Does 2 inches make that big of a difference?

[SixPapaCharlie]

I clearly don't understand power.

If I have 2 planes that are the same airframe but with a different engine. Cirrus 20 vs 22, Grumman Cheetah vs Tiger, Comanche 180 vs 250.
I think those are all examples of the same airframe with a different engine.

2700 RPM is 2700 RPM. It isn't as if the 180 can't get to redline and the 250 can.

If I have an Cheetah and a Tiger, both at 3500 feet, both planes running at 2500 RMP but one is 30 knots faster and the only difference is 74 inches vs 72 on the prop.
I don't get why having more HP, Torque, etc matters if I am not swinging the prop any faster.

It would make a lot more sense to me if the 150 HP plane could only run 2400 RPM and the 180 could run 2700 but that's not the case.
Is it really just the prop size? I get that the higher we go, the more power is needed and the lower power engine can't do as well up there but there is something else at play that I don't understand.

Why would a Tiger beat a Cheetah a couple thousand feet above the ground if both are running the same RPM and carrying the same airframe regardless of HP?

 

[iamtheari]

Swinging the prop the same speed only produces the same thrust if it’s the same prop at the same pitch and same airspeed. Changing any of those (and probably dozens of other variables—I’m not an aerodynamics scholar, myself) means that the same RPM no longer equals the same thrust.

 

[NoHeat]

Why would a Tiger beat a Cheetah a couple thousand feet above the ground if both are running the same RPM and carrying the same airframe regardless of HP?

Different prop. You mentioned diameter, and that’s one thing. Pitch is another.

About diameter, what’s important for how much air is moved is the area that’s swept out by the prop, and that’s proportional not to diameter but diameter squared. So a few extra inches makes a difference.

 

[dmspilot]

If I have an Cheetah and a Tiger, both at 3500 feet, both planes running at 2500 RMP but one is 30 knots faster and the only difference is 74 inches vs 72 on the prop.
I don't get why having more HP, Torque, etc matters if I am not swinging the prop any faster.

That is not the only difference. They're also pitched differently.

It would make a lot more sense to me if the 150 HP plane could only run 2400 RPM and the 180 could run 2700 but that's not the case.

The Cessna 172R/S are like that. 160hp vs. 180hp. Different redlines. Different props. Same engine.

 

[midlifeflyer]

So you think a modern sleek body LSA with a Rotax turning a prop at 5500 RPM in cruise should be faster than an SR22?

 

[Dana]

So you think a modern sleek body LSA with a Rotax turning a prop at 5500 RPM in cruise should be faster than an SR22?

A Rotax engine may turn 5500 rpm, but the prop doesn't turn that fast, there's a reduction drive.

 

[midlifeflyer]

A Rotax engine may turn 5500 rpm, but the prop doesn't turn that fast, there's a reduction drive.

You’re right. Just woke up. But at 2400, just as fast?

 

[Dana]

You’re right. Just woke up. But at 2400, just as fast?

It could, depending on the design. But it wouldn't be LSA any more.

 

[Tools]

We teach enough principle in aviation to facilitate learning, and that’s it. Realize there are DOZENS of other variables.

This leads to the perception this is magic, art, or whatever. The real discussion, which frequently manifests itself here (always AMAZED by the expertise of this group) is LONG and COMPLICATED. Once in a while you just gotta press the I believe button and move on! Or knuckle down and get your doctorate in aerodynamics, and, and, and.

My eyes glaze over when I see two airplanes, same engine, same prop, same rpm, acting WAY differently… uhhhh… wha?
And then just realize one is my twin son, and the other is my other twin son.

 

[nauga]

My eyes glaze over when I see two airplanes, same engine, same prop, same rpm, acting WAY differently… uhhhh… wha?
And then just realize one is my twin son, and the other is my other twin son.

One of your twins has more drag than the other. Check his rigging and gap seals.

Nauga,
and his vector sum

 

[texasclouds]

Dodge Ram with manual transmission …2500 rpm in 2nd gear = weee ….2500 rpm in 5th gear = yeeyee

In the airplane…The higher manifold pressure is pushing a coarser pitch (or larger diameter) than the lower mp at same rpm.

