Rate of climb vs altitude in Turbo normalized aircraft

[Kiddo's Driver]

If HP is held constant by a turbo normalized engine and indicated airspeed is held constant, will rate of climb increase as you climb into thinner air?
Thinner air should reduce drag.
The engine will keep putting out the same HP.
That should result in higher climb rates, right?

Higher TAS will induce more drag.
TAS +2% per 1000 ft.
Drag -3.3% per 1000 ft.
But drag is affected by the square of speed... my poor brain...

 

[Checkout_my_Six]

But the prop loses as you climb.

 

[Pinecone]

IHigher TAS will induce more drag.

Drag is based on IAS, not TAS.

And drag increases 4x speed.

 

[Ted]

Your climb rates will be more or less equal when you first take off for the first part of climb. As you get higher eventually the prop losing efficiency (because thinner air) and the engine losing horsepower (because hotter induction air temps due to the pressure ratio going up and thus more heat at the turbo outlet) will impact that.

Regardless, your performance (both climb and cruise) will be better than naturally aspirated of the same engine.

 

[luvflyin]

Your climb rates will be more or less equal when you first take off for the first part of climb. As you get higher eventually the prop losing efficiency (because thinner air) and the engine losing horsepower (because hotter induction air temps due to the pressure ratio going up and thus more heat at the turbo outlet) will impact that.

Regardless, your performance (both climb and cruise) will be better than naturally aspirated of the same engine.

Would wing efficiency be a factor to, like the prop?

 

[Kiddo's Driver]

Your climb rates will be more or less equal when you first take off for the first part of climb. As you get higher eventually the prop losing efficiency (because thinner air) and the engine losing horsepower (because hotter induction air temps due to the pressure ratio going up and thus more heat at the turbo outlet) will impact that.

Regardless, your performance (both climb and cruise) will be better than naturally aspirated of the same engine.

100% agree. Trying to quantify what the turbo does for you.

 

[Checkout_my_Six]

100% agree. Trying to quantify what the turbo does for you.

In my case. V35A TC with a 285 HP TC. I see 1,100 fpm fully loaded at sea level....and it drops off to 800-900 fpm as we climb into the teens.

My Cherokee Six could barely get to 12,000....lightly loaded. The last couple thousand feet took a while.....climbing slowly.

 

[Ted]

100% agree. Trying to quantify what the turbo does for you.

The answer will depend on the plane, but in my experience you normally will see sea level climb performance up through around 10k ft, and then it starts to taper off. With a naturally aspirated engine you will notice climb tapering off pretty much immediately.

The specifics will vary dependent on aircraft as some engines will hold power to a higher altitude than others. Also some propellers are better designs and will therefore maintain pretty close to similar efficiency up higher.

My opinion is that turbochargers are not worth the extra cost, weight, maintenance, etc. unless you are operating out of the rockies, and even then you can question how beneficial they truly are. The exception is in cases where you have turbocharged engines and the turbos provide you with more horsepower, which the aircraft needs - like with virtually every cabin class aircraft.

A fun tool to play with is the Borg Warner Matchbot:

https://www.borgwarner.com/matchbot/

All aircraft turbochargers are way older than anything that Borg Warner makes today. However it will show you some of the changes in just how hot intake air can get as you get up in altitude, and the resultant changes in horsepower. I've often wondered what sort of performance improvements we'd see with a proper modern turbocharger. On my bus just going from a 2000 model year turbo (which is realistically probably early 90s tech) to a modern turbo (released in the early 2010s) yielded a noticeable reduction in intake temps and better performance/lower EGTs. I'm sure aircraft would benefit similarly, as the turbo tech is from the 60s.

 

[Kiddo's Driver]

What lead me to the question is two part.

1. Background is that I am evaluating a couple options to repower my Tampico TB9. Two of them are turbocharged.
2. I've been plotting/comparing the climb data from my pilot's information Manual (PIM) and comparing it to the "lose 3.5% of HP per 1,000ft climbed" rule of thumb. The climb rate does not drop off at the rate predicted by the loss of HP. That means that for the book numbers to be true the HP required for level flight must be going down as you climb so that your excess HP doesn't degrade as fast as the loss of HP due to thinner air would predict.

This makes me want to understand what happens as you climb better so that I can better quantify my predicted performance.

 

[Kiddo's Driver]

In my case. V35A TC with a 285 HP TC. I see 1,100 fpm fully loaded at sea level....and it drops off to 800-900 fpm as we climb into the teens.

My Cherokee Six could barely get to 12,000....lightly loaded. The last couple thousand feet took a while.....climbing slowly.

