the exact formula for figuring out how much horsepower is lost per 1,000 feet in altitude?

Thanks.

Rob

#### 13brv3

##### Gold Member
Good question. The best answer I can come up with is about 3% per 1000 ft in "standard atmospheric conditions". That fits almost exactly with the aircraft rule of thumb that you can only get 75% power at 8000 ft.

The other major factor will be mixture control. That 3% per 1000 ft will be assuming you can keep the mixture optimized by reducing the amount of fuel flow to match the reduced air density. If you can't do that, then you'll be running richer than optimal, which will reduce power even more.

Cheers,
Rusty

#### C. Beaty

##### Gold Supporter
The power an engine can produce is directly proportional to barometric pressure and inversely proportional to absolute temperature. So the relationship between power and altitude is not a simple one.

Aviation handbooks give the following approximation for available power without supercharging:

Altitude, feet…..…….…..0………5k…….10k……15k……20k…….25k
Fixed pitch prop………...1……0.82…..0.66….0.52.…0.41…..0.30
Constant speed prop….1……0.85…..0.71….0.59….0.48…..0.38

#### Vance

##### Gyroplane CFI
Hello Rob,

The short answer is that there is no set loss.

In my experience different engines respond differently to a change in air density.

When we build an engine for Bonneville, which is at 4,400, we use a lot of compression and short cams.

When we build an engine for forced induction we use less compression and longs cams.

The idea is to keep the brake mean effective pressure in the best range.

Typically, an aircraft will go further on a galleon of fuel at altitude because there are many things that change as the air thins out.

Thank you, Vance

#### Heron

##### Platinum Member
Will there be differences for carb and injection?
Guess yes!
Heron

#### Vance

##### Gyroplane CFI
As the air gets thinner the size of the carburetor venturi needs to be smaller to get a good signal and good fuel atomization.

With fuel injection their needs to be something to tell the injectors to inject less fuel as the engine takes in less air.

How well these things are addressed has a lot to do with how well the systems work at altitude.

One system is not inherently better than the other.

An altitude compensating turbocharger changes all of this.

Thank you, Vance

#### ventana7

##### Gold Member
Thanks Vance and Chuck and Rusty,

Rob

#### Aussie_Paul

##### A reforming stirrer!!!!!
Does anyone know the exact formula for figuring out how much horsepower is lost per 1,000 feet in altitude?

Thanks.

Rob

Until you need oxygen, none with a turbo Rob

Aussie Paul.

#### Fl90

##### Gold Member
Turbo with a waste gate Paul, and then you still need to read the manufacturers chart for critical altitude.

#### birdy

##### Active Member
A 914 dont have alt limits.

#### davidnane9

##### Newbie
Question: RV 4 equipped with Rotax-914 engine and variable pitch propeller.
Problem observed during flight above 7000 feet , vertical speed become 0, rpm over shoot for 2,3 second up 6300 when turbo engaged for gaining height.i did effort for Turbo but not gaining height. Feels like under power. But take off was normal. Air Box target pressure normal. All other engine parameters also normal.
So come back to pavilion.

#### Vance

##### Gyroplane CFI
I have had similar issues in two different Cavalons with 914.

One would over boost briefly and then reduce to very little boost.

It turned out to be a sticking waste gate.

The other had to do with the inlet temperature sender. It would reduce the boost to zero and then, after a while it would go back to working normally.

I don’t recall what manifold pressure I was running. It fit the definition of cruise power in the POH. I was around 9,000 density altitude on a warm day in Utah near Cedar City.

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