 

[William Pete Hodges]

Swinging the prop the same speed only produces the same thrust if it’s the same prop at the same pitch and same airspeed. Changing any of those (and probably dozens of other variables—I’m not an aerodynamics scholar, myself) means that the same RPM no longer equals the same thrust.

The quick answer to your question is the prop size, shape, and profile bridge the gap between engine performance and airframe power demand. The prop diameter is the limiting factor for engine RPM, as the prop tip speed must be kept below the speed of sound, reducing prop efficiency and effectiveness. That's why most GA airplane engines red line at about 2700 RPM.

Constant speed props limit engine RPM to maximum and allow the operator to select engine speed for cruise. If comparing a Mooney M20C 180HP to an M20E 200HP, the only difference is the engine HP as the CS prop can adjust between the engines with pitch, but I am not sure if the blades are different or not.

Matching a propeller to an engine/airframe is not an exact science, as there are compromises involved. The best fixed pitch prop for maximum takeoff and climb will not be a good cruising prop and a good cruising prop will not be a good takeoff and climbing prop. Variable pitch props help with this but are not perfect either. The blade twist only perfectly matches the airflow over the prop in one condition of power, airspeed, and RPM, but the airframe demands a wide range of power vs airspeed over the airframe power demand curves. It is impossible to match propeller performance to airframe demand under all conditions with the highest efficiency, so the designer must select a prop profile that is flexible enough to supply power demand as needed, with loss of efficiency as a result.

The best option is to make a reasonable comprimise and hope the overall performance is satisfactory.

 

[MauleSkinner]

Once in a while you just gotta press the I believe button and move on!

I believe! I believe!

 

[455 Bravo Uniform]

The real discussion, which frequently manifests itself here (always AMAZED by the expertise of this group) is LONG and COMPLICATED. Once in a while you just gotta press the I believe button and move on! Or knuckle down and get your doctorate in aerodynamics, and, and, and.

Yeah, there are a few propeller-heads in here that when they start conversing in the combination of physics and aero I lose hold of the tow-rope. You know who you are (and I know who I am, lol!).

 

[Checkout_my_Six]

We can run thru the maths.....but those 2 extra inches are biting more air....moving more air....and producing more thrust.

And you thought she wouldn't notice those 2 inches......

 

[eman1200]

girth. it's all about the prop girth.

 

[MauleSkinner]

girth. it's all about the prop girth.

Don’t underestimate the shape of the spinner.

 

[Sac Arrow]

A Rotax engine may turn 5500 rpm, but the prop doesn't turn that fast, there's a reduction drive.

I'm just curious about Rotax engines, and I can't seem to find the answer online. 5,500 - 6,000 rpm is fast enough that I think the engine would have to have an automatic spark advance. Do Rotax engines have that? I -think- some have electronic ignition, so it shouldn't be difficult to incorporate.

 

[SixPapaCharlie]

So you think a modern sleek body LSA with a Rotax turning a prop at 5500 RPM in cruise should be faster than an SR22?

If I don't think too hard about it... yes.

 

[SixPapaCharlie]

Sooooooo.

If I have 2 Cessna 172s of identical shape, size, weight and one has 100HP engine and the other 200HP but they are spinning the same prop, would they have identical performance?
I feel like the answer to that is yes.

 

[AvNavCom]

Sooooooo.

If I have 2 Cessna 172s of identical shape, size, weight and one has 100HP engine and the other 200HP but they are spinning the same prop, would they have identical performance?
I feel like the answer to that is yes.

If the props are identical and are spinning at the same RPM, then my thinking is that they are pushing the same mass of air at the same rate, and therefore the resultant thrust should be identical. Presumably the HP part of the equation is the engine's ability to maintain that RPM under load (i.e. climbs) or at higher altitudes. I look forward to being smashed into oblivion by the mechanical and aeronautical engineers reading this thread.

 

[jsstevens]

Sooooooo.

If I have 2 Cessna 172s of identical shape, size, weight and one has 100HP engine and the other 200HP but they are spinning the same prop, would they have identical performance?
I feel like the answer to that is yes.

I think if the 100hp one is able to turn the prop at max RPM then the 200hp one will be RPM limited.

