Fun question for all you gear heads and nerds out there.

[drew03tx]

So i was driving along the highway the other day and for some reason a question popped in my head that i couldnt quite figure out the answer too. Maybe im just too stupid but here it is. If you took 2 identical planes, same frame, same prop, same weight, same weather conditions...yada yada yada, give one plane a 300hp engine and the other a 600hp engine. Obviously the 600 hp plane will go faster but what i cant narrow down is why. Yes i know more power means more speed, BUT if both planes are identical, prop RPM is the same, say 2500rpm, why would the one plane go faster than the other? Ill put my flame suit on but i seriously want an answer. Drop some knowledge fellas!

[AndyJWest]

Fixed-pitch prop? Because If a 300 hp engine can turn a fixed-pitch prop at 2500 rpm, that is all the power it needs to turn it. Another potential 300 hp is only going to be of any use if you can increase the pitch or the rpm.

[drew03tx]

Lets go with constant speed, so the higher power engine with have a greater prop pitch at 2500 rpm? And just for grins your saying that if it was FIXED pitch that the additional 300hp would yield no performance gain over the less hp engine?

Edited March 24, 2017 by drew03tx

[AndyJWest]

If the pitch is fixed, the amount of power needed to turn it at constant rpm (assuming the airframe remains the same) is also fixed. If it takes 300 hp to do it, that's what it takes. Extra power can only turn the prop faster: which would of course make the plane go faster too*. With a constant-speed propeller, more power will increase the pitch, while the rpm stays the same: making the plane go faster.

 

*there are limits to how fast you can make a propeller go before you get problems with the tips going supersonic, which is not only noisy, but incredibly inefficient.

[HagarTheHorrible]

For propeller driven aircraft it's the physical limitations of application. Regardless of engine power a propeller can only do so much work or push so much air. You can try to make the propeller as efficient as possible but compromises creep in and eventually it' becomes a case of just rearranging the deck chairs with diminishing returns.

 

That's my take on it anyway, probably utterly and moronically wrong.

 

https://en.m.wikipedia.org/wiki/Fastest_propeller-driven_aircraft

 

might help answer

Edited March 24, 2017 by HagarTheHorrible

[Dakpilot]

If a car is geared to top out at 120mph, and is able to, adding another 100hp will not allow it to go any faster, with comparable engine rev limit, it will accelerate quicker though

 

similar but totally different 

 

Cheers Dakpilot

[drew03tx]

If a car is geared to top out at 120mph, and is able to, adding another 100hp will not allow it to go any faster, with comparable engine rev limit, it will accelerate quicker though

 

similar but totally different 

 

Cheers Dakpilot

 

Its funny you mentioned this because i was trying to use the same method to compare the two. So im thinking now that the propeller is like the transmission on a car being that its the gear that allows the difference in speed, and the more horsepower you put behind it allows it to accelerate faster. but if the engines only spin too 2500 rpm then still why the quicker accelertation?  is it because the prop can have a greater pitch allowing a bigger bite into the air?

[Dakpilot]

 

 

 but if the engines only spin too 2500 rpm then still why the quicker accelertation?  is it because the prop can have a greater pitch allowing a bigger bite into the air?

 

 

Essentially yes,

 

though in effect, if you were going to have an engine with twice the power fitted to an airframe it would have to have a different prop, designed to match the larger power output

 

Cheers Dakpilot

[-TBC-AeroAce]

Thrust of a propeller is equal to it's efficiency times its power normally written T=n*p. However the efficiency of a fixed pitch propeller will only be maximum for one speed hence the need for variable pitch propellers. Remember that power is just a measure of a force times a speed. So in a way more power can be seen as applying more force. The efficiency Then dictates how much of that power is turned into thrust. As the changes of efficiency with speed will be low for a sensible speed range the change in power will be the significant factor. I.e more power more speed

[69th_chuter]

All things equal except power (and with constant speed props), the higher power aircraft will go faster (as Andy said) because the propeller would have to run at a higher pitch to hold the same rpm limit (2500).  With the horsepower of the two otherwise identical aircraft given as 300 and 600 hp, one might assume that the top speed of the higher powered aircraft to be 50% greater than the lower powered one.  Unfortunately, propellers increasingly lose efficiency as the pitch coarsens so the increased speed will be a fair amount less than the anticipated 50%.  A change in propellers might bring back some, if not most of the difference but a larger diameter would be required to do it completely and one might assume the prop was already as large (diameter) as possible for the 300hp aircraft and therefore the 600hp aircraft would have to make due with the less efficient fix of more and/or wider blades (which would have a measurable destabilizing effect on the aircraft's flight characteristics).

[Ribbon]

600hp plane will burn more fuel so in given time it's weight will be lower so it will be faster

[drew03tx]

600hp plane will burn more fuel so in given time it's weight will be lower so it will be faster

There it is....solved the case lol. But seriously the prop pitch info makes a lot more sense

[Guest deleted@30725]

Today I learned something that I think I already knew, but it was good to have it in black and white.

[NO.20_W_M_Thomson]

There it is....solved the case lol. But seriously the prop pitch info makes a lot more sense

Wait a sec, wouldn't the 600 hp weigh more than the 300 hp. so to start with the 300 hp would be faster until the fuel empties  to equal amounts then the 600hp goes faster but by then it has to catch up to the 300 hp, by then it has not enough fuel to go any further and the 300 hp reaches it's destination and the 600 hp doesn't.

