Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?

Question

As a general rule, I'm told GA planes generate 4 lbs of thrust per horsepower. (Source: Zenith School of Airplane Design, Flying on your own Wings)

So if you use a 4 bladed prop, does it generate anything close to 8 lbs of thrust per hp? In other words, say for an ultralight, where you're only flying 60 mph, and prop drag isn't an issue, can you use an engine with half the horsepower, or some other fraction thereof?

Will that shorten my takeoff roll? Conceptually, takeoff roll is just the distance to accelerate from zero to my minimum takeoff speed.

Since F=ma, then a=F/m, so if I want to accelerate twice as fast, I need twice the force, or thrust ( probably more than twice the thrust, to overcome rolling drag, etc.)

Is this correct, at least conceptually?

Just to summarize:

In the GA world, I'm told as a general rule, props develop 4 lbs of thrust per hp. So, for 160 hp, I get 640 lbs of thrust, or is this not correct?

In the STOL world, with 8' props, I'm told, they get 8 lbs of thrust per hp. So, for 160 hp, I get 1,280 lbs of thrust.

Well, I'd rather have a 160 HP engine and prop that produces 1,280 lbs of thrust than 640lbs of thrust.

So... if I take my 8 ft 2 bladed prop, chop it in half to get a 4' 4 bladed prop ( same overall "wing area"), I'll still get 1,280 lbs of thrust ( less a few % for inefficiency), instead of 640 lbs of thrust. Is this correct? If not, what value of thrust should I expect, roughly?

Or are these rough estimates incorrect?

Related: https://aviation.stackexchange.com/questions/37720/how-does-blade-solidity-ratio-relate-to-thrust-power-torque-of-a-propeller — AEhere supports Monica, Aug 06 '19 at 13:46

Therac · Accepted Answer · 2019-08-08T04:02:54.317

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At the same size and rpm, a 4-bladed prop will require twice the horsepower to drive, approximately.

Due to blade interference, it will always generate less than twice the thrust. So lbs thrust per hp will generally slightly decrease, as long as the propeller is in its optimal tip airspeed band.

If the propeller was overly fast initially, slowing it down and going with more blades to compensate can result in small thrust/hp gains (+X percent, not X times). This will generally apply to aircraft originally designed with much less power and later modified.

There are a few legitimate cases where design size is less than the aerodynamically optimal size. One is clearance to the ground or between the props (in quads). Another is turbofans: the drag of the nacelle overcomes the benefits of lower disc loading. Even for airliners there's a push for wider fans, but the "open fan" idea (basically a turboprop) was scaled down to geared fans and the Ultrafan for noise reasons.

In helicopters, at some point, the blades become so long and so far apart that adding more blades beats the weight and bulk of longer blades. The more blades, the worse, so going up to 4 is not such a big deal as doubling it again to 8. For small aircraft with low power/weight ratios, where 2 blades fit easily, that's generally the best configuration.

