Strange Question about thrust
 

I have been debating this with myself since i started working with these little engines but have never asked.

If, for example, there is a engine with 1 horsepower turning a 14-4 prop 10,500 rpm WOT and another engine with 2 horsepower turning a 14-4 prop 10,500 rpms WOT wich one is producing more thrust, or is it the same?

RE: Strange Question about thrust
 

The static thrust will be the same with both props

RE: Strange Question about thrust
 

Cameron,


There is one thing basically wrong with your question; manufacturers' claimed horsepower numbers don't make any thrust.
Not even one ounce; not a gram in fact...

Using my trusty Reivers PropPower calculator, corrected for APC props, 1 HP is not enough to spin a standard 14x4 at 10,500 RPM.
The amount of output needed is 1.104 HP.

With an actual 2 HP, this prop will spin at 12,769 RPM; not just 10,500.

Horsepower is a product of torque and RPM and represents the amount of work (the physical definition - a force applied over a distance) an engine can do, per unit of time. Actually, one horsepower it is 33,000 lbs. ft. per minute.

When output is in HP and torque is in lbs. ft.; HP is equal to torque multiplied by RPM and divided by 5,252.1.
This is why torque and HP curves that are advertised for cars, always intersect at 5,252.1 RPM.

For an engine, since torque varies and RPM varies, the HP output has different values at different RPM levels.

What is supposed to be quoted is the peak HP. This isn't always true, for various causes...


As to thrust; a given prop, at a given RPM, in given atmospheric conditions, will always make the same thrust, but static thrust is not possible to accurately calculate from HP.

RE: Strange Question about thrust
 

In order for the propeller to produce thrust it must be rotating which takes work per unit time, i.e., power. How fast the propeller will turn depends how much torque the engine produces as a function of RPM.
I often read the statement that "it is not power that produces thrust, but torque". That statement is rather messed upp. It becomes clear if one consider a propeller mounted to an electric motor. If we apply current to the motor the propeller is subject to a torque tending to rotate the propeller. If we hold the propeller stationary torque is maximized, but of course no useful thrust is produced. It is not until we let the propeller rotate that it can perform work on the air surrounding it. This process absorbs power (work per unit time) from the motor.

BTW in order to avoid confusion, gram is unit for mass, not a force unit. In the SI system Newton is the force unit and in Imperial units lbf is often used :-)!

/Red B.

RE: Strange Question about thrust
 

all else being equal the 2hp engine should spool up twice as fast.

RE: Strange Question about thrust
 

The question is which engine is producing more thrust. The answer is neither. Thrust is produced when the prop converts the rotational energy of the engine into lift (forward, but still lift) along the prop blade. The total thrust produced is the integral of the lift function along the entire length of the prop. Therefore, it is easy to see that at a given rotational speed, it is the prop diameter that determines total thrust. Increasing the engine power will result in a higher peak RPM for the prop, which means more thrust will be produced as speed increases.

Another point to be made is the diffference between thrust and speed. Maximum thrust is produced with the airplane not moving through the air. As the speed of the plane increases, drag increases along the wing and air frame. At the same time, due to the relative motion between the prop and the air stream, thrust is reduced. At some point, where the drag on the airframe equals the thrust produced by the prop, is the constant airspeed of the plane (see image below for a graphical representation)

Now in the real world, we don't change engines. We change props. For a given engine, increasing the prop diameter for the same pitch increases the load on the engine, which results in a lower peak RPM. Depending on where your particular setup is in relation to the engines power curve, this may or may not result in an increase in thrust. If you reduce the pitch of the same diameter prop, the engine RPM will increase. This will generally result in an increase in the static thrust of the prop, but will probably result in a lower top speed of the plane, again depending on how much the RPM actually increases. Normally, the objective is to keep the prop loading at some optimum value (I normally prop for a peak RPM on the ground between 12,000 and 13,000 rpm) If I need more thrust (better vertical, quicker takeoffs), it will increase the prop diameter by 1 inch and reduce pitch by 1 or 2. If I want more speed, I'll decrease prop diameter by 1 and increase pitch by 1 or 2.

Brad

RE: Strange Question about thrust
 

Wow! Mechanical Engineering 101 explained in detail for the novice. What a web site. What an outstanding international group of enthusiasts.

Cheers,

Chip

RE: Strange Question about thrust
 

Red,


Yes, a gram is a unit of mass, as is a kilogram.

But it is customary to use kilograms to describe thrust of jet engines, like kilo-Newtons are used, as it is to amount torque in kg-meters and not only in Newton meters.

A force of 1 Newton will accelerate a mass of 1 kg at 1 meter per second squared...

RE: Strange Question about thrust
 

Cambo
What you proposed isn't really possible in the way you've worded the question. You can't have two engines running at WOT and developing completely different HP when peaked out at the same revs on the same prop. With the same prop at the same revs they must be developing the same HP.

