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Static thrust

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BrianW

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If you want to know engine power, a stationary dynamometer will give you power versus RPM. If you want to know thrust in climb and cruise, you need to fly accurate timed climbs no wind, and timed cruise distance. The RPM you make in these two phases of flight tells you a lot about the prop you chose....
 

fly2kads

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I think what the problem here is, is that no one wants to invest (spelled waste) the time and money building several propellers for their airplane in order to tune performance.
I don't know that would call that a waste (I think of it as an R&D expense), but I think your point is essentially correct. The math and procedures for design and analysis of propellers is pretty well defined. One principal challenge is that it is fairly complex, and many people find it intimidating. Another challenge is that it is very much a "garbage in, garbage out" process, in that the quality of results is dependent on the quality of your input data. Further complicating that challenge is that some of your most important input data are often unknowns: how much power does your specific powerplant installation actually produce; how much drag does your airframe really have, etc.? To get the best results, you need to either obtain better data, be prepared to iterate on a solution, or some combination of both.

There is a strong temptation to look for an "easy button" to simplify this process. I understand this desire. Static thrust is relatively easy to understand and measure, so people may find it tempting to look for a way to use it, either directly, or as a proxy of some sort. Unfortunately, as we have discussed here, that's not particularly valuable in most normal cases. Another "easy" solution is a ground adjustable propeller. Start with something that gets you in the ballpark, and tweak the blade angle to make it better. Will it provide an optimum solution? Most likely not, for the exact same reasons that constant speed props aren't universally applicable, as discussed above.

Each builder needs to decide how much they want to optimize their setup, and how much resources (time and money) they want to put into obtaining it. If you want a cookie-cutter solution, the most popular airframe/engine combinations have been worked out pretty well. If you are building some unique combination, an airplane built in limited numbers, or something altogether new, you have some decisions and work ahead of you.
 

Dana

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Another challenge is that it is very much a "garbage in, garbage out" process, in that the quality of results is dependent on the quality of your input data. Further complicating that challenge is that some of your most important input data are often unknowns: how much power does your specific powerplant installation actually produce; how much drag does your airframe really have, etc.? To get the best results, you need to either obtain better data, be prepared to iterate on a solution, or some combination of both.
Thatone

When putting a new prop on my Hatz, even with performance data for the existing prop it was a crapshoot, the new propeller was reworked three times before I was (mostly) satisfied.
 

Jsample40

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An easy and relative inexpensive solution is to acquire an IvoProp ground adjustable propellor. I have a 54" two blade unit and have recently replaced a 25hp KFM 107ER twin cylinder light sport aircraft engine with the next size up. The KFM Maxi is a 30 hp engine which maxes out at approx 6500 rpm.
The Ivo prop allows me to "try / test" various prop pitch settings and see the actual result by either using a large pull scale, or flight testing (including differences in take off, cruise speed, and top speeds). The pitch can be easily and quickly adjusted with a wrench and measuring tape (to check for the central neutral pitch position.
An additional benefit of the IvoProp is if one of the blades is damaged, it can be replaced relatively cheaply, as compared to having to buy an entire new propellor. For those folks attempting to "tune & tweak" for the optimum prop pitch with various engines, this may be helpful. And lastly.. if the prop is determined to be too long, it can simply be cut off to appropriate lengths with a typical hand or power saw.

Disclaimer: I own an Ivoprop for my Ridge Runner 1, but am not compensated by the manufacturer for my recommendations.
 

Armilite

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Prop Lenght and rpm turned sets your Static Thurst, and Prop Pitch used sets your Speed. Like I recently emailed Warp Drive for a Prop for a Rotax 277 rated 26hp@6250rpm with the 2.0 Gear Drive. It was also rated 28hp@6400rpm. They recommended a 54" x 18 Prop. A 54" x 18 prop, tuened 6250/2= 3150rpm Pitched for -100rpm = 3050rpm at 77F and using .82CF makes 171.07 lbs Static Thrust and needs 23.602 hp. Estimated flying speed = 51.5 mph or 44.7 knots! using the same Data but changing the Pitch.

Recommended 54" x 18, 171.07 lbs Static Thrust and needs 23.602 hp. Estimated flying speed = 51.5 mph or 44.7 knots.

A 54" x 19, 171.07 lbs static Thrust and needs 24.913 hp. Estimated flying speed = 54.4 mph or 47.2 knots.

A 54" x 20, 171.07 lbs static Thrust and needs 26.224 hp. Estimated flying speed = 57.2 mph or 49.7 knots.

A 54" x 21, 171.07 lbs static Thrust and needs 27.535 hp. Estimated flying speed = 60.1 mph or 52.2 knots.

A 54" x 22, 171.07 lbs static Thrust and needs 28.847 hp. Estimated flying speed = 63.0 mph or 54.7 knots.

Part 103 is 55 knots / 63.2 mph!

