# Static Thrust Calculator

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#### n3puppy

##### 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? #### cblink.007 ##### Well-Known Member 988 pounds of thrust from just 82hp? Way too optimistic. What prop are you using? This is from a real-time test with a Yamaha conversion... These numbers are in line with numbers given to me from Sensenich with their 3-blade prop for the Rotax 912/914 in the pusher configuration. Last edited: #### n3puppy ##### Well-Known Member 988 pounds of thrust from just 82hp? Way too optimistic. What prop are you using? If 988 lbs of thrust with 82hp is optimistic - 988Lbs with 4hp is downright magic !! Yet the calculator says it will happen That's why I don't believe the numbers - nothing seems realistic. I'm not using any specific brand prop - just entered a generic 72 inch prop at 3300RPM to see the answer. I don't believe the numbers- but the calculator has been used/quoted so many times without being challenged that I am afraid I might be using it wrong. #### N804RV ##### Well-Known Member HBA Supporter What is that pitch number again? My little Sterba prop is a 52/42. #### poormansairforce ##### Well-Known Member 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)
Congrats, you created a helicopter! But try to rotate it 90° and fly it horizontally....
The calculator probably doesn't work so well with outliers but for average systems it works well. I put my specs in for my Minimax and it was perfect. It's easy to create large amounts of thrust at stand still but to do it with respectable velocity takes a lot more power. There are posts on here about contra-rotating props making impressive amounts of thrust for the horsepower but I keep pointing out that I'd like to fly faster than 50 mph.

#### Starflight

##### Well-Known Member
N3puppy...the "standard propeller" setting choice is NOT lit up. Those calculations were made for a model prop which does not exist. Model prop makers do not list diameters above 36 inches (giant scale). No need to put anything in the lower box of propeller type. N804RV's Sonerai IIL has twice the H.P. and displacement as my V-twin and has twice the pitch of my prop, and exactly the same diameter. It works! Forgot to mention; his cruise speed is twice mine...lol

#### N804RV

##### Well-Known Member
HBA Supporter
I thought maybe the ".9" CF was taken from the prop's published specs. Though, if my propeller only moved me 0.5" (or even 5") per revolution, I think I'd be looking for another prop.

#### Dana

##### Super Moderator
Staff member
Remember that while lots of people talk about static thrust because it's easy to measure, it's really not a useful measurement unless you're building a VTOL.

#### bmcj

##### Well-Known Member
HBA Supporter
Remember, it doesn’t take much engine to lift a hovercraft.

#### N804RV

##### Well-Known Member
HBA Supporter
Remember that while lots of people talk about static thrust because it's easy to measure, it's really not a useful measurement unless you're building a VTOL.
A guy on another board is claiming his airplane is turning a 53/42 prop at 3100 RPM on takeoff. He claims wing incidence and flight control rigging has been verified and that W&B is within limits. Yet, he says the aircraft will not accelerate to a safe climb speed. Static thrust has been suggested as a factor to rule out the engine and prop.

This is on an experimental design with a long, successful history

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#### n3puppy

##### Well-Known Member
Congrats, you created a helicopter! But try to rotate it 90° and fly it horizontally....
The calculator probably doesn't work so well with outliers but for average systems it works well. I put my specs in for my Minimax and it was perfect. It's easy to create large amounts of thrust at stand still but to do it with respectable velocity takes a lot more power. There are posts on here about contra-rotating props making impressive amounts of thrust for the horsepower but I keep pointing out that I'd like to fly faster than 50 mph.
Actually I was thinking of only a few degrees rotation - not 90

They needed two 40 horse engines to make enough thrust
With the calculator only would have needed a couple 5hp briggs. (Two counter rotating props)

Like you say - calculator might work within some range of numbers but which ones? The thrust number is the same no matter what prop pitch 1-99 inches
Almost like a broken clock - right twice a day

#### TFF

##### Well-Known Member
There can be a tail off when parameters are on the ends. A friend had access to an engine program at a hot rod computer chip maker. We raced karts at the time so we put our engine in and crunched the numbers. The exhaust pipe it recommended was much smaller than normal. We made one up. We went slower. We were also accused of cheating because we were different than everyone else. That part was worth it. It did run smoother.