 

[William Pete Hodges]

Sooooooo.

If I have 2 Cessna 172s of identical shape, size, weight and one has 100HP engine and the other 200HP but they are spinning the same prop, would they have identical performance?
I feel like the answer to that is yes.

The correct answer is MOSTLY yes or mostly no. It depends on which engine the prop is designed to match. If the prop matches the 100 HP engine, and both of them are set up for 2700 RPM Max power, the 200 HP will overload the prop and it won't absorb all the extra power available. This will force the operator to run at reduced power to prevent engine damage, and the airframe will accelerate a bit better, but overall performance will be similar.

If the prop is matched to the 200 HP engine, the 100 HP won't be able to get to rated RPM because the engine will be overloaded at low RPM, further reducing power available, and the airframe will be grossly underpowered and may not even takeoff.

The real question is how much HP can the airframe accept? Then match the engine power to that and set up the prop to match engine power and designed airspeed of the airframe.

Hope this helps

 

[AV8R_87]

And you thought she wouldn't notice those 2 inches......

She said those extra two inches made all the difference.

 

[455 Bravo Uniform]

I'm just curious about Rotax engines, and I can't seem to find the answer online. 5,500 - 6,000 rpm is fast enough that I think the engine would have to have an automatic spark advance. Do Rotax engines have that? I -think- some have electronic ignition, so it shouldn't be difficult to incorporate.

I don’t have a Rotax answer, but in go kart racing parlance, the timing on a little Kawasaki KT-100 is fixed and we wring ours out to at least 14,000 rpm.

 

[Checkout_my_Six]

She said those extra two inches made all the difference.

yup....this is what an extra 2 inches looks like.

 

[nauga]

Sooooooo.

If I have 2 Cessna 172s of identical shape, size, weight and one has 100HP engine and the other 200HP but they are spinning the same prop, would they have identical performance?
I feel like the answer to that is yes.

Define "performance." Thrust is produced by the propeller(*), regardless of what is turning it. If the airframes are identical, including weight and balance, for any given flight condition both airplanes can attain, drag will be identical; and if the props are identical, thrust at any given RPM and airspeed will be identical. Soooo...if your definition is based only on speed at RPMs available to the 100hp motor performance will be the same. The throttle setting and fuel flow to get that RPM will be different so range and endurance will be different, and the 200hp airplane will be able to get that RPM at higher altitudes, so performance there will be different.

(*) Spare me the hair-splitting of pressure recovery, exhaust thrust, etc.

Nauga,
and his Ps map

 

[IK04]

The Cheetah is a dirtier airframe than the Tiger. If both airplanes had the same engine and prop and produced the same thrust at a given RPM (e.g. 2500 with a fixed pitch prop), the Tiger would always go faster.

The thrust to drag ratio is different and thus the speed difference. Different engine RPM and prop diameters and produce the exact same thrust.

 

[ArrowFlyer86]

I clearly don't understand power.

If I have 2 planes that are the same airframe but with a different engine.... [Example:] Comanche 180 vs 250.

2700 RPM is 2700 RPM. It isn't as if the 180 can't get to redline and the 250 can.

... both are running the same RPM and carrying the same airframe regardless of HP?

It's probably important to only change 1 variable at a time to see what's going on. Switching or comparing airframes/prop-types/prop sizes/engines/etc and looking at resulting performance conflates multiple potential issues.

I'd consider a really simple case. Just think about flying your Comanche with the current engine/CSP you have installed. Imagine flying at 3500 MSL. Why would you ever fly at 22" MP and 2400 RPM if you could get away with 17" MP and still produce 2400 RPM? If RPMs were the only thing that mattered in terms of thrust (for a given airframe/engine/propeller speed setting/etc), why would you increase throttle/fuel burn to achieve the same result?

 

[Dana]

I don’t have a Rotax answer, but in go kart racing parlance, the timing on a little Kawasaki KT-100 is fixed and we wring ours out to at least 14,000 rpm.

Apropos of nothing, I once owned a Quicksilver ultralight powered by a KT100 engine. I don't remember what the rpm or reduction ratio were, but it spun a tiny little toothpick of a prop.

