Force or Torque in Engine (1 Viewer)

[Ted griffin]

Hello all
Greetings
I have a question but didn't find a clear answer yet about engine HP for all old aircraft. As known, most of the engine are more than 1,000 hp.
My question is, do we need this high HP to get high RPM ? or force? Or torque?
If yes, what is the startup ideal RPM for the propeller ? I have heard abou 1.3 RPM which is low when we camper it with can engine.
If the the high HP is to convert the force into torque, in this case, mechanically the speed will be reduced as you know.
Please advise me and sorry if i confused you

 

[cvairwerks]

You need both the HP and the RPM's to get the prop to the desired speed to achieve the needed thrust. The actual HP and RPM's required will vary based on the prop diameter, the particular blade profile, blade pitch angle and the tip design. Ideally, the RPM's are set to keep the blade tip speed below supersonic for efficiency and noise control.
A T-6 on takeoff will have the prop tips well into the supersonic region, but the pilot reduces the blade pitch and the needed RPM's very quickly after takeoff, as not as much power is need to drive the prop at that point. It's a huge balancing game with some black art to it.

 

[Ted griffin]

You need both the HP and the RPM's to get the prop to the desired speed to achieve the needed thrust. The actual HP and RPM's required will vary based on the prop diameter, the particular blade profile, blade pitch angle and the tip design. Ideally, the RPM's are set to keep the blade tip speed below supersonic for efficiency and noise control.
A T-6 on takeoff will have the prop tips well into the supersonic region, but the pilot reduces the blade pitch and the needed RPM's very quickly after takeoff, as not as much power is need to drive the prop at that point. It's a huge balancing game with some black art to it.

Really worth and rich explaining
Additional concerned, some ww2 aircraft has transmission or gearshift and other the connected directly to the prop such as in radial engine, does that related to torque convertion or just shift the engine crankshaft motion?
From your info, what is the max RPM allowed for P-51 mustang or A6M zero aircraft or any other good example you knwo

Also, when i camper, the hp for A6M zero is 940 hp from Nakajima Sakae 14 cylinder engine with 554 k/m as speed and Cessna 172 i see with 160 hp and 4 cylinder only and 226 km as speed
I am wondering, does this high hp was required at that time due to the limited of technology in other words, can i make by today same P-51 mustang or A6M zero aircraft with same performance with lower hp engine?

 

[Michael Hope]

I do not know of any engine with a transmission to shift gears between the engine and the propeller. Yes there were reduction gear boxes to run the propeller at a lower RPM than the engine was turning.

 

[GregP]

Horsepower = (Torque x rpm) / 5252.

Torque is higher below 5,252 rpm, and horsepower is higher above 5,252 rpm. So, diesels are usually oriented toward torque and petrol engine are usually oriented toward horsepower. Going back in forth is easy. At 5,252 rpm, torque = horsepower.

Torque in pound-feet.
rpm = revolutions per minute.
hp = 550 foot-pounds / second.
1 horsepower = 745.7 Watts.

Conversions are easy in Google.

 

[MiTasol]

Also, when i camper, the hp for A6M zero is 940 hp from Nakajima Sakae 14 cylinder engine with 554 k/m as speed and Cessna 172 i see with 160 hp and 4 cylinder only and 226 km as speed
I am wondering, does this high hp was required at that time due to the limited of technology in other words, can i make by today same P-51 mustang or A6M zero aircraft with same performance with lower hp engine?

Every different shape of aircraft has different power requirements to reach the same speed and drag increases roughly on the square of the speed.

As a VERY basic rule of thumb you therefore have to square the power to increase speed so to double the speed you need 4 times the power and to triple the speed you need 9 times the power.

Naturally when you double the speed you must also increase the strength of the aircraft or it will break and that makes it heavier which makes it slower. Also your 4x more power engine weighs about 4 times as much and that also slows you down.

 

[Jugman]

As a VERY basic rule of thumb you therefore have to square the power to increase speed so to double the speed you need 4 times the power and to triple the speed you need 9 times the power.

