# Static Thrust Calculator

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#### Armilite

##### Well-Known Member
Can someone help me understand how to use the Static thrust calculator that is often quoted when discussing engine hp numbers?
Static Thrust Calculator - STRC

This is a common format I see when results are tabulated
3300 rpm with 72" Prop = 988.7 lbs of Static Thrust at 77 Degrees with a (3) blade prop at .9 CF
72" x 10 = 988.7 lbs Static Thrust. Needs 82.671 hp
72" x 5 = 988.7 lbs Static Thrust. Needs 41.335 hp
72" x 2= 988.7 lbs Static Thrust. Needs 16.534 hp
72" x 1 = 988.7 lbs Static Thrust. Needs. 8.267 hp
72" x 0.5 = 988.7 lbs Static Thrust. Needs 4.133 hp

Instinct tells me that the same 988lbs thrust for any pitch is not plausible.
Every formula (not calculator) I have seen always has pitch as a variable when calculating thrust. Change the pitch - thrust changes. Example formula below. (Set Vo to zero for static thrust)

If I am to believe the output of this calculator.....
I could build VTOL ultralight with a 500lb MTOW
Hang it from a 72in 3 blade prop with a virtually flat pitch (0.5inch)
Prop makes 988 lbs thrust according to calculator
Climb vertically at almost 2G (Thrust/Weight ratio 1.98 to 1)
All while using a \$120 Harbor Freight 6.5hp motor (only need 4.133 hp)

Am I interpreting the results wrong?
Or is the calculator wrong?
View attachment 98739View attachment 98740View attachment 98741
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Sadly this calc website is down now, but you have the wrong Data Inputed. Most Props have a CF of .75, not .9 which makes a Big Difference in your Numbers. Putting 0.5 for a Pitch is just ridiculous. The Rotax B Gear Box can only handle a 68" (2) Blade Prop and a 64" (3) Blade Prop. So a 72" is out of the Scope to be used. Warp Drive recommended a 54" x 18 for my 277UL 28hp. The 185UL 9.4hp used a 34" x 10. So a Smaller hp engine will use an even smaller Prop.

Prop Lenght used sets the Static Thrust made, Pitch used sets the Speed. Once you Input the right CF Number your Numbers will change greatly. Once you Dial it in for say Part 103 55 knots your hp needed will change also.

Some examples of Props for Rotax's. You will Notice not (1) 72" Prop listed!

#### n3puppy

##### Well-Known Member
Published thrust formulas indicate pitch affects thrust. (Not just speed)

#### Lendo

##### Well-Known Member
I found this interesting and can be confirmed on an Internet search; when considering P/D = (Pitch / Diameter) where1 is efficient and 1.3 is 90% or more efficient.
George

#### Lendo

##### Well-Known Member
Armilite, sad isn't it - I guess that was directed at my comments about Jack Norris book, however I'm having a lot of success with 72" in JC's Prop Program and 1.3 P/D it is. Could be the Rotary has more power?
George

#### Armilite

##### Well-Known Member
Published thrust formulas indicate pitch affects thrust. (Not just speed)

View attachment 110429
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Thrust is what gives you your Speed from a Prop. I would think that would be a given! Without PITCH you have No Thrust so you have No Speed! Just as HP & TORQUE go hand in hand, but without rpm, you have neither.

That Calc if you Input the right DATA it showed you the hp needed and gave you an Approximate Speed once you Dialed in the Pitch.

Like Warp Drive, Props suggested a 54" x 18 for the Standard Rotax 277UL. It Dynoed 25.4hp@6000rpm!
 Rotax 277UL 2.0 to 1 Ratio Pitched 3025rpm
54" X 18
 THRUST = 187.76 HP Needed = 25.9 SPEED = 51.5 KNOTS = 44.8

For a 277UL turned 6400rpm making 28hp you could use a 54" x 22 Prop which would also put you closer to the Max 55 knots.

Interesting Calc: Static Thrust Calculator – Bandit Airdrives
25hp 54" 75% = 175.7 lbs Static Thrust!
25hp 56" 75% = 180.0 lbs Static Thrust!
25hp 58" 75% = 184.3 lbs Static Thrust!
25hp 60" 75% = 188.5 lbs Static Thrust!
25hp 62" 75% = 192.6 lbs Static Thrust!

26hp 54" 75% = 180.4 lbs Static Thrust!