It's possible though to have an engine that could potentially develop 2HP only drive that prop at the same revs either by throttling back (in which case it's no longer at WOT) or by severely overpropping it way down in it's torque curve so it wasn't able to turn that prop any faster. It would then be making the same HP as the other engine and the prop would have exactly the same thrust.

RE: Strange Question about thrust
 

Been a while since I did any engineering in the Metric system, but I thought grams was weight not mass. Newtons is mass. Just as pounds is weight but slugs is mass in the English system.

RE: Strange Question about thrust
 

I suppose it could be the same engine turning two brands of props of the same size and pitch, or the same brand and size but with a wide blade prop. A wider blade will push more air and be more efficient at lower speeds with more thrust. They would also take both more HP and thrust at any given speed. But I doubt you could make a prop wide enough to absorb twice the power.

RE: Strange Question about thrust
 

Sport Pilot;

Newtons (N): force (and weight which is the force of gravity)
kilograms (kg): mass

When weight is "measured" in grams/kilograms (or ounces/pounds), it is a way of simplifying things, although it is wrong. The mass of one kilogram will have a gravity of roughly 9.80665 Newtons. It has become a "bad habit" to say that the weight is 1 kg. The weight is 9.80665 N, and the mass is 1 kg.

RE: Strange Question about thrust
 

Force is newtons mass is Kg's. Torque is N.m, Force x distance. Power is Torque x rpm x Pi /60 or work done / time.

RE: Strange Question about thrust
 

Rupert, Load on a prop goes up by the sqaure of the speed because you are incrasing the static thrust as well the velocity it is being generated at. So to double the rpm of a given prop, you would need four times the power. also drag ie friction goes up by the square of the speed as well.

RE: Strange Question about thrust
 

Okay, I think i am getting it know
I understand the numbers probabley didn't exist but i was comparing something like a 70 4-stroke to a 90 2-stroke. 70's produce around 1 horsepower if i am not mistaken and a .90 is above 2. Arn't most 90's designed around 14 inch props. I don't have a 90 and could be wrong but am concidering one for my next plane.

Again, thanks for clearing this up for me. I almost feel embaressed asking because i have been running these little engines for some time now.

RE: Strange Question about thrust
 

There's nothing to be embarassed about. Just because you use something doen't necessarily mean you have to know how it all works. I'm typing this on a computer but for all I know there's probably a little green imp with an abacus inside it doing all the work :D

RE: Strange Question about thrust
 

Lyn,


You have to get your terms straight...

Are you referring to 'power' as in horsepower, which is a unit of output, or as in torque, which is force?

If you are referring to torque, you are correct.
Since air resistance and thus prop drag, rises as a function of the air-speed squared, spinning a prop twice as fast would require four times the torque.

If you are referring to horsepower, than you are wrong, since eight (8) times the horsepower would be required to spin the same prop twice as fast... The required torque is four times greater and this force must be applied over twice the distance per unit of time, so the required work per time unit is eight times greater at double the RPM.

At three times the RPM it is 27 times greater and at four times the RPM, 96 times greater.

RE: Strange Question about thrust
 

:D

RE: Strange Question about thrust
 

Another question on topic
Why can a 4c turn a larger prop without rpm loss if they have less horsepower than an equivlent size 2 stroke?
I understand that have a better torque curve, but why?

RE: Strange Question about thrust
 

Cameron,


With a reasonably sized prop, this simply isn't true.

If you take a .91 sport two-stroke engine and a .91 (non-supercharged) four-stroke engine and put a 14x6 APC on both, the two-stroke engine will have an advantage.

If you use a much larger prop that will load the two-stroke engine, to significantly below its peak torque RPM and the four-stroke will still be at, or above its own peak torque, the two-stroke engine could be scavenging so inefficiently that the four-stroke engine will make more torque and spin that big prop a bit faster.


It is all in the timing. If you port and time the two-stroke for low RPM, it will retain an advantage to low RPM...

RE: Strange Question about thrust
 

low@slow What I meant, and should have said was it will accelerate to the same speed twice as fast.

RE: Strange Question about thrust
 

Actually both are used for both force and mass. However Newtons and Slugs are simply adjusted for the force of gravity. 1 Newton is equal to 9.8 Kg, and one Slug is 32.2 pounds.

RE: Strange Question about thrust
 

Actually 9.81N is the force a mass of 1Kg exerts on earth. Force is a vector it has magnitude and direction, mass on the other hand is a scalar and only has magnitude. They are not actually interchangeable but people do anyway

RE: Strange Question about thrust
 

Hugh,


I think you got this reversed... a bit less than 10 Newtons is one kg.

RE: Strange Question about thrust
 

Hugh,


I think you got this reversed... A bit less than 10 Newtons is one kg.