RPM/MP & turbos

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Tommar98

Pre-takeoff checklist
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Tommar98
Ok I’m a truly confused by the physics involved in airplane engine with turbos. I’ve read and seen videos on how turbo charging works - more air increases power as sea level pressure decreases with altitude, etc. However if the prop is spinning at 2400 rpm at 5000 ft and 2400 at 12000 feet, how does that translate to more speed through the air? The prop is still only spinning at 2400 rpm and still has less air to cut through? I understand how the engine benefits from the turbo but I don’t understand how that translates to the prop performing better and thereby increasing speed at higher altitudes.


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Tommar98 said:
Ok I’m a truly confused by the physics involved in airplane engine with turbos. I’ve read and seen videos on how turbo charging works - more air increases power as sea level pressure decreases with altitude, etc. However if the prop is spinning at 2400 rpm at 5000 ft and 2400 at 12000 feet, how does that translate to more speed through the air? The prop is still only spinning at 2400 rpm and still has less air to cut through? I understand how the engine benefits from the turbo but I don’t understand how that translates to the prop performing better and thereby increasing speed at higher altitudes.


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Prop pitch, number of blades, size of blades.... You can move a lot more air at same RPM by changing prop configuration
 
In fact, turbo planes tend to be less efficient at low altitudes than their normally aspirated cousins. This is mostly due to them having propellers designed for higher power at higher altitudes.
 
I’d suggest it has to do with the prop controller.

Consider the non-turbocharged aircraft: the rpm in your example is being maintained be the controller flattening the prop blade angle, to reduce load on the engine (to maintain rpm). Reduced load also means reduced thrust generation, therefore less speed.
 
Tommar98 said:
. However if the prop is spinning at 2400 rpm at 5000 ft and 2400 at 12000 feet, how does that translate to more speed through the air? The prop is still only spinning at 2400 rpm and still has less air to cut through? I understand how the engine benefits from the
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Would be true in an airplane with a fixed pitch prop, but I've never seen a turbocharged airplane with one.
 
Ever gone canoeing?

Paddle with the blade slicing through the water, and then turn it 90 degrees. Which one pushes you through the water faster when taking the same strokes per minute?

That 2400 RPM is a different blade angle with the same MP at different altitudes.
 
Tommar98 said:
Ok I’m a truly confused by the physics involved in airplane engine with turbos. I’ve read and seen videos on how turbo charging works - more air increases power as sea level pressure decreases with altitude, etc. However if the prop is spinning at 2400 rpm at 5000 ft and 2400 at 12000 feet, how does that translate to more speed through the air? The prop is still only spinning at 2400 rpm and still has less air to cut through? I understand how the engine benefits from the turbo but I don’t understand how that translates to the prop performing better and thereby increasing speed at higher altitudes.


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Thinner air will eat up less horsepower through drag and other parasitic forces so the engine will naturally spin faster for a given amount of horsepower. A plane with a constant speed prop will adjust itself to maintain RPM. This means that it will have to increase the pitch more than it would at lower altitude so that the extra horsepower will be used and maintain load on the engine. Ok, if that's confusing enough for you....
 
genna said:
In fact, turbo planes tend to be less efficient at low altitudes than their normally aspirated cousins. This is mostly due to them having propellers designed for higher power at higher altitudes.
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I think it's actually mostly due to lower CR pistons which require more fuel per cycle to produce the same power, and poor cooling designs that lead to operators attempting to cool the engine with extra fuel not used in the combustion process.

Turbo-normalizing goes a long way in regaining this efficiency, as does intercooling (minus the added weight penalty of the hardware).
 
As you go higher the air is less dense, offers less resistance to the airframe, so you go faster. It also offers less resistance to the engine, so it turns faster too. So, you coarsen the propeller pitch to keep that from happening and there you have it.
 
hindsight2020 said:
I think it's actually mostly due to lower CR pistons which require more fuel per cycle to produce the same power, and poor cooling designs that lead to operators attempting to cool the engine with extra fuel not used in the combustion process.

Turbo-normalizing goes a long way in regaining this efficiency, as does intercooling (minus the added weight penalty of the hardware).
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I didn’t mean fuel efficiency. I meant performance. Sorry

If you compare SR22NA and SR22T, you’ll see that NA performs better at sea level. Climb and takeoff performance are better in NA. My understanding is that it’s mostly due to prop being more optimized for higher altitude cruise
 
Tommar98 said:
Ok I’m a truly confused by the physics involved in airplane engine with turbos. I’ve read and seen videos on how turbo charging works - more air increases power as sea level pressure decreases with altitude, etc. However if the prop is spinning at 2400 rpm at 5000 ft and 2400 at 12000 feet, how does that translate to more speed through the air? The prop is still only spinning at 2400 rpm and still has less air to cut through? I understand how the engine benefits from the turbo but I don’t understand how that translates to the prop performing better and thereby increasing speed at higher altitudes.
Click to expand...

