Aerodynamics Discuss the concepts of aerodynamics here

Thrust to Speed?

Old 05-20-2008, 04:10 AM
Subsonic Ken
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Dear stinkweed007,
these are the moods I went through reading this thread.
It's like trying to change a tire in the dark while it is raining and you refuse to use the flashlight and umbrella that is in your glove box. I don't mean to seem sarcastic just trying to make you see the big picture, it's not very big.

Please buy an rpm meter like the one you can find here
I am sure there are cheaper ones out there, but this is the one I use and new where to find it quickly. I believe there are ones that use a lazer that you point at the prop. If you don't know what the RPM is under the load of the prop you have attached to it then you need an RPM guage to find out. Then use the link below and put in the pitch your prop has and also the RPM you found the running motor to have. This will calculate the pitch speed or the speed at which your motor and prop would travel through the air if it were in space, (not having any drag)

There is no way you can tell what the pitch speed of a prop by just the thrust it puts out with out knowing anything else period....

That being said if you know the size of your prop and pitch I am sure there is a formula out there to find the pitch speed since there would be a direct correlation between pitch and size of the prop and how fast it would have to go to get the thrust you know it to have (720grams) using a prop calc you could then when the 720grams of thrust is reached find the RPM and then find the pitch speed and enter it in the above speed calc.

Please for the love of god WHAT IS THE SIZE and PITCH OF YOUR PROP YOU ARE USING. I will find out the theoretical speed of which the air is moving through it, not exact, but close enough.

Sorry for the sarcasm

Please someone tell me I'm not going crazy, I did spend 4 hours in the sun yesturday flying
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Old 05-20-2008, 07:00 AM
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Default oh brother... again?

Ok, so we've measured static thrust and want to solve for velocity (although I'm not sure why)... correct?
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Old 05-20-2008, 01:34 PM
Subsonic Ken
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Originally Posted by stinkweed007 View Post
Is there a formula for thrust to velocity?

ie, I have a motor/prop which, when at full throttle, produces 720 grams of thrust, what is the speed which that air is moving?

Yes here is the first post by stinkweed007. From what I know of how things work here on earth there is know way to know pitch speed if all you know is static thrust unless you know the size of the prop, both pitch and length. You could figure out how fast the prop would have to spin to get to 720 grams of thrust to find the RPM. Then easily put it in to the a pitchspeed calc or do the math your self with this formula
RPM x Pitch /1056 = Mph

Stinkweed007 here are some examples to help you see why you can't get the pitch speed just by the thrust alone I don't care if a rocket scientist tells you other wise.

1st a 10x4 prop at 8573 rpm will give you 718grams thrust and 32.47 mph
2nd a 9x5 prop at 9386 rpm will give you 720grams thrust and 44.44 mph
3rd a 7x4 prop at 13158 rpm will give you 721 grams thrust and 79.84mph
4th a 4.7x4.7 prop 27073 rpm gives you 718grams thrust and 120.50mph

Hope you see the picture I am trying to paint for you, you see you have to know the prop size at least to figure pitch speed from thrust. All of these setups have virtually the same amount of thrust with huge differences in pitch speed especially between the 1st and 4th example.

Please tell me what you prop size is
I will get you in the ball park.

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Old 05-20-2008, 02:49 PM
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Default Rocket scientist? Rockets don't use propellers...

Propeller thrust is determined by mass flow rate. From NASA -

"Mass flow rate is the amount of mass moving through a given plane over some amount of time. Its dimensions are mass/time (kg/sec, slug/sec, ...) and it is equal to the density r times the velocity V times the area A. Aerodynamicists denote this parameter as m dot (m with a little dot over the top).
m dot = r * V * A
Note: The "dot" notation is used a lot by mathematicians, scientists, and engineers as a symbol for "d/dt", which