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What determines which prop you use????


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I was wondering ...

 

If you were building a plane and intended to use an unusual engine for a plane, hypothetically, a 1000 cc 4-cylinder engine from a Morris Minor, how would you determine the pitch and diameter of the propeller you could attache to it?

 

 

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That would depend on direct or reduction drive. If direct you would have about 30 hp at say 2800rpm similar to a Rotax 377 at the flange, had a 60x34 on my Thruster single. People like Bolly would have a suitable ground adjustable prop.

 

 

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I was wondering ...

 

If you were building a plane and intended to use an unusual engine for a plane, hypothetically, a 1000 cc 4-cylinder engine from a Morris Minor, how would you determine the pitch and diameter of the propeller you could attache to it?

 

I reckon I’d pitch it as far as I could!

 

loved my MM and Mini but definitely not suitable for aviation...all revs, no grunt, too heavy, ....better off with a VW or other boxer motor.

 

 

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I would have thought that you need to get it to rev high enough to produce max power. Tip speed needs to be lower than 0.8 of the sped of sound.

 

 

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Tip speed needs to be lower than 0.8 of the sped of sound.

 

Speed of sound @ 20C = 343 m/s

 

0.8 x 343 = 274.4 m/s

 

For the sake of easy arithmetic, let RPM = 2400

 

2400 RPM = 2400/60 revolutions per second = 40 revolutions per second

 

What is the maximum blade length so that Tip Speed = 274.4 m/s?

 

Diameter to a circle - pi x Diameter

 

Therefore  pi x D x 40 = 274.4

 

D = 274.4/(40 pi)

 

D = 274.4 / 125.66

 

D = 2.18 metres

 

So, the length of the propeller blade has to be less than 2.18 metres.

 

 

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Speed of sound @ 20C = 343 m/s

 

0.8 x 343 = 274.4 m/s

 

For the sake of easy arithmetic, let RPM = 2400

 

2400 RPM = 2400/60 revolutions per second = 40 revolutions per second

 

What is the maximum blade length so that Tip Speed = 274.4 m/s?

 

Diameter to a circle - pi x Diameter

 

Therefore  pi x D x 40 = 274.4

 

D = 274.4/(40 pi)

 

D = 274.4 / 125.66

 

D = 2.18 metres

 

So, the length of the propeller blade has to be less than 2.18 metres.

 

Yes, but only if you are stationary. You will need to factor in forward speed as well. Culver Props has an online calculator. I'm sure you could find others if you look...

 

http://www.culverprops.com/pitchselection.htm

 

 

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Yes, but only if you are stationary. You will need to factor in forward speed as well. Culver Props has an online calculator. I'm sure you could find others if you look...

 

http://www.culverprops.com/pitchselection.htm

 

 

 

I knew there'd be an app for that?

 

 

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Wooden props need an even lower max tip speed as the blades are thicker in section.. You have to add forward speed to the circumferential speed using the long side of a triangle, and introduce the effect of slip to the froward speed, which you need to maintain THRUST. That's usually a %.

 

. HP is the key IF you have /use a REDRIVE which also affects tip speed. Your engine must operate at the rpms that allow it to reach it's max Horsepower and the properly designed prop converts that to THRUST. Another limit on the prop is ground clearance( or structural clearances with a pusher) The thrust the prop delivers allows the plane to overcome the drag it has at the load, density altitude and speed it flies at. It was common to tether the plane  to a fence or heavy vehicle and use a spring gauge to determine what thrust was available before flying it. You can make your own wooden prop if you want to. A laminated 2 blade is not too difficult if you've ever made them for models . Nev

 

 

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I knew there'd be an app for that?

 

Warp Drive has one too, but after posting I found that neither of them factor forward speed.

 

That calculation can be done using your rotational speed and you forward speed to get a vector.

 

Generally you want the largest diameter you can get allowing for ground clearance and tip speed, then look at pitch to get the right airspeed. This get s a bit variable as draggy aircraft aren't as efficient, so some experience helps.

 

 

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HP is the key

 

Yes and no.

 

Horsepower is related to the speed of the crankshaft (RPM) and the toque so that:

 

HP = (Torque x RPM)/ 5252

 

The constant, 5252 has been derived from converting circular motion to linear motion to satisfy the definition of horsepower. It also takes into account any units conversions that need to be corrected.