 

[flyingcheesehead]

Define "performance." Thrust is produced by the propeller(*), regardless of what is turning it. If the airframes are identical, including weight and balance, for any given flight condition both airplanes can attain, drag will be identical; and if the props are identical, thrust at any given RPM and airspeed will be identical.

RPM, airspeed, and prop pitch.

@SixPapaCharlie with the Comanche example, you have a 180hp O-360 vs a 250hp O-540. At the same RPM and MP, the volume of fuel and air that can be combusted is 50% higher on the 540, and that extra power has to go somewhere. The change in diameter from 72" to 74" accounts for a 5.63% difference, the rest of the power will have to be absorbed by running the prop at a higher pitch. Presumably there are some other differences in the props too, such as planform shape and airfoil, that will contribute, but I think the big one is just going to be pitch.

Looking at this further, I pulled up the Type Certificate Data Sheet for the PA24 line. It appears that the PA24-180 can have a Hartzell 70.5"-72.25" diameter prop with a pitch range from 13º to 27º or a McCauley 72-74" with a pitch range from 12.7º to 27.5º, while the PA24-250 can have a Hartzell 76-77.4375" at 14.5-33º pitch or a McCauley 73"-74" at 15.7º-32º.

 

[William Pete Hodges]

RPM, airspeed, and prop pitch.

@SixPapaCharlie with the Comanche example, you have a 180hp O-360 vs a 250hp O-540. At the same RPM and MP, the volume of fuel and air that can be combusted is 50% higher on the 540, and that extra power has to go somewhere. The change in diameter from 72" to 74" accounts for a 5.63% difference, the rest of the power will have to be absorbed by running the prop at a higher pitch. Presumably there are some other differences in the props too, such as planform shape and airfoil, that will contribute, but I think the big one is just going to be pitch.

Looking at this further, I pulled up the Type Certificate Data Sheet for the PA24 line. It appears that the PA24-180 can have a Hartzell 70.5"-72.25" diameter prop with a pitch range from 13º to 27º or a McCauley 72-74" with a pitch range from 12.7º to 27.5º, while the PA24-250 can have a Hartzell 76-77.4375" at 14.5-33º pitch or a McCauley 73"-74" at 15.7º-32º.

The basic job of a propeller is to move air to create thrust. In so doing the propeller is turned by the engine and absorbs its power, and transmits that power into the air. The air pushes back on the propeller, to pull it through the air, providing thrust while the air is being accelerated. In a very real sense the propeller is a lever between the engine and the air.

The problem is air doesn't make a good fulcrum for the lever to push against. Air is flexible and it can only push against itself. So to get any meaningful amount of power we have to move a large amount of it and change its relative speed alot. Add to that problem that air density and its ability to push back changes with altitude, temperature, pressure, and humidity. From sea level to 10,000 feet air loses 40% of its density or weight, and comparing winter to summer air it could lose over 50-60% at the same altitude. Propeller diameter, pitch, blade area, blade plan shape, airfoil shape and variable blade angles are all things about the propeller that change how the propeller interacts with the air. These properties are changing propeller effectiveness and efficiency in relation to the engine and the airspeed as it moves.

It is an oversimplification to say a 2" increase in diameter is the only change between two propellers, especially two propellers designed to absorb 180 HP in one and 250 HP on the other. There are likely other changes too.

 

[flyingron]

It's not the size, it's how you use it.

 

[AV8R_87]

"Oh, honey, it's not the size of the boat that matters, it's the motion of the ocean."

It takes a long time to get to England in a rowboat.

 

[Pinecone]

2 inches might not seem like much, but....

72 inch prop has 4071 square inches of swept area.

74 inch proper has 4300 square inches.

So that 2 inches in diameter is 5.6% greater area. So at the same pitch and same RPM, there should be about 5.6% more thrust.

 

[DJTorrente]

Bear in mind that the extra 2" of prop diameter, or 1" of blade length, are on the outer edge of the prop disk. That's the part of the blade that is moving the fastest at any given rotational speed (RPM). There's obviously more to it than that, primarily prop twist, but the blade tip is moving 4 times as fast as the midpoint of the blade, and that's a lot more speed giving a lot more opportunity to move a lot more air a lot more quickly than might be suggested by the 3% increase in blade length.