If you maintain the same amount of engine power and decrease the resistance on the prop by making the air thinner, you merely need to increase the angle of attack of the prop to move more air and create more thrust and more resistance to keep the RPM the same.

hindsight2020 said:
I think it's actually mostly due to lower CR pistons which require more fuel per cycle to produce the same power, and poor cooling designs that lead to operators attempting to cool the engine with extra fuel not used in the combustion process.

Turbo-normalizing goes a long way in regaining this efficiency, as does intercooling (minus the added weight penalty of the hardware).
Click to expand...

There's also the increased back pressure of having the turbo in the exhaust and spinning it, which is going to absorb some of your power, whereas on the NA engine that power is going to the prop.
 
flyingcheesehead said:
There's also the increased back pressure of having the turbo in the exhaust and spinning it, which is going to absorb some of your power, whereas on the NA engine that power is going to the prop.
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No....power is the same at the prop at sea level conditions. Fuel flow is not. The turbo takes more fuel (+0.75-1.0 gph) to spin the turbo....and is less "fuel" efficient.

Power is made up from exhaust back pressure with "more" manifold pressure.....
 
Tommar98 said:
Ok I’m a truly confused by the physics involved in airplane engine with turbos. I’ve read and seen videos on how turbo charging works - more air increases power as sea level pressure decreases with altitude, etc. However if the prop is spinning at 2400 rpm at 5000 ft and 2400 at 12000 feet, how does that translate to more speed through the air? The prop is still only spinning at 2400 rpm and still has less air to cut through? I understand how the engine benefits from the turbo but I don’t understand how that translates to the prop performing better and thereby increasing speed at higher altitudes.
Click to expand...

There are two kinds of witchcraft going on here. Above, others have addressed the magic of the constant speed propeller and variable blade angles. As air gets thinner, blade angle gets steeper [it takes a larger bite on each revolution], which pushes the plane faster.

The other magic comes from power. As an airplane climbs, air pressure is reduced, making less oxygen available to produce power. The handy turbocharger shoves more air inside the cylinders, so it makes more power at any altitude. The higher the altitude, the more difference it makes until reaching the critical altitude, where the turbo can no longer make 100% power. FYI, my plane can't make 100% power at any density altitude higher than sea level. Some turbocharged engines will make 100% power up to 18,000 msl or higher, giving them the ability to steepen propeller blade angle even more, taking an even larger bite and going even faster.
 
Some people thing about constant speed propellers like shifting gears in cars. The analogy sort of works.. in that as the plane goes faster (or slower) you can change the pitch of the propeller to keep it spinning at the same rate by "grabbing" more or less air. Fixed pitch props are kind of like having your car stuck in third gear.. people find a balance between climb and cruise performance, but usually all this means is that you end up with a plane that doesn't climb that great, and can't really cruise much over 110-120 TAS.

The turbo works by keeping the engine producing power levels similar to what you get at sea level (usually) even up at altitude where the air is thinner and you'll usually only make around 55% power even with WOT. Super chargers do the same thing, the difference being that turbo's use the expanding exhaust gases to spool them up and force air into the intake manifold while super chargers use a mechanical drive from the engine. Turbo's do add some back pressure on the exhaust, but I'm of the conjecture that it's more efficient than a super since you are utilizing expanding exhaust gases that would otherwise have gone to waste

So, putting the two together (turbo / constant speed prop) you have an engine that can continue to make full power up at altitude, and the adjustable pitch prop keeps the propeller in an optimum power band, so you can "shift gears" as you go faster and keep the prop somewhere around 2,200 to 2,400 RPM

Also.. air does get thinner as you go up, but it tends to loosely "balance out" in favor of the airplane. IE, sure the propeller has less air to grab onto, but the plane also has less air to fly through.. that's why given a constant engine man press and RPM you'll pick up a few knots of TAS the higher up you

*PS.. faster spinning prop is not always better.. in fact, turboprop planes spin relatively slow. So there's an optimum speed that works for the aircraft props too. One criticism of Cirrus no-blue-knob is that the 2,500 RPM they're set at is a little too fast. I trust the engineers know what they did, but there are people out there who believe 2,200 - 2,400 is a better cruise regime
 
1. Air density decreases causing less drag
2. Available horsepower remains constant instead of decreasing.
3. Propeller pitch changes.
 