edited Aug 08 '19 at 04:02

answered Aug 06 '19 at 13:47

Therac

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I guess so in theory. That assumes the prop was optimized in the first place, but in the GA world, I see ground clearance limiting prop diameter in many cases. Case in point, many float pilots put on a larger prop once they are on floats, my soaring club wanted better climb performance from their tow plane, so they bolted on a 2nd prop, offset 90deg to the first one. Lazair did the same thing, all without increasing horsepower. I'd say there is probably at least a 20-30+% improvement, or else no one would bother doing it. Of course cruising speed would suffer. – Fred Aug 06 '19 at 16:32
This is probably the case of a prop optimized for cruise at full course pitch where you need full power. I'm thinking they couldn't get their prop pitch fine enough and there was extra power to be used. – Fred Aug 06 '19 at 16:39
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@Fred If you're diameter limited, then optimization may be achieved at more than 2 blades. This is why large airliners can have 40 blades in their fan. – Therac Aug 06 '19 at 17:07
Will blade interference not make it require less than twice the horsepower to drive? – rackandboneman Aug 07 '19 at 19:13
@rackandboneman Can require a little bit less due to the spin. But very nearly twice. – Therac Aug 07 '19 at 19:56
Blade interference makes it so that more horsepower is required for more blades. More blade profile resistance to swirl around. – Koyovis Aug 07 '19 at 21:37
@Fred: Yes, size restriction is generally the only reason people increase number of blades. Same is true for helicopters but usually the size restriction there is structural strength (strength is the same for airplane wings - also the reason for biplanes: increasing the "blades" of wings) – slebetman Aug 08 '19 at 01:04
Does this suggest that a single blade with a counterweight would theoretically be better than two? – Reid Aug 14 '19 at 15:35
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@Reid Yes, in terms of aerodynamics only. Overall it will be less efficient, as the counterweight is wasted weight. Some gliders do use single-blade propellers, but only for the ease of stowage. – Therac Aug 14 '19 at 17:12

score 17 · Answer 2 · answered Aug 06 '19 at 16:30

This is not conceptually correct, for the following reason: Doubling the number of blades on your prop will not double the thrust unless your engine is powerful enough to drive the prop at the same speed as the 2-bladed case.

For small planes with fixed-pitch props, a prop and engine combination is chosen for a given aircraft so that when the engine is running at its maximum RPM setting and full throttle, the prop is absorbing the full power output of the engine. If one then adds some more blades to the prop, the engine will be loaded down too heavily for it to run at its maximum power setting and the power output of the engine will go down and so will the thrust generated by the prop.

score 2 · Answer 3 · answered Aug 07 '19 at 20:40

There is no free lunch here. Adding blades does not somehow cause the engine to produce more power. It just means that each blade gets a smaller fraction of that power and thus produces less thrust, and total thrust would remain constant.

Actually, due to interference, total thrust actually goes down a bit when you add blades. The main reason to accept that inefficiency is that any fewer couldn't absorb all the power coming from the engine, typically due to ground clearance or tip speed, so you don't really have a choice.

To my understanding, one single blade would be most efficient. However, for obvious balancing issues, it is never used. — Tero Lahtinen, Aug 08 '19 at 06:37

Chris · Answer 4 · 2019-08-07T22:52:15.963

As I read this there are really two questions being asked here.

1) Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?

Basically yes, if the propeller rotation rate, blade length, inflow velocity, and blade pitch all remain the same. There will be some losses for interference between blades, but those can be neglected to first order.

Going from 2 blades to 4 will also, those same things being equal, double the required torque, which will also double the shaft power required to spin the prop at that same RPM. If you double your number of propeller blades AND double the size of the engine, you will nearly double your thrust. This should significantly reduce the takeoff roll.

2) Can I add blades to my propeller to compensate or allow for a reduction in engine power?

The answer to this is a more nuanced no.

At the most basic level, the useful flow power of a propeller $P_{flow} = TV_\infty = P_\mathrm{shaft}\eta_{p}$

Restated, $T = \frac{P_\mathrm{shaft}\eta_{p}}{V_\infty}$ This shows that, for a given amount of engine power at some airspeed, the useful thrust you can get is driven by your propeller efficiency at that flight condition.

In general, a well-designed 4-bladed propeller is about as efficient as a well-designed 2-bladed propeller. More blades is desirable when you need more thrust in a small diameter, less blades gives you a slight efficiency benefit because of the reduced losses from blade-blade interactions. But one won't give 2x the efficiency of the other.

Prop efficiency varies widely, depending on the operating condition and design details. So a specific 4-bladed propeller might very well give more thrust than a 2-bladed propeller on the same engine, depending on how well-matched it is to the engine and the design details of the propeller. The converse may also be true.

QPROP is a good, open-source tool for playing around with the effect of changing the number of propellers blades, operating condition, and other design details. The theory document also lays out the fundamental relationships clearly.

Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?

4 Answers4