Exhaust Thrust

Shortround6 · Apr 13, 2018

Zipper730 said:
Intuitively that makes perfect sense, but compressibility effects were known since the 1700-1800's on some level, but rarely factored into early aircraft designs because they flew nowhere near fast enough to cause trouble (except the props).

I'm wondering if something like that applied to attempting to extract exhaust thrust from the engine system, or at the very least not really pursuing it in any degree because of the fact that

Most early engines didn't produce all that much horsepower, so the amount of thrust to be gained by a creative exhaust pipe set-up wouldn't have been worth it

Most early engines didn't have particularly high manifold pressures, so it would have interfered with scavenging

Most early engines were naturally aspirated or used minimal supercharging, and were not capable of propelling an aircraft high enough to make use of it

Most early aircraft were not capable of flying fast enough to make use of such a thing

You pretty much have it, go back to your chart on hp and Look at the 100-200mph range, then figure that superchargers were actually pretty rare until the early 1930s so cylinder pressure was low and that 500-600hp for a an engine was pretty hot stuff, racing engines aside.

Okay
I'm confused about something. Some things increase to the square of velocity, others are proportional.

I keep mixing things up

mass times velocity gives you momentum, Mass times velocity squared gives you energy.

we can break it down a bit by looking at a rifle. Bullet weight times velocity squared gives us the energy of the bullet (it's ability to do work, like punch through an obstacle) but when figuring recoil we take the weight of the bullet times velocity (NOT squared) plus the weight of the propellent times the escape velocity of the gas at the muzzle and then divide by the weight of the rifle to give us the velocity of the rifle moving backwards. Then we use the mass/weight of the rifle times it's recoil velocity squared to get the recoil energy.

Drag gets a bit stranger

Zipper730 · Apr 13, 2018

swampyankee said:
One way to sort out whether velocity is squared or not is to sort out the units.

Mass flow = mass/time; Velocity = distance/time; multiplied mass-distance/time^2?

Work and energy are force times distance (torque is a vector, and torque and energy are not interchangeable!); both have the units of joules, or (mass times [distance squared]) divided by (time squared). It's measured in joules. Power is energy per unit time, so its units are joules per second, or watts. Dimensional analysis is a useful technical skill.

What's a vector? As for joules, you could call it a newton-second right?

A newton is (kg)/(m/s^2), so a joule is [(kg)/(m/s^2)]*(second), which is kg*s^2/m (provided I can take the reciprocal, reverse it and multiply it)

Shortround6 said:
You pretty much have it, go back to your chart on hp and Look at the 100-200 mph range, then figure that superchargers were actually pretty rare until the early 1930s so cylinder pressure was low and that 500-600hp for a an engine was pretty hot stuff, racing engines aside.

Provided you have a top-speed of 150 mph, 500-600 horsepower, and 2.5 pounds of thrust per horsepower from the propeller, that comes out to a propeller thrust of 1250-1500 pounds, an exhaust thrust of 50-66 pounds?

Bullet weight times velocity squared gives us the energy of the bullet

Yup

when figuring recoil we take the weight of the bullet times velocity (NOT squared) plus the weight of the propellent times the escape velocity of the gas at the muzzle and then divide by the weight of the rifle to give us the velocity of the rifle moving backwards. Then we use the mass/weight of the rifle times it's recoil velocity squared to get the recoil energy.

So you factor the force of the bullet shoving the rifle back and then the mass of the rifle times the velocity it goes flying back squared

Shortround6 · Apr 13, 2018

Zipper730 said:
Provided you have a top-speed of 150 mph, 500-600 horsepower, and 2.5 pounds of thrust per horsepower from the propeller, that comes out to a propeller thrust of 1250-1500 pounds, an exhaust thrust of 50-66 pounds?

Probably not even that. The Merlin was making 1362hp at the crankshaft (but not including friction) but 236 hp was being used to drive the supercharger so while 1126hp went to the prop you had the air/fuel mass needed for 1460-1500hp (including friction. rough guess) going out the exhaust.

1920s engine without supercharger only made 500-600hp near sea level. It was smaller in displacement (mostly) and turned fewer rpm. Approximately 80% of the friction on an engine comes from the pistons/piston rings and friction goes up with the square of the speed. A 2400 rpm engine has about 64% of the friction of a 3000rpm engine with everything else being equal (cylinder sizes, bearing sizes, etc).
With no supercharger such an engine might only be using 1/3 of the fuel/air a 1100hp engine is at 20,000ft

It is certainly going have a much different exhaust thrust ratio ( lower compression plus no boost in the intake fighting higher back pressure if flying lower) than a 1940 engine.

They were much more interested in keeping the exhaust out of the cockpit than getting thrust from it.
They also found that some of the engines tended to warp valves using short stacks. Low rpm engines had a longer time between exhaust pulses and cold air could be drawn in hitting the valve stem between exhaust pulses. 1920s valves were not what 1940s valves were nor was coolingo f the whole exhaust valve area.

swampyankee · Apr 13, 2018

Joules are newton-meters, that is kg-m^2/s^2. Foot-pounds is also a unit of energy. Confusingly, torque is also measured in foot-pounds, which is part of why I think there is an inane argument about torque vs power: people conflate energy and power all the time, and the units make it easy to conflate energy and torque.