Checkout_my_Six said:
constant speed prop....tips don't like to go mo fasta.
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Yeah, but you can design 'em to be slower. That was my point. You don't have to turn 2500 RPM to get good performance from a prop.

1RTK1 said:
I thought for most propellers they were most efficient right before they reach the speed of sound....
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I think maybe you're joking... But when the props get close to mach 1, they start to lose efficiency very quickly. I'm not sure how far below that the optimum speed is, though.
 
James_Dean said:
I'm thinking about it. That '74 Machen conversion over on BT looks like fun.
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I guess when you get a plane that's fast enough, you have to start buying more planes to go slower, right? :D

I'd love to have a Globe Swift for fun... Alas, I can only afford one airplane. :( First world problems! :rofl:
 
flyingcheesehead said:
I guess when you get a plane that's fast enough, you have to start buying more planes to go slower, right? :D

I'd love to have a Globe Swift for fun... Alas, I can only afford one airplane. :( First world problems! :rofl:
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I'm very happy with 200 kts true....:D
 
EdFred said:
Ever gone canoeing?

Paddle with the blade slicing through the water, and then turn it 90 degrees. Which one pushes you through the water faster when taking the same strokes per minute?

That 2400 RPM is a different blade angle with the same MP at different altitudes.
Click to expand...

Best explanation ever!
 
flyingcheesehead said:
I think maybe you're joking... But when the props get close to mach 1, they start to lose efficiency very quickly. I'm not sure how far below that the optimum speed is, though.
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Not joking, flying RC I remember reading an article that said the faster you could turn a prop without breaking the speed of sound the more efficient.
Wish I could remember the formula. Thought it was about 75-80% speed of sound.
 
1RTK1 said:
Not joking, flying RC I remember reading an article that said the faster you could turn a prop without breaking the speed of sound the more efficient.
Wish I could remember the formula. Thought it was about 75-80% speed of sound.
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That's about right. Remember that the propeller is a rotating wing, with an airfoil that accelerates the air over the cambered surface. At those percentages you cite the airflow over the prop is considerably closer to the speed of sound than the prop tip is. Increasing RPM starts the formation of shockwaves and increasing drag on the prop that just eats horses without giving much extra performance in return.

And since the speed of sound is dependent only on temperature, and the temps at altitude are pretty low, tip speeds become critical.

Forward speed figures into the tip speed as well. Rotational tip speed squared plus forward speed squared equals overall tip speed squared. So turboprop airliners use big, slow-turning props. 900-1300 RPM or so.
 
flyingcheesehead said:
I guess when you get a plane that's fast enough, you have to start buying more planes to go slower, right? :D

I'd love to have a Globe Swift for fun... Alas, I can only afford one airplane. :( First world problems! :rofl:
Click to expand...

What I really want is a Harmon Rocket. :)


Edit: with the 400hp IO-540. I’ll call it ‘The Ted’.
 
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Checkout_my_Six said:
I'm very happy with 200 kts true....:D
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I'm happy with my 175 kts true! But, I'm also happy with getting that at 12 gph, so if I just want to go up and tool around for fun, I'm not worried about operating costs.

1RTK1 said:
Not joking, flying RC I remember reading an article that said the faster you could turn a prop without breaking the speed of sound the more efficient.
Wish I could remember the formula. Thought it was about 75-80% speed of sound.
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75-80% I would buy. But when you say "without breaking the speed of sound" I'm thinking in the neighborhood of 99%. ;)

And while the prop may be more efficient when faster, the engine is less efficient. So, the trick is to get your 75-80% at a lower RPM. I run at 2300 RPM, so if my math is correct, at 175 KTAS my prop tips are going about Mach .725. Probably close enough. ;)

James_Dean said:
What I really want is a Harmon Rocket. :)

Edit: with the 400hp IO-540. I’ll call it ‘The Ted’.
Click to expand...

You can't call it "The Ted" unless there's a second engine on it, along with a couple of bolted-on turbos. :D
 
flyingcheesehead said:
I'm happy with my 175 kts true! But, I'm also happy with getting that at 12 gph, so if I just want to go up and tool around for fun, I'm not worried about operating costs.
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yup....mine will do that too. It has a throttle and gas knob....to go slow. o_O
 
I can only imagine what the fuel burn is and how often cylinders get cooked to go 200 knots true at 3000 MSL.
 
Checkout_my_Six said:
yup....mine will do that too. It has a throttle and gas knob....to go slow. o_O
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So, what do you fly anyway?

EdFred said:
I can only imagine what the fuel burn is and how often cylinders get cooked to go 200 knots true at 3000 MSL.
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I can't do 200 at any altitude, but at 3000 I can do somewhere between 160-165 KTAS on 12 gph at 65%, not cookin' anything.
 
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