Naturally when you double the speed you must also increase the strength of the aircraft or it will break and that makes it heavier which makes it slower. Also your 4x more power engine weighs about 4 times as much and that also slows you down.

Drag goes up with the square of speed. Power goes up by the cube of speed. Remember you're traveling twice the distance in the same amount of time.

 

[PStickney]

You need both the HP and the RPM's to get the prop to the desired speed to achieve the needed thrust. The actual HP and RPM's required will vary based on the prop diameter, the particular blade profile, blade pitch angle and the tip design. Ideally, the RPM's are set to keep the blade tip speed below supersonic for efficiency and noise control.
A T-6 on takeoff will have the prop tips well into the supersonic region, but the pilot reduces the blade pitch and the needed RPM's very quickly after takeoff, as not as much power is need to drive the prop at that point. It's a huge balancing game with some black art to it.

It's a bit backward, up there. Horsepower is the product of (Torque x RPM) / Some constant to square up the units.
So, for a given torque output, the higher the RPM, the higher the Power, and Vice versa.
Uhm, on your T-6 example, and for any Constant Speed / Controllable Pitch propeller combination -
Manifold Pressure (Throttle) controls Torque.
Propeller Pitch, particularly with a Constant Speed prop controls RPM.

For the T-6 specifically -
Takeoff Power is 36" Manifold Pressure, and 2250 RPM, developing 600 HP.
After takeoff, you's pull back to Normal Power for the climb out - 32.5" and 2200 RPM. developing about 550 HP.
Managing Power is a bit of a Finger Dance. When increasing power, you advance the fuel/air mixture (If necessary) -> Rich Mixture; Propeller to desired RPM; then Advance Throttle to desired Manifold Pressure.
Power Reduction is the reverse - Pull back the Throttle to the new Manifold Pressure, Prop to new RPM, then Mixture as required.
Note that with a controllable pitch propeller, increasing pitch lowers RPM, decreasing pitch increases RPM.
Fun T-6 note - with the Direct Drive R1340, on a Hot Florida Day, A T-6's prop tips aren't quite supersonic, but as it gains speed on takeoff roll, you can hear the transition as the tips go transonic, since the propeller tip speed is the sum of rotational speed and airspeed.

 

[Michael Hope]

It's a bit backward, up there. Horsepower is the product of (Torque x RPM) / Some constant to square up the units.
So, for a given torque output, the higher the RPM, the higher the Power, and Vice versa.
Uhm, on your T-6 example, and for any Constant Speed / Controllable Pitch propeller combination -
Manifold Pressure (Throttle) controls Torque.
Propeller Pitch, particularly with a Constant Speed prop controls RPM.

For the T-6 specifically -
Takeoff Power is 36" Manifold Pressure, and 2250 RPM, developing 600 HP.
After takeoff, you's pull back to Normal Power for the climb out - 32.5" and 2200 RPM. developing about 550 HP.
Managing Power is a bit of a Finger Dance. When increasing power, you advance the fuel/air mixture (If necessary) -> Rich Mixture; Propeller to desired RPM; then Advance Throttle to desired Manifold Pressure.
Power Reduction is the reverse - Pull back the Throttle to the new Manifold Pressure, Prop to new RPM, then Mixture as required.
Note that with a controllable pitch propeller, increasing pitch lowers RPM, decreasing pitch increases RPM.
Fun T-6 note - with the Direct Drive R1340, on a Hot Florida Day, A T-6's prop tips aren't quite supersonic, but as it gains speed on takeoff roll, you can hear the transition as the tips go transonic, since the propeller tip speed is the sum of rotational speed and airspeed.

The standard length blade on a T-6 is a basic 9 feet in diameter, plus 7/16 inch for the 12D40 hub.

 

[PStickney]

I do not know of any engine with a transmission to shift gears between the engine and the propeller. Yes there were reduction gear boxes to run the propeller at a lower RPM than the engine was turning.