27hp 54" 75% = 184.9 lbs Static Thrust!

28hp 54" 75% = 189.5 lbs Static Thrust!

29hp 54" 75% = 194.0 lbs Static Thrust!

30hp 54" 75% = 198.4 lbs Static Thrust!

31hp 54" 75% = 202.8 lbs Static Thrust!

32hp 54" 75% = 207.1 lbs Static Thrust!

33hp 54" 75% = 211.4 lbs Static Thrust!

34hp 54" 75% = 215.7 lbs Static Thrust!

35hp 54" 75% = 219.9 lbs Static Thrust!

36hp 54" 75% = 224.0 lbs Static Thrust!

#### n3puppy

##### Well-Known Member
Published formulas indicate prop static thrust varies with changes in pitch.

Examples of questionable Static Thrust from calculator - using correct data.
Calculator produces static thrust numbers that do not change with pitch or HP.

3150rpm, Pitched -100rpm for 3050rpm with a 58" Prop = 282.26 lbs of Static Thrust at 77 Degrees with a (2) blade prop.
58" x 21 = 45.808 hp needed. 282.26 lbs Static Thrust.
58" x 20 = 43.627 hp needed. 282.26 lbs Static Thrust.
58" x 19 = 41.445 hp needed. 282.26 lbs Static Thrust.
58" x 18 = 39.264 hp needed. 282.26 lbs Static Thrust.
58" x 17 = 37.082 hp needed. 282.26 lbs Static Thrust.
58" x 16 = 34.901 hp needed. 282.26 lbs Static Thrust.
58" x 15 = 32.720 hp needed. 282.26 lbs Static Thrust.

60" x 21 = 52.461 hp needed. 323.25 lbs Static Thrust.
60" x 20 = 49.963 hp needed. 323.25 lbs Static Thrust.
60" x 19 = 47.464 hp needed. 323.25 lbs Static Thrust.
60" x 14 = 34.974 hp needed. 323.25 lbs Static Thrust.

62" x 21 = 59.813 hp needed. 368.55 lbs Static Thrust
62" x 20 = 56.965 hp needed. 368.55 lbs Static Thrust.
62" x 19 = 54.117 hp needed. 368.55 lbs Static Thrust.
62" x 12 = 34.179 hp needed. 368.55 lbs Static Thrust.

.

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#### rv6ejguy

##### Well-Known Member
Most props within the normal realm of pitch and diameters on fixed wing aircraft produce 4 to 4.5 pounds of thrust per hp, according to actual pull testing with scales.

100hp will give you around 400-450 pounds of thrust. Static thrust figures are not that useful in reality on fixed wing aircraft.

#### n3puppy

##### Well-Known Member
Most props within the normal realm of pitch and diameters on fixed wing aircraft produce 4 to 4.5 pounds of thrust per hp, according to actual pull testing with scales.

100hp will give you around 400-450 pounds of thrust. Static thrust figures are not that useful in reality on fixed wing aircraft.
Exactly.
The reason this thread was started close to a year ago was the calculator was giving questionable results for static thrust. Made HP numbers questionable too since thrust numbers were off.

Your experience based rule of thumb seems more useful in real life

#### Lendo

##### Well-Known Member
JC's thrust formula is-
Thrust (Lbs.) = HP *550*efficiency/ Velocity (ft./sec.
OR
Thrust (Kg) = HP *75* efficiency/ Velocity (M/s).
Hope that helps.
George

#### Armilite

##### Well-Known Member
Published formulas indicate prop static thrust varies with changes in pitch.

Examples of questionable Static Thrust from calculator - using correct data.
Calculator produces static thrust numbers that do not change with pitch or HP.

3150rpm, Pitched -100rpm for 3050rpm with a 58" Prop = 282.26 lbs of Static Thrust at 77 Degrees with a (2) blade prop.
58" x 21 = 45.808 hp needed. 282.26 lbs Static Thrust.
58" x 20 = 43.627 hp needed. 282.26 lbs Static Thrust.
58" x 19 = 41.445 hp needed. 282.26 lbs Static Thrust.
58" x 18 = 39.264 hp needed. 282.26 lbs Static Thrust.
58" x 17 = 37.082 hp needed. 282.26 lbs Static Thrust.
58" x 16 = 34.901 hp needed. 282.26 lbs Static Thrust.
58" x 15 = 32.720 hp needed. 282.26 lbs Static Thrust.