 

The torque produced by an engine is a rotational force determine by the offset (s) of the centreline of the rod journal centreline measured from the main journal centreline of the crankshaft multiplied by the Force (F) provided by the combustion of the fuel. 

 

In its simplest form Torque = F.s Therefore the amount of torque an engine can produce per cylinder is dependent on the size of the "bang" (affected by cylinder diameter and compression ratio) and the offset of the rod journal. Thus the torque an engine can produced is limited by its construction. That's why the graphic depiction of torque is fairly flat over a wide RPM range

 

155HPND-vs-167HPNC2-hypothetical-torque%2Bv2.png

 

The RPM of an engine depends on how quickly the crankshaft can be rotated, which is determined by how quickly a cylinder can get through the 4 cycles of suck, squeeze, burn and blow. A big factor here is the design of the intake and exhaust manifolds, and the amount of fuel/air mixture that can be got into the cylinder during the "suck" cycle.

 

Horsepower in its simplest form is (F.s)/t, ie Work per unit of time.

 

When we look at our aircraft engines and propellers we must remember that moving a propeller is "work". We want that work to be done quickly (high HP) on take-off as the distance we have available for surface running is restricted. However, once in flight, we don't need maximum HP, so we throttle back for best torque to swing the prop to produce Thrust to just balance Drag

 

(Newton No 1 Law).

 

Is it any wonder that the Wright Bother's engine with a 4 inch by 4 inch bore/stroke pattern producing 12 HP was able to swing those 8 foot diameter props?

 

 

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Jeez I wish people  would leave and quote my statement in context. Re" HP is the key" IF you have /use a REDRIVE.( reduction drive which multiplies the torque).              Anyhow, in the  final outcome, the HP will determine your top speed IF you use it properly.( Convert it to useful thrust in an efficient manner.)  Too fine a pitch will limit speed as you run out of available revs ( Like being stuck in too low a gear. in a car/bike) Too coarse and the engine will Labour and not get to where it makes all of it's  potential Horsepower which is determined by the engines own particular rpm and torque figures. The diameter, blade area, RPM and angle (Pitch) of the prop are adjusted /designed to achieve the best result. That's what we aim for  but not always achieve as quite a few things have to be right

 

  To continue to provide thrust the prop has to move the air at a faster speed than the plane is going through the air at. Ie IF you don't PUSH it, it  won't provide an equal reaction.. Newton's Law.  This is the magic "slip" factor that varies from one plane to another depending on the drag of the total thing, and keeps the blades at a positive angle of attack to THEIR relative airflow and maintains the thrust and absorbs the engines POWER.. and keeps everything in equilibrium for your flight condition, level, climbing descending with some power.etc  Nev

 

 

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Props change their section as the diameter changes.  The Reynolds number also changes according to, among other things, that is a function of lineal speed the position on the prop.  These things change the drag of the prop at a point along the shaft, the overall torque required by the prop is the integral of the torque along the entire length of the prop.  This should match, or be slightly lower than the available engine torque. This should be analysed for the aircraft to ensure that the aircraft can get into the air.  At max aircraft speed the prop angle of attack to the air vector, this should be the best lift/drag ratio for  the aircraft ( lots of factors affect what ratio is best).

 

The best technique for prop design is to use finite element analysis for the aerodynamics of the prop.  I created a crude finite element program some years ago.  It generally produces results that are close to operating wings (actually it was designed for helicopter blade design but is the same as prop design). It also calculates stress along the prop.  An important  factor.

 

 

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Jeez I wish people  would leave and quote my statement in context. Re" HP is the key" Horsepower which is determined by the engines own particular rpm and torque figures.

 

I'm saying that you've put the cart before the horse. in saying that HP is the key

 

HP is a product of the amount of torque an engine can produce at a reference point - let's say the crankshaft centreline. It is also a product of the rate at which that torque can be generated - RPM

 

Anyhow, in the  final outcome, the HP will determine your top speed IF you use it properly.

 

I'd say that it is torque that will determine your speed. But torque has to be used properly. To use the analogy of a bicycle, if the bike only has a main sprocket and a wheel socket, it's like a plane with a fixed pitch prop. Add a gear system and it's like a variable pitch prop. These are the means by which torque is manipulated to achieve the best outcome in terms of speed.