The 54" x 22 needs 28.847 hp - 26 hp = 2.847 hp you would need to find. Just a Bigger 36mm to 38mm Carb change can give you a 10+% Increase. 26hp +10% = 28.6hp! Porting the Case & Cylinder can give you 10-15% Increase. 28.6hp + 10% = 31.46hp@6250rpm! A better Exhaust Muffler can Gain you more hp also!

Static Thrust Calc.
 

Dan Thomas

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Prop Lenght and rpm turned sets your Static Thurst, and Prop Pitch used sets your Speed.
One can get awesome static thrust from a very low pitched prop, one that lets the engine turn up to redline, but now as soon as the airplane starts moving forward the engine overspeeds. Static thrust is not really a useful number, since airplanes don't fly at zero MPH.

The manufacturers of fixed-pitch-prop airplanes choose the pitch that will let the engine run up to redline and no further at full throttle in level flight at sea level. (I have found that to be true at 6000 feet as well. As the HP decreases with altitude, the air density and therefore its drag also decreases with altitude at roughly the same rate up to some unknown altitude that I haven't tried.) A prop pitched like that gets the best average performance. If one wants to take off shorter and climb faster he can use a prop with less pitch (provided it's legal; this is for certified airplanes) but he has to be ready to give up some cruise speed. If he wants better cruise he needs more pitch but has to put up with lousy takeoff and climb.
 

Swampyankee

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Having worked in prop and helicopter rotor aero for several years, I can say that we never talked about torque. The structures and gearbox guys did, because, well, bolts, bending moments, and sizing shafts and things like that.
 

Armilite

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One can get awesome static thrust from a very low pitched prop, one that lets the engine turn up to redline, but now as soon as the airplane starts moving forward the engine overspeeds. Static thrust is not really a useful number, since airplanes don't fly at zero MPH.

The manufacturers of fixed-pitch-prop airplanes choose the pitch that will let the engine run up to redline and no further at full throttle in level flight at sea level. (I have found that to be true at 6000 feet as well. As the HP decreases with altitude, the air density and therefore its drag also decreases with altitude at roughly the same rate up to some unknown altitude that I haven't tried.) A prop pitched like that gets the best average performance. If one wants to take off shorter and climb faster he can use a prop with less pitch (provided it's legal; this is for certified airplanes) but he has to be ready to give up some cruise speed. If he wants better cruise he needs more pitch but has to put up with lousy takeoff and climb.
============================

That's what I was trying to show. Static Thrust doesn't change with different Pitches used and since many people use a Fixed Pitch Prop, it's important to figure out what is the Best Pitch for your needs. it's easier to upgrade the Engine to make more hp! Engines all have a Max rpm they are turned at and All Engines are rated at that max rpm at Sea Level. Different Airplane Designs also put a Limit on How Long they can use, just as some Reduction Drives also have limits. Since my Post was about a 277UL which was used on probably 85% of the Part 103 Ultralights and many Small Kitplanes and used the Rotax A&B Gear Drives, and they are limited to a Max (2) Blade 68" Prop and a (3) Blade 64" Prop. Sure you can put on a (2) Blade 68" x 8 using .82CF, and 77F, Pitched for 3,025rpm, and see some High Static Thrust Numbers 430.16 lbs and only needs 26.377 hp! But the Estimated flying speed = 22.9 mph = 19.8 knots. Since Part 103 is limited to 55 knots, it would take a 68" x 22 to go 54.7 knots = 63 mph and needs 72.537 hp which a 277UL can't attain that hp. Even if you did most of the bells and whistles, a Big Bore 277 (82mm x 66mm) 348.6cc/7cc = 49.8hp@6500rpm is the best it can do with a Good Tuned Pipe. Case & Cylinder Fully Ported, 49.8 + 15% = 57.27hp@6500rpm. Even if you used a Stroker Crank 70mm, (82mm x 70mm) 369.8cc/7cc = 52.8hp + 15% = 60.7hp@6500rpm. That's the Max hp you could get without turning it higher rpm.

Prop Recommendations Off Ultralight News.
 

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Gregory Perkins

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Saying that the pitch of the prop does not affect static thrust is like saying the AOA of a wing does not affect lift. The prop is an airfoil. Consult your L/D AOA charts for your prop foil. The formula shown above is just that. No relationship to the real world. The maximum thrust for any forward speed - prop rpm condition will always be when the most effective section of the prop blade is at max L/D AOA. These statements assume subsonic tip speeds.
 

Vigilant1

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The maximum thrust for any forward speed - prop rpm condition will always be when the most effective section of the prop blade is at max L/D AOA.
Given your input conditions (a particular forward speed and prop RPM), the maximum thrust will be when the prop blade is at its critical AoA (that is, the AoA that produces the highest Cl, usually just before the airfoil stalls). That's often in the range of 15 degrees or more, where ClMax is achieved. The max L/D AoA for an airfoil is typically much lower (closer to 4 degrees). When the prop is at that pitch it is highly efficient (high thrust per amount of engine power) but if we could increase the pitch more and the engine could maintain the same RPM (your given condition), we'd get more thrust.