[Ribbon]

Wait a sec, wouldn't the 600 hp weigh more than the 300 hp. so to start with the 300 hp would be faster until the fuel empties  to equal amounts then the 600hp goes faster but by then it has to catch up to the 300 hp, by then it has not enough fuel to go any further and the 300 hp reaches it's destination and the 600 hp doesn't.

i thought about that too but OP said shape, weight and rest of stuff are complete same on both planes, just HP is diff

[Willy__]

Wait a sec, wouldn't the 600 hp weigh more than the 300 hp. so to start with the 300 hp would be faster until the fuel empties  to equal amounts then the 600hp goes faster but by then it has to catch up to the 300 hp, by then it has not enough fuel to go any further and the 300 hp reaches it's destination and the 600 hp doesn't.

 

Weigh has very little influence on top speed, it will greatly effect the acceleration, though.

[=ARTOA=Bombenleger]

The number of prop blades is dictated by the power of the engine.
The stronger the engine gets the larger the prop blades need to become, in order to deliver the engines power to the air.
But bigger blades become ineffective at some point, because parts of the prop that are outwards spin supersonic.
Supersonic props cause a lot of friction and slow the plane down, so they put more prop blades, instead of larger ones, to avoid going supersonic.

EDIT:
I forgot to answer the actual question :D
Engines became more powerful and so the number of prop baldes increased too.

I dont think private aircraft these days have so powerful engines, as those that were used in WW2, so I guess most of them will have 2 blades, except very heavy duty planes.
Also private aircraft today mostly use turboprop engines, which is a turbine driving the propellor.
 

Edited April 1, 2017 by =ARTOA=Bombenleger

[AndyJWest]

The increase in blade numbers was down to the need to make use of increasing engine power. If you are starting with a blank slate, the best way to do this is to increase the diameter of the propeller, but that isn't usually practical with an existing design, since the limitation is usually ground clearance. Changing the gearing for increased RPM isn't likely to be an option either, since as I noted above, propellers get increasingly inefficient as the tip speed approaches the speed of sound. Which then leaves either increasing the blade chord, and/or increasing the number of blades, and accepting the drop inefficiency. The only other approach I can think of which met with any real success were the contra-rotating props as used with Griffon engines - which had the added benefit of mostly eliminating the torque issues which high-powered single-propeller aircraft have always suffered from.

 

As for modern propeller-driven private aircraft, I don't think they generally have the sort of power output which merits more than a three-blade propeller.

[JG13_opcode]

If anyone has the time and inclination, could I ask why pre-war and WWII aircraft (engines) appeared to move from 2 prop blades to 3, then some had 4, mostly later aircraft of the war from my limited knowledge, but not many. Is 3 blades still the 'norm' for private aircraft these days?

 

Sorry for hijacking the thread.

To see why we can't just keep adding more blades, it's helpful to look at it from the thermodynamics point of view.

 

All aircraft propulsion systems work from the same principle:  You have air come in, and you use some kind of process to increase that air's kinetic energy.  The system accelerates the air backwards, and because of momentum this pushes your aircraft forwards.  In general:

 

thrust = [mass flow of air through the system] * [Vs - Vf]

 

Where Vs is the velocity in the slipstream imparted by the system, and Vf is the freestream velocity, aka your airspeed.  The quantity (Vs-Vf) is called the "velocity increment".  So there's two ways to increase your thrust:  You can pull more air through the system (increase the mass flow), or you can increase the velocity increment (make your system impart more speed to the air like a jet).

 

With some fancy math, we can show that the efficiency of a propeller can be simplified to:

 

efficiency = 1 / ( 1 + (Vs - Vf)/2Vf )

 

Because propellers have large diameters, they usually produce a high mass flow rate and a low velocity increment, which is why you often hear that props are more efficient than jets at low speed.

 

So if we want to put a more powerful engine into our fighter, and take advantage of that power, we can either increase the mass flow rate (diameter of prop) or we can increase the velocity increment (amount of "push" that the prop imparts on the air passing through it)

 

As others have said, there are two limits on the diameter of the prop: Ground clearance and tip speed. Once you hit either of these limits, you can only add more blades to get more thrust power.  Unfortunately, adding more blades increases the velocity increment, and because it is in the denominator of the efficiency equation, decreases the efficiency, so at some point you get into the "diminishing returns" zone where each blade you add is just adding more weight to the aircraft and the extra thrust gained is not worth the loss of efficiency.

 

-------

 

In terms of aircraft design, the rule of thumb I've seen quoted most often is that you want the helical prop tip speed to be less than 300 m/s or so at sea level to avoid shockwaves forming:

 

static tip speed = [pi] * [prop RPM/60] * [prop diameter]
helical tip speed = sqrt( [static tip speed squared] * [airspeed squared] )

 You can work backwards from there to figure out what the maximum diameter of your prop is.  If your prop is too big, you can reduce the diameter and add a blade.  To figure out the equivalent diameter for a given power setting, you can use:

 

D = k * [ 4th root of power ]

Where in British units, k is 1.7 for 2 blades, 1.6 for 3 blades, 1.5 for 4 blades, etc. etc.

 

Looks like this:

 

 

Hope that helps.

Edited April 2, 2017 by JG13_opcode