 

I agree with this statement: "The diameter, blade area, RPM and angle (Pitch) of the prop are adjusted /designed to achieve the best result"

 

This statement: "To continue to provide thrust the prop has to move the air at a faster speed than the plane is going through the air at", can be confusing. Consider the plane at rest with the engine idling. The propeller is converting {(torque x RPM)/5252} into a directional force we call Thrust. However this Thrust cannot overcome all the other forces resisting the forward movement of the plane. Now do a magneto check and run the RPM up to about 1000. The thrust is now greater than all the resisting forces and the plane will try to move forward - if you don't have the brakes on. While the plane is held stationary with brakes, the prop is still producing Thrust. 

 

Now we get into that old chestnut argument of Newton -v - Bernoulli.  Is Thrust a Newtonian force created by the propeller pulling the air mass similarly to the way an Archimedian Screw pulls water, or is it a Bernoullian force created by the movement of an aerofoil shape through air?

 

image.png.f6aa89983ac084e4fd8deed168a2e0bd.png

 

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I really do not agree with  your approach as a means of explaining the situation, but you are certainly free to do it as you wish with the topic, but I will (Have to) defend mine.

 

     I have applied  standard and accepted Newtonian physical laws . Thrust is needed to oppose drag and provide acceleration (change of speed). You can also have thrust and not move.  ie pushing against a brick wall or a vehicle with a high static rolling friction . W = FxS ( Force times distance the "S" is standard for distance)) You can push/pull but UNTIL something moves, no WORK is done.' The RATE at which the work is done is where the concept of horsepower comes in.. The time/rate of doing work

 

  There is no way that the horsepower won't be the determinant of your final achievable velocity. where drag increases as the square of speed  the HP required  rapidly becomes  equal to what you have. This is how the 'gearing" of vehicles is calculated that are used for speed runs.. The gearing  is what must be done correctly or the speed you achieve will be less than the optimum for the engines POWER... The prop and reduction gear in a plane does the same as the gearbox in a land vehicle.  Both have efficiency losses which must be considered. Nev

 

 

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Nev, I reckon you and I are on the same line of argument. We aren't really in conflict, it's just that I'm say that without torque, you have no horsepower, and you are saying that without horsepower no work can be done. I fully agree with that. 

 

Either way, we both get to the point of having Force to apply to the propeller. 

 

 

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Nev, I reckon you and I are on the same line of argument. We aren't really in conflict, it's just that I'm say that without torque, you have no horsepower, and you are saying that without horsepower no work can be done. I fully agree with that. 

 

Either way, we both get to the point of having Force to apply to the propeller. 

 

You are correct OME.

 

An engine has to produce torque before anything happens.

 

Power or horsepower is simply torque with a time element added in.

 

Your formula is correct for horsepower..

 

The metric fprmula is: P = Mn/9549

 

where:

 

P=kW

 

M=Nm

 

n=rpm

 

0549 - coefficient

 

So for an engine with 245 Nm torque:

 

At 1000 rpm it will produce 25.66  kW

 

At 5000 rpm it will produce 128.27  kW

 

In motor vehicles:

 

A vehicle with the highest torque will climb a hill fastest with equal gearing

 

A vehicle with a smaller engine with less torque can pass that vehicle by shifting down a gear and developing more power at faster rpm

 

Over 80 km/hr on a flat road at a constant speed, power is the biggest factor for pushing the wind out of the way (and goes up exponentially with speed.

 

So they interchange in importance depending on the job.

 

 

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The metric formula is: P = Mn/9549

 

where:

 

P=kW

 

M=Nm

 

n=rpm

 

0549 - coefficient

 

Here's a case where the non-Metric terms are easier to understand than the Metric

 

HP = (Torque x RPM)/5252

 

 

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Here's a case where the non-Metric terms are easier to understand than the Metric

 

HP = (Torque x RPM)/5252

 

Kw = (Torque x RPM)/9549 

 

Same easy to understand formula, just a different  coefficient.  

 

 

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Go to a drag strip and talk about a car's power in kilowatts and torque in Newton-metres. The rev-heads will direct you to the next village that's looking for an idiot.

 

Honestly, how do you visualise a kilowatt or a Newton-metre?

 

 

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