See, PHAK, Ch. 7, p. 7-5.

 

[Jon Wilder]

I clearly don't understand power.

If I have 2 planes that are the same airframe but with a different engine. Cirrus 20 vs 22, Grumman Cheetah vs Tiger, Comanche 180 vs 250.
I think those are all examples of the same airframe with a different engine.

2700 RPM is 2700 RPM. It isn't as if the 180 can't get to redline and the 250 can.

If I have an Cheetah and a Tiger, both at 3500 feet, both planes running at 2500 RMP but one is 30 knots faster and the only difference is 74 inches vs 72 on the prop.
I don't get why having more HP, Torque, etc matters if I am not swinging the prop any faster.

It would make a lot more sense to me if the 150 HP plane could only run 2400 RPM and the 180 could run 2700 but that's not the case.
Is it really just the prop size? I get that the higher we go, the more power is needed and the lower power engine can't do as well up there but there is something else at play that I don't understand.

Why would a Tiger beat a Cheetah a couple thousand feet above the ground if both are running the same RPM and carrying the same airframe regardless of HP?

Prop length, pitch, twist...all of this dictates how much air the propeller can process, or "bite". The more air processed per second, the faster you go. However, more air bite = greater engine load. The engine must have the power to pull that propeller to redline RPM.

If you had, say, two propellers, and "Propeller A" processes more air per second at 2700 RPM than "Propeller B", it's going to take an engine with more power to spin Propeller A to 2700 RPM. Throwing numbers out for sake of argument, a 150hp engine may be able to spin Propeller B to 2700 RPM, but can only spin Propeller A to 2300 RPM. Versus a 180hp engine that can spin Propeller A to 2700 RPM (it would probably overspin Propeller B, requiring the pilot to fly it at a reduced power setting, which means less air processed per second and a slower speed).

It's not just the peak horsepower RPM, but the power/torque curve that determines what RPM it will top out at with a given propeller.

Constant speed propeller is a whole different ball of wax as the governor adds more blade when necessary to keep the engine loaded at redline with the governor control to the firewall (there's an actual "set & forget" adjustment for this).

 

[Dan Thomas]

The basic job of a propeller is to move air to create thrust. In so doing the propeller is turned by the engine and absorbs its power, and transmits that power into the air. The air pushes back on the propeller, to pull it through the air, providing thrust while the air is being accelerated. In a very real sense the propeller is a lever between the engine and the air.

The problem is air doesn't make a good fulcrum for the lever to push against. Air is flexible and it can only push against itself. So to get any meaningful amount of power we have to move a large amount of it and change its relative speed alot.

Newton's Third Law:

1. Whenever one object exerts a force on another object, the second object exerts an equal and opposite on the first.

The air doesn't have to be a good fulcrum. Just pushing it back creates a forward force. It comes down to the same old argument about what causes lift: is it Newton or Bernoulli? Newton says it's a reaction as per his third law. Bernoulli says that the propeller is a rotating wing where the pressure on the curved side of the blade (actually called the "back" of the prop) is less that the "face," the flat side we see from the cockpit. They're both right.

One of the things that made the Wright brothers successful was in treating the propeller like a wing. Other designers treated it as some form of ship's screw.

Add to that problem that air density and its ability to push back changes with altitude, temperature, pressure, and humidity. From sea level to 10,000 feet air loses 40% of its density or weight, and comparing winter to summer air it could lose over 50-60% at the same altitude. Propeller diameter, pitch, blade area, blade plan shape, airfoil shape and variable blade angles are all things about the propeller that change how the propeller interacts with the air. These properties are changing propeller effectiveness and efficiency in relation to the engine and the airspeed as it moves.

Yes the density falls with altitude, and it affects the prop, the engine (which gets less air for combustion) and the airframe, which experiences much less drag so that less power is needed. That decreased drag is why airliners go so high.

 

[midwestpa24]

I have to say POA, I'm impressed with how well this thread stayed on track given 6PC's thread's title. I'm not sure whether to be proud or disappointed, both!

 

[Checkout_my_Six]

size does matter.