Just after the War, there was some work done on dual-speed Propeller Reduction Gearboxes. The idea was that they'd allow propellers to be run more efficiently in different sections of the flight envelope - for example, Takeoff and Initial Climb, and Cruise at 40,000'. Some prototypes were built - the one that comes to mind was an R4360 variant for the B-36. In the end, the slight gains that could be obtained just weren't worth the complexity, weight, and inherent unreliability that comes with increasing the parts count.

 

[PStickney]

The standard length blade on a T-6 is a basic 9 feet in diameter, plus 7/16 inch for the 12D40 hub.

Yep. There are some T-6s around with geared engines - the difference in noise level, and noise quality is striking.

 

[PStickney]

Really worth and rich explaining
Additional concerned, some ww2 aircraft has transmission or gearshift and other the connected directly to the prop such as in radial engine, does that related to torque convertion or just shift the engine crankshaft motion?
From your info, what is the max RPM allowed for P-51 mustang or A6M zero aircraft or any other good example you knwo

Also, when i camper, the hp for A6M zero is 940 hp from Nakajima Sakae 14 cylinder engine with 554 k/m as speed and Cessna 172 i see with 160 hp and 4 cylinder only and 226 km as speed
I am wondering, does this high hp was required at that time due to the limited of technology in other words, can i make by today same P-51 mustang or A6M zero aircraft with same performance with lower hp engine?

It's not a gearshift or torque converter - it's a fixed reduction gear to slow the rotational speed of the propeller to allow it to produce maximum thrust, using the Power Available, Tis usually requires a large diameter propeller, which at high rotational and forward speeds (They add together) can quickly get the outer portions of the propeller into the transonic and supersonic speed ranges, which take a big bite in their efficiency. Reduction Gearing between the engine crankshaft and the propeller shaft gives a slower tip speed, delaying this problem, and the propeller pitch is designed to operate most efficiently at the lower RPMs.

 

[PStickney]

Really worth and rich explaining
Additional concerned, some ww2 aircraft has transmission or gearshift and other the connected directly to the prop such as in radial engine, does that related to torque convertion or just shift the engine crankshaft motion?
From your info, what is the max RPM allowed for P-51 mustang or A6M zero aircraft or any other good example you knwo

Also, when i camper, the hp for A6M zero is 940 hp from Nakajima Sakae 14 cylinder engine with 554 k/m as speed and Cessna 172 i see with 160 hp and 4 cylinder only and 226 km as speed
I am wondering, does this high hp was required at that time due to the limited of technology in other words, can i make by today same P-51 mustang or A6M zero aircraft with same performance with lower hp engine?

The quick answer is "Not Really". The more accurate answer is "That Depends" - which is the True Answer to all Aeronautical Questions - everything depends on context.
To explain it better, we need to start by looking at the relationship between Engine Horsepower and Propeller Thrust. To save typing, I'll paste in a snippet from a piece I did on Usenet (rec,aviation.military) many years ago:

Thrust: How much force is being exerted by the aircraft's powerplant to push it through the air.
This is a toughie. Reciprocating engines don't produce thrust directly, they produce power. (Which is defined as Torque * rotational Speed - don't sweat that). Power doesn't directly translate into thrust, so we'll have to do a bit of arithmetic:
1 HP = 550 ft-lb/sec. Now, thrust is lbf (pounds force), since we're on Earth, we can safely assume 1G - 32.2 ft/sec^2 and not sweat it) So, to get lbs out of a horsepower number, we divide by 550 ft/sec. (Hey ft/sec, that's speed!) so, if we do a bit more figuring, to get the speed part down, we end up with T (Thrust in lbs) = HP * 550/v (v in ft/sec). As you can see, at low speeds, we get bags of
thrust per horsepower. At high speeds, the thrust decreases.
Here's another table that shows this: (remember that 550 ft/sec = 375 mph)
Speed Thrust (1 HP) HP (1# of thrust)
100 mph 3.75# 0.266
200 mph 1.88# 0.533
300 mph 1.25# 0.800
400 mph 0.94# 1.067
500 mph 0.75# 1.333