As others have said, static thrust measurements are largely meaningless to real conventional airplanes. As soon as the plane begins to move, the effective AoA of the prop decreases. A prop that was optimized for static thrust begins to lose Cl. Another prop that might have been optimized for higher speed may have had an AoA that was above the critical AoA under static conditions (so the blades were stalled= high drag and low thrust). As the plane picks up speed and the effective AoA of the prop blades decreases (smaller "bites" as the relative wind increases), the Cl of that prop actually increases and the drag on the prop decreases. This allows the engine to turn at higher RPMs and produce even more thrust (if the engine isn't already at red line).

1601471032272.png
 
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mcrae0104

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:)That's an unusual one: pitch and diameter in inches, airspeed in m/s, force expressed in Newtons. If needed, 1 Newton = .225 pounds force
Perhaps we should rename the elfish unit of force (as pictsidhe once called it) the Keebler, to keep it in the same theme as the Newton.
 
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Gregory Perkins

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Given your input conditions (a particular forward speed and prop RPM), the maximum thrust will be when the prop blade is at its critical AoA (that is, the AoA that produces the highest Cl, usually just before the airfoil stalls). That's often in the range of 15 degrees or more, where ClMax is achieved. The max L/D AoA for an airfoil is typically much lower (closer to 4 degrees). When the prop is at that pitch it is highly efficient (high thrust per amount of engine power) but if we could increase the pitch more and the engine could maintain the same RPM (your given condition), we'd get more thrust.
That is the point. The engine cannot maintain the same RPM as drag increases, thusly the max thrust is being made when the setup conditions are most optimum and most efficient. As you sorta said, the most thrust for the amount of power available...........
Having reread my statement, you are correct. I did not say engine power was assumed to be maximum. It would be possible as you said in a less than max power scenario, you could increase the pitch of the prop and maintain rpm by increasing throttle and increase thrust. You must be some kind of nerd to pick up on that. It takes one to know one :)
 

Speedboat100

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Prop Lenght and rpm turned sets your Static Thurst, and Prop Pitch used sets your Speed. Like I recently emailed Warp Drive for a Prop for a Rotax 277 rated 26hp@6250rpm with the 2.0 Gear Drive. It was also rated 28hp@6400rpm. They recommended a 54" x 18 Prop. A 54" x 18 prop, tuened 6250/2= 3150rpm Pitched for -100rpm = 3050rpm at 77F and using .82CF makes 171.07 lbs Static Thrust and needs 23.602 hp. Estimated flying speed = 51.5 mph or 44.7 knots! using the same Data but changing the Pitch.

Recommended 54" x 18, 171.07 lbs Static Thrust and needs 23.602 hp. Estimated flying speed = 51.5 mph or 44.7 knots.

A 54" x 19, 171.07 lbs static Thrust and needs 24.913 hp. Estimated flying speed = 54.4 mph or 47.2 knots.

A 54" x 20, 171.07 lbs static Thrust and needs 26.224 hp. Estimated flying speed = 57.2 mph or 49.7 knots.

A 54" x 21, 171.07 lbs static Thrust and needs 27.535 hp. Estimated flying speed = 60.1 mph or 52.2 knots.

A 54" x 22, 171.07 lbs static Thrust and needs 28.847 hp. Estimated flying speed = 63.0 mph or 54.7 knots.

Part 103 is 55 knots / 63.2 mph!

The 54" x 22 needs 28.847 hp - 26 hp = 2.847 hp you would need to find. Just a Bigger 36mm to 38mm Carb change can give you a 10+% Increase. 26hp +10% = 28.6hp! Porting the Case & Cylinder can give you 10-15% Increase. 28.6hp + 10% = 31.46hp@6250rpm! A better Exhaust Muffler can Gain you more hp also!

Static Thrust Calc.

I love this STC chart...I can go 178 mph with just 6 kilowatts with it.
 

Vigilant1

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Perhaps we should rename the elfish unit of force (as pictsidhe once called it) the Keebler, to keep it in the same theme as the Newton.
I like it!
Those elves take a lot of flack, but it turns out they are pretty clever. Only those elves have walked on the moon, and they left flags that were 3 feet by 5 feet.
A little annoying blast from the past that some may remember. I'd get a lot more done if my cerebral RAM wasn't full of 40 year old TV commercials:Dancing Fig Newton Commercial
 

Lendo

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N3puppy,
A real mixture of values in that formula, I assume (m/s) is meters a second.
If I may ask, where does that formula originate?
George
 

Lendo

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N3puppy, never mind I had that reference, but didn't remember it. The answer is in Newtons.
 
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