60" x 21 = 52.461 hp needed. 323.25 lbs Static Thrust.
60" x 20 = 49.963 hp needed. 323.25 lbs Static Thrust.
60" x 19 = 47.464 hp needed. 323.25 lbs Static Thrust.
60" x 14 = 34.974 hp needed. 323.25 lbs Static Thrust.

62" x 21 = 59.813 hp needed. 368.55 lbs Static Thrust
62" x 20 = 56.965 hp needed. 368.55 lbs Static Thrust.
62" x 19 = 54.117 hp needed. 368.55 lbs Static Thrust.
62" x 12 = 34.179 hp needed. 368.55 lbs Static Thrust.
===========================

So what have you learned in a Year?

Prop Length used at a Specific RPM = Static Thrust!
'
Prop Pitch used = HP Needed and also Approximate SPEED made!

Many of these Older Airplanes are not Propped or Pitched for Optimin Performace.

Many also have the wrong Reduction Ratio's. Look what Rotax used on most of these Smaller Engines 277 to 582, 2.0, 2.24, 2.58. With 2.58 being the most popular ratio used. SPEED = NOISE, but also more THRUST!

26hp at 6250rpm/2.0 = 3125rpm <-- Most Thrust, and the Higher the Speed!
28hp at 6400rpm/2.0 = 3200rpm <-- Most Thrust, and the Higher the Speed!

26hp at 6250rpm/2.24 = 2,790.2rpm
28hp at 6400rpm/2.24 = 2,857.1rpm

26hp at 6250rpm/2.58 = 2422.5rpm
28hp at 6400rpm/2.58 = 2,480.6rpm

Your Examples above are not for a Rotax 277UL what we were talking about, but we can apply your Data in which 34.901hp (377UL) is the lowest and 59.813 hp is the Highest you give. Your Data doesn't give the Approximate SPEED that you can attain with any of them. Maybe the original Static Thrust Calc was too complicated for you to understand. Temperature and Altitude also affect your Static Thrust made!

Your 58" x 18 = 39.264 hp needed. 282.26 lbs Static Thrust would work for a 447UL. Your 60" x 20 = 49.963 hp needed. 323.25 lbs Static Thrust would work for a 503UL.

We know what hp Rotax Rates their different Engines and most of us know those hp numbers are not Exact as portrayed like the Rotax 277UL turned 6250rpm is not making 26hp, but makes Max 25.4hp@6000rpm. MATH does not lie, only Humans do. Could someone fine Tune a Rotax 277 Engine to make 26hp at 6250rpm, Yes! Could an Engine Builder person fine Tune a 277 engine to make even more hp with some upgrades, Yes.

Warp Drive recommended a (2) Blade 54" x 18 Prop for the Standard 277UL rated 26hp@6250rpm with a 2.0 Reduction. With that first S/T Calc, I fine-tuned it to 54" x 22 to get me close to the 55 knots I wanted for a Part 103. I figure I could improve the 277 enough to make the hp needed to meet the requirements. I doubt even Warp Drive makes every Prop Size & Pitch.

We have lost Tennesse Prop and I think Culver Props also. So our choices are getting less.

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Talked to Culver props yesterday about making a new prop for me for a flywheel drive VW engine.

#### n3puppy

##### Well-Known Member
Published thrust formulas indicate pitch affects thrust (Not just Diam and RPM)
Not all calculators agree on relation of pitch/diam/RPM on thrust.

EXAMPLE - Original Godollo calculator in question and the new one above from Bandit.
Using Same Diameter, Same HP's - Different results.
Godollo has identical thrusts - Bandit varies thrusts.

Original Godollo Airport Calculator
60"x 21 = 52.461 hp needed. 323.25 lbs Static Thrust.
60" x 20 = 49.963 hp needed. 323.25 lbs Static Thrust.
60" x 19 = 47.464 hp needed. 323.25 lbs Static Thrust.
60" x 14 = 34.974 hp needed 323.25 lbs Static Thrust.

New. Bandit Airdrives Calculator @75%.
60 inch - 52.461 hp = 308.9 lbs static thrust.
60 inch - 49.963 hp = 299.0 lbs static thrust.
60 inch - 47.464 hp = 289.0 lbs static thrust.
60 inch - 34.974 hp = 235.8 lbs static thrust.

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