As you can see, as speed goes up, thrust drops off alarmingly. But, of course, there's more to it. A reciprocating engine generates its thrust by turning a propeller, which is basically a set of rotating wings, which turn torque into thrust by moving a large volume of air from in front of the propeller disk to behind it. This of course, isn't 100% efficient. While the propeller can be considered a set of wings, it moves through a complicated airmass. The airspeed that a propeller's airfoil sees is defined by the rotational speed of the propeller, and by the forward motion of the airplane. There are also, of course, altitude effects. An airplane propeller, like a wing, stalls at low speeds, and has transonic problems at higher airspeeds. The efficiency of a propeller isn't onstant. At low speeds, it can be rather poor, and it drops off at high speeds. The altitude effects also mean that a given propeller setting is only most efficient at a particular combination of Torque, RPM, airspeed, and altitude. This led to problems in the 1930s, when airplanes with wide speed ranges were beginning to be developed, and supercharged engines, which produced their best power at higher altitudes, were introduced. As an example, the Boeing Monomail transport prototype, with a supercharged Pratt & Whitney Hornet engine, couldn't take off with its propeller set for the cruising design point of the airframe/engine combination -
the propeller efficiency was too low. When the propeller was set for takeoff performance, there was a hefty hit on cruise speed. This was resolved by producing variable pitch and constant speed propellers. Basically, the pitch change allows the peak efficiency to be maintained over a wide combination of engine power/ airspeed and altitude combinations. What this means for this analysis is that the
thrust produced for a particular horsepower isn't dependant on altitude.

Oh, yeah, there's one other factor as well. Because the combination of airplane airspeed and the propeller's rotational speed can get quite high, there's a loss of efficency as the propeller's blades approach the speed of sound. To get past that, the propeller shaft is geared down to keep the total speed low. The Mustang's V1650 engine had a gear ratio of 0.479. For every 1000 engine RPMs, the propeller turned
479.

The efficency of an airplane propeller is best referenced by the Advance Ratio, or 'J'. 'J' is defined as J = V(true airspeed) / n(rotational speed)* d (diameter). For a typical WW 2 fighter airplane propeller, the highest efficiencies are reached at Js between 1.5 and 3.5.

So, as we can see, as speed increases, partularly beyon 300 MPH, Available thrust drops off sharply. As other posters have noted, when you push an airplane through the air more quickly, the drag increases by the square of the speed - so, to double the speed, you need 4 times the thrust, triple it, you need 6 times - all things being equal.
But they aren't.
The airflow over the wing of an airplane increases in speed as it flows over the wing's surface. At higher speeds, the eccelerated airflow starts reaching the Speed of Sound. (Mach 1) When you reach that point, Shock Waves form over wings and tail (Their airfoils, too), and Drag increases sharply - as in a nearly vertical climb to over 10x the subsonic drag in the span of about 30 mph - 50 or so km / h. At the speed and altitude range of a late World War 2 fighter, that's roughly 500 mph True Airspeed. There's a reason why the "Superprops", like the P-51H, or the Spiteful, or whatever, had to put in a lot of effort, and a _LOT_ of Horsepower for a small increase in level flight performance.
Jets, on the other hand, are esentially Constant Thrust - which means the faster they go, the more Power they generate. That's why everybody essentially halted Piston Engined fighter development in the later part of 1944, (The airplanes developed around that time wree basically available at, or just after, the War's end) and all the attention, and most of the engine development, went into Jet Engines and Jets.
We have made some improvements in reducing drag in General Aviation airplanes - it's easier to produce optimized seamless shapes with modern composites, but not as much as you'd think. There are some small useful gains between, say, 120 and 300 mph.

 

[SaparotRob]

It's not a gearshift or torque converter - it's a fixed reduction gear to slow the rotational speed of the propeller to allow it to produce maximum thrust, using the Power Available, Tis usually requires a large diameter propeller, which at high rotational and forward speeds (They add together) can quickly get the outer portions of the propeller into the transonic and supersonic speed ranges, which take a big bite in their efficiency. Reduction Gearing between the engine crankshaft and the propeller shaft gives a slower tip speed, delaying this problem, and the propeller pitch is designed to operate most efficiently at the lower RPMs.