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Automation Aero Challenge - Round Four Results Are Up!


#21

That’s propeller pitch. I can’t help you with the others. It’s how much the propeller blades are angled to cut into the air.


#22

but i thought that’s blade angle?


#23

(I don’t know koolkei, I’m just an idiot with a computer)
@koolkei they seem to be the same thing.

Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller or helicopter rotor into or out of the wind to control the production or absorption of power


#24

From a quick Google:

“Pitch is defined as “the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood.” For example, a 21-pitch propeller would move forward 21 inches in one revolution”

I’m sure there must be some maths to calculate your ideal pitch given other propeller characteristics


#25

okay so, in some sort of more comples version of this
blade angle + propeller diameter = propeller pitch.

i suppose?


#26

That makes more sense… but hey, what do I know :joy:


#27

As mentioned above, propeller pitch is the distance that the propeller, under theoretically ideal conditions, would move forward through one full rotation without any slippage. So for example, if you see a propeller marked as having 54 inches of pitch, it will move forward 54 inches for each rotation under ideal conditions. It is directly related to the actual blade angle of the propeller.

Advance ratio is a bit more complicated; it is the ratio between an aircraft’s speed through the air, how quickly the propeller is turning and the diameter of the propeller. It is expressed by the following equation:

Advance Ratio (J) = Speed (units per time) / [propeller revolutions per time * propeller diameter in units]

There is no ideal advance ratio; it varies based on airspeed, propeller blade angle and diameter; a low-speed aircraft with a small, fast-turning propeller - like a motor glider - might have an advance ratio of 0.4-0.6, while a high-speed prop-driven airliner with a very large, slow-turning propeller can have an advance ratio as high as 2.5 or more.

Advance ratio, along with the engine power, aircraft drag, propeller diameter and atmospheric conditions, are fed into the most heinously complicated series of tables and equations I’ve ever had to deal with in Excel (seriously, it takes up some 25-odd columns and several hundred rows), to generate the overall efficiency of the propeller in question.


#28

This is just remarkable. Fantastic. I’m in.


#29

okay i do not know how this is possible, but it is


#30

A prop diameter of 26.15 inches makes my prop weight 0.00 pounds.

Something else I don’t understand: I edited my engine that had a TOB of 5200 hours, I lowered the RPM, raised the reliability, lowered the cost, lowered the production units, yet my TBO went from 5200 hours down to 4600 hours. Also, this engine cost more than another engine I made that has a higher material cost and more production units.


#31

the engine costs more or the powerplant costs more?


#32

Engine cost on the worksheet.

Another note, these 2 engines have exactly the same reliability.

I just played around a little with the worksheet, apparently, the engine’s maximum horsepower affects both TBO and engine cost.


#33

Yes, for the purposes of this challenge, engine horsepower affects TBO to some degree; the effect is more extreme at either end of the horsepower spectrum, the idea being at the high end of the power spectrum that it would bring values more in line with what you’d expect in the real world. In the power range we will be spending most of our effort in future challenges, it will have a less noticeable effect. I may yet revisit that part of the calculator for future rounds of the challenge going forward.

As for the propeller weight dropping to zero at extreme values, I realise two things now - first, I need to specify a minimum diameter in the challenge, and second, we will almost certainly never be working down in that range anyways. The smallest propeller I am aware of in any manned aircraft is 28 inches, on the Colomban Cri-cri, and it’s tasked with absorbing some 15 horsepower per engine. I also realise now that in some of my calculations, certain values break the calculations too because I did not put in proper minimum bounds.

Propeller cost and weight is based on a fairly extensive survey of real-world propellers, where it was found that the diameter and the engine power were the biggest drivers of cost and weight - and the equations I generated to simulate these seem break down at sizes below about 40 inches, which is about the smallest widely available propeller size.

I should mention that a very small propeller turning at high speeds will tend towards very poor efficiency in one category or another; they either have very low or very high advance ratios at cruise speed, and may find themselves extremely compromised in a very critical area of performance as a result. Unfortunately it’s something I can’t easily simulate in the powerplant calculator without making it frighteningly complicated to use, unless I find a way to massively simplify the calculations…

Additionally, any propellers with tip velocities exceeding Mach 0.8 suffer extreme efficiency loss when plugged into the propeller simulation model. As an example, a propeller which has a peak efficiency of 0.86 at its design cruise speed and a tip speed of Mach 0.7 (which are pretty typical values for your average propeller) sees its efficiency drop to roughly 0.80 when turned at Mach 0.75, and it is just below 0.75 when turning at Mach 0.80.

The reason is because of how a an airfoil (which a propeller very much is) works:

As we no doubt learned in school, one of the reasons why an airplane flies (and by extension, how a propeller generates thrust) is that the air flowing over top of an airfoil needs to move faster in relation to the air below it, which according to Bernoulli’s Principle results in lower pressure. Now, as the airfoil goes faster and faster through the air, at some point the airflow over the top will start to approach and even exceed supersonic velocities, creating a standing shock wave over the top of the airfoil. This standing shockwave decreases the lift produced by the airfoil, increases drag and in the case of a propeller, dramatically increases noise production - something I think I might try to find a way to simulate for the next round…after all, who would want to fly an aircraft for hours on end with an unnecessarily loud, droning propeller?

The important thing to take away from this is that transition to supersonic airflow begins well before the airfoil’s speed itself approaches supersonic speed. Typically, most airfoil shapes (and it is very dependent on shape) experience this transition at a forward speed between Mach 0.65 and Mach 0.8, with the airfoil shape I chose as the default for the propeller simulation, it happens at Mach 0.75.


#34

well with 37 inches diameter propeller, i am getting exactly 0.75 tip mach and 0.75 advance ratio, is that a good setup?


#35

For a 37-inch, fixed-pitch, 2-blade propeller, I’m struggling to absorb more than 50 horsepower with the planned speed at 120 knots. Just curious, but are you sure you’re using a propeller that fits in the bounds of the competition?


#36

yeah actually it’s just over 50, at 50.4hp.

something tell me it’s not optimal


#37

The desired engine power range is 80-100 HP for this competition.


#38

yes, but it’s still not answered yet. that 80-100hp rating is for the pure engine power or the flat-rated power?


#39

80 to 100 horsepower is the desired power at the propeller, so after any flat-rating.


#40

owh
so i’m way below then…