#### BrianW

##### Well-Known Member
Back in the land of reality, it is true that power = thrust times airspeed
OK so if its true I expect 100 HP (74600 W) = thrust X 100 mph (44.7 m/s)
will give me 374 lb. ( 1669 newtons)
so 10 mph should give me 3740 lb. Does it? Hell no!
Two factors: the prop's angle of attack changes - so it will stall dropping thrust dramatically.
OR
When the fuselage is moving at 10 mph, the airflow over the prop is going much faster.

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#### henryk

##### Well-Known Member
They needed two 40 horse engines to make enough thrust
-I suppose, 50 HP (R 503) will be anoth,
with 2m CR propellers in good duct...=circa 200 kG vertical thrust, in hoovering mode.

#### Dana

##### Super Moderator
Staff member
A guy on another board is claiming his airplane is turning a 53/42 prop at 3100 RPM on takeoff. He claims wing incidence and flight control rigging has been verified and that W&B is within limits. Yet, he says the aircraft will not accelerate to a safe climb speed. Static thrust has been suggested as a factor to rule out the engine and prop.

This is on an experimental design with a long, successful history
Yours is a 52x42 turning the same rpm and flies OK? I assume we're talking about same HP engine and same type plane?

#### N804RV

##### Well-Known Member
HBA Supporter
Yours is a 52x42 turning the same rpm and flies OK? I assume we're talking about same HP engine and same type plane?
Yep. 120mph all day long at 3100 rpm and 23"map (cruise power) on an 1835cc VW engine rated for 60hp @ 3400rpm.

#### Dana

##### Super Moderator
Staff member
Well, if he's turning the same 3100 with a 53x42 and there isn't something drastically wrong with the plane then either the prop isn't a 53x42 and/or the tachometer is drastically off. An inch of diameter should give about a 100 rpm difference. He didn't mount the prop backwards, did he? (Don't laugh, it's been done.)

#### N804RV

##### Well-Known Member
HBA Supporter
Well, if he's turning the same 3100 with a 53x42 and there isn't something drastically wrong with the plane then either the prop isn't a 53x42 and/or the tachometer is drastically off. An inch of diameter should give about a 100 rpm difference. He didn't mount the prop backwards, did he? (Don't laugh, it's been done.)
He said he's verified the engine tach with an optical tach. And, the fact that he got it to go fast enough to get airborne at all leads me to believe something else is going on. But, its **** hard to troubleshoot something like that via internet.

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Well, if he's turning the same 3100 with a 53x42 and there isn't something drastically wrong with the plane then either the prop isn't a 53x42 and/or the tachometer is drastically off. An inch of diameter should give about a 100 rpm difference. He didn't mount the prop backwards, did he? (Don't laugh, it's been done.)
Yes, I know of a Piet with a Corvair engine with the prop on backwards. Would barely move and engine would get hot.

#### n3puppy

##### Well-Known Member
N3puppy...the "standard propeller" setting choice is NOT lit up. Those calculations were made for a model prop which does not exist. Model prop makers do not list diameters above 36 inches (giant scale). No need to put anything in the lower box of propeller type.
Got to thinking about your comment that the calculator might not be as accurate for props over 36 inch as used in Giant scale

As far as inputs I use - when you first open up the calculator it says "Choose!" Prop type - I normally choose Standard . Other options are manufacturer names like APC and Bambula.

For fun I tried an APC propeller for Model planes - 20x16 and 20x10 both at 8000 rpm
Same results - both props the same 29lbs of thrust.
APC publishes data sheets and they say different thrust for the 2 props at 8000rpm.
Performance Data | APC Propellers

In my opinion - The calculator is wrong 99% of the time for THUST numbers (even RC models)-
Same Diam. Same thrust - for any pitch 1-99 inch. 1 is probably correct - Which one?
Don't use it if you want thrust numbers for a prop

For HP required? Maybe but that would take more investigation