If you keep the IAS steady, does it keep 1,100 fpm through 12K feet? Are you making any power changes in there?

The answer will depend on the plane, but in my experience you normally will see sea level climb performance up through around 10k ft, and then it starts to taper off. With a naturally aspirated engine you will notice climb tapering off pretty much immediately.

The specifics will vary dependent on aircraft as some engines will hold power to a higher altitude than others. Also some propellers are better designs and will therefore maintain pretty close to similar efficiency up higher.

My opinion is that turbochargers are not worth the extra cost, weight, maintenance, etc. unless you are operating out of the rockies, and even then you can question how beneficial they truly are. The exception is in cases where you have turbocharged engines and the turbos provide you with more horsepower, which the aircraft needs - like with virtually every cabin class aircraft.

A fun tool to play with is the Borg Warner Matchbot:

https://www.borgwarner.com/matchbot/

All aircraft turbochargers are way older than anything that Borg Warner makes today. However it will show you some of the changes in just how hot intake air can get as you get up in altitude, and the resultant changes in horsepower. I've often wondered what sort of performance improvements we'd see with a proper modern turbocharger. On my bus just going from a 2000 model year turbo (which is realistically probably early 90s tech) to a modern turbo (released in the early 2010s) yielded a noticeable reduction in intake temps and better performance/lower EGTs. I'm sure aircraft would benefit similarly, as the turbo tech is from the 60s.

I'm evaluating the Deltahawk DHK180 (180hp for 5 minutes, 135hp continuous after that well up into the teens) and the Rotax 916isc C24 (160hp for 5 minutes, 137hp continuous after that well up into the teens).
I'll go check out your link.

Thanks!

 

[Checkout_my_Six]

If you keep the IAS steady, does it keep 1,100 fpm through 12K feet? Are you making any power changes in there?

I'm evaluating the Deltahawk DHK180 (180hp for 5 minutes, 135hp continuous after that well up into the teens) and the Rotax 916isc C24 (160hp for 5 minutes, 137hp continuous after that well up into the teens).
I'll go check out your link.

Thanks!

It's been a while since I've climbed that high...but to my recollection....I think the power settings are not touched (this has an altitude compensating fuel pump). We are trimmed for a climb attitude. IAS is somewhat constant....and as altitude affects the aircraft the climb speed slows.

 

[Sac Arrow]

My Turbo Arrow (which was turbo supercharged, not normalized) had a pretty dismal rate of climb from sea level. But it wasn't any more dismal at a 10,000 foot DA. Climb performance would however start to fall off climbing above 10,000 feet.

 

[Kiddo's Driver]

It's been a while since I've climbed that high...but to my recollection....I think the power settings are not touched (this has an altitude compensating fuel pump). We are trimmed for a climb attitude. IAS is somewhat constant....and as altitude affects the aircraft the climb speed slows.

Based on what I'm hearing it looks like what I get at take off is about what I will keep all the way up to where the turbo drops off. I'll modify that by saying that since both of the turbo engines I am looking at drop down after a 5 minute higher power run, the climb will be based on the lower continuous power setting.
That actually helps a lot. I can find the 135/137hp altitude of my stock engine in the PIM and see what the climb rate is there. It should hold that climb rate up to where the turbo drops off. (Realistically, I'm probably not going to be flying up where O2 is required so I'll never get to where the turbo can't keep up.)

 

[Checkout_my_Six]

The turbo will get you to altitude faster....faster above 8-10,000 feet where the HP starts to drop off. If you are not flying above 8K ft...the turbo doesn't buy you anything. And...it will cost you fuel down low (up high too). It takes energy to spin the turbo.

 

[Kiddo's Driver]

The turbo will get you to altitude faster....faster above 8-10,000 feet where the HP starts to drop off. If you are not flying above 8K ft...the turbo doesn't buy you anything. And...it will cost you fuel down low (up high too). It takes energy to spin the turbo.

My stock engine is an O-32-D2A, 160HP. It makes 135 at 4,000ft. Everything above 4,000 feet the turbo will produce more power. Both of the turbo engines will keep their max continuous to 15-17,000 feet.

Edit:
Rotax 916 ISc Takeoff power available up to 15,000ft. Rotax "critical altitude" is 23,000ft.
Deltahawk DHK1800 Take off power available up to 12,000ft. Continuous available up to 17,500ft.

 

[Ted]

I'm evaluating the Deltahawk DHK180 (180hp for 5 minutes, 135hp continuous after that well up into the teens) and the Rotax 916isc C24 (160hp for 5 minutes, 137hp continuous after that well up into the teens).