A vector is a quantity with an associated direction: going 4 meters north is a vector, as there's a quantity (4 meters) with a direction (north). Everybody uses them all the time.

Recoil is conservation of momentum: the mass times velocity of the bullet (plus the mass times velocity of the gun gas) is equal and opposite to the mass times velocity of the recoiling gun; this is one of the basic laws of physics, the law of conservation of momentum. A ballistic pendulum, which used to be used to measure muzzle velocity, relies on this law and the law of conservation of energy. That energy is (1/2)mv^2 and momentum is mv is why a high-velocity, light-weight bullet may produce less recoil than a heavy, slow bullet with the same energy: if you cut the mass in half and double the velocity, the momentum (recoil) is the same but the muzzle energy is twice as much.

wuzak · Apr 13, 2018

Zipper730 said:
A newton is (kg)/(m/s^2), so a joule is [(kg)/(m/s^2)]*(second), which is kg*s^2/m (provided I can take the reciprocal, reverse it and multiply it)

wtf!

F = m * a
N = kg*m/s^2

Work (energy) = F * displacement
J = N * m = kg*m/s^2*m = kg*m^2/s^2

wuzak · Apr 13, 2018

swampyankee said:
A vector is a quantity with an associated direction: going 4 meters north is a vector, as there's a quantity (4 meters) with a direction (north). Everybody uses them all the time.

Velocity is a vector.
Speed is a scalar (quantity without direction).

wuzak · Apr 13, 2018

swampyankee said:
Joules are newton-meters, that is kg-m^2/s^2. Foot-pounds is also a unit of energy. Confusingly, torque is also measured in foot-pounds, which is part of why I think there is an inane argument about torque vs power: people conflate energy and power all the time, and the units make it easy to conflate energy and torque.

In SI units energy is J and torque is Nm. While they are dimensionally the same, they are not. Completely different concepts.

Torque is the rotational equivalent of force.
Power = Torque * angular velocity

is equivalent to
Power = Force * Velocity

Edit: Isn't one foot-pounds and the other pound-feet?

wuzak · Apr 13, 2018

Indeed:

Foot-pounds and pound-feet—what's the difference?

foot-pound is unit of work.
pound-foot is unit of torque.

Zipper730 · Apr 22, 2018

wuzak said:
wtf!

when dividing a fraction, you could treat it as multiplying the reciprocal, so kg/(m/s^2) could be treated as kg(s^2/m) though I might very well have screwed some things up as that looks like it'd come out to kg*s^2 / kg/m

Velocity is a vector.

A distance covered over a period of time in a given direction?

Speed is a scalar (quantity without direction).

Velocity without a specified direction?

pbehn · Apr 22, 2018

Zipper730 said:
when dividing a fraction, you could treat it as multiplying the reciprocal, so kg/(m/s^2) could be treated as kg(s^2/m) though I might very well have screwed some things up as that looks like it'd come out to kg*s^2 / kg/m
A distance covered over a period of time in a given direction?
Velocity without a specified direction?

I am getting a bit nervous about flying this plane you are building Zipper, do you know any other pilots?

Zipper730 · Apr 22, 2018

Fortunately, I'm not planning on building any planes

XBe02Drvr · Apr 22, 2018

Zipper730 said:
A distance covered over a period of time in a given direction?

Not quite. This sounds like nitpickin' semantics, but think of it as a linear rate of change of position in a specific direction. It's a rate, not a distance.
Cheers,
Wes

pbehn · Apr 22, 2018

XBe02Drvr said:
Not quite. This sounds like nitpickin' semantics, but think of it as a linear rate of change of position in a specific direction. It's a rate, not a distance.
Cheers,
Wes

That is the difference between theoretical mathematics and real life situations. In college I had lots of theoretical problems concerning speed, velocity etc. Almost all of them concerned a constant speed or velocity or a change in accordance with another mathematical formula. In life of course very few things behave so uniformly. I remember several problems regarding bullets, but this was always as if in a vacuum, only gravity was considered not air resistance.

A police speed trap using time between distance may record an average speed of 70MPH which can mean the car travelled at a constant 70MPH ......or went for a 180MPH blast then stopped for a cigarette.

XBe02Drvr · Apr 22, 2018

pbehn said:
A police speed trap using time between distance may record an average speed of 70MPH which can mean the car travelled at a constant 70MPH ......or went for a 180MPH blast then stopped for a cigarette.

Ah, the wondrous VASCAR speed trap system! Boy does that bring back memories. Only works well if you can hide the cruiser or if you have an aircraft. In the Keys it was hard to find a place to hide a cruiser, and they didn't have aircraft, so as soon as you spot the cruiser, get on the brakes and they can't touch you. Years: 4, Tickets: Zero.
Cheers,
Wes

Exhaust Thrust

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