Given that you aren't looking at standard LyContisaur engines, I need to backpeddle a bit and say I don't know what Rotax or Deltahawk uses for turbochargers, or how those translate into actual altitude performance. Diesels tend to run at higher pressure ratios and I'm sure that the Deltahawk engine is no different, but if they're claiming 135 HP into the teens, that seems reasonable.

I can say that between the two, the Deltahawk is the only one I would consider if I was going to look at a repower.

 

[Kiddo's Driver]

Given that you aren't looking at standard LyContisaur engines, I need to backpeddle a bit and say I don't know what Rotax or Deltahawk uses for turbochargers, or how those translate into actual altitude performance. Diesels tend to run at higher pressure ratios and I'm sure that the Deltahawk engine is no different, but if they're claiming 135 HP into the teens, that seems reasonable.

I can say that between the two, the Deltahawk is the only one I would consider if I was going to look at a repower.

I just added an edit above your post that says:
Edit:
Rotax 916 ISc Takeoff power available up to 15,000ft. Rotax "critical altitude" is 23,000ft.
Deltahawk DHK1800 Take off power available up to 12,000ft. Continuous available up to 17,500ft.

There is also a 155 pound weight difference between the two engines with the Rotax being lighter (and cheaper).

 

[MRC01]

If HP is held constant by a turbo normalized engine and indicated airspeed is held constant, will rate of climb increase as you climb into thinner air?
...

One simple approach to this: Rate of climb depends on excess power. Vy is the speed of maximum excess power, so it is your max rate of climb. As the airplane climbs from sea level, Vy gradually changes. It decreases as IAS but increases as TAS. Either way, if you start at Vy sea level and hold airspeed constant, you are gradually flying different than the optimum airspeed. That means you are climbing less efficiently, or straying from the best climb rate.

Even if you kept it optimal by adjusting the airspeed to maintain Vy, I think the rate of climb would gradually decrease because of the shape of the curve of "power required" versus altitude, which gradually increases. That means at higher altitude, even if your engine is producing the same power, more power is required for level flight, so there is less excess power, and that's what determines your climb rate.

 

[EdFred]

If you keep the IAS steady, does it keep 1,100 fpm through 12K feet? Are you making any power changes in there?

It's been a while since I've climbed that high...but to my recollection....I think the power settings are not touched (this has an altitude compensating fuel pump). We are trimmed for a climb attitude. IAS is somewhat constant....and as altitude affects the aircraft the climb speed slows.

The turbo will get you to altitude faster....faster above 8-10,000 feet where the HP starts to drop off. If you are not flying above 8K ft...the turbo doesn't buy you anything. And...it will cost you fuel down low (up high too). It takes energy to spin the turbo.

So, out of curiosity, why keep the turbo?

 

[Checkout_my_Six]

So, out of curiosity, why keep the turbo?

It was cheap....I didn't choose it. It chose me. It was a deal too good to refuse. No one wanted the factory turbo in my setup. It's not the preferred config. If I land an extra couple Mil and need to swap out the engine....I might think about changing it.

It's not bad....but it does take a little bit more to maintain (I don't mind...I'm an A&P). And it takes about 1-1.5 gph more fuel than the other braggarts.

It moves nicely....here's one of my crab runs to the eastern shore around BWI class B. I prolly had a 10-15 kt tail wind.

 

[Pinecone]

What a turbo gives you, if you are willing to use O2, is to fly higher.

1) Higher true airspeed on the same fuel flow.
2) Above more of the weather
3) If IFR less frequency changes because you are talking to centers, not every approach control.
4) Almost no traffic. Most GA is lower than 10,000. The turbines and pressurized twins are above 20,000.
5) Higher winds, which can be great (Sunday I have 22 knots tail wing), but also can be brutal head winds.

I fly in the teens for anything over about 1.5 hour flights, wind permitting.

 

[Kiddo's Driver]

Given that you aren't looking at standard LyContisaur engines, I need to backpeddle a bit and say I don't know what Rotax or Deltahawk uses for turbochargers, or how those translate into actual altitude performance. Diesels tend to run at higher pressure ratios and I'm sure that the Deltahawk engine is no different, but if they're claiming 135 HP into the teens, that seems reasonable.

I can say that between the two, the Deltahawk is the only one I would consider if I was going to look at a repower.

On the engine choice: ideally they will be bringing a 200-210 takeoff hp engine to market soon and it will be able to do 160HP continuously. They are showing that it is available in up to 250HP, but only the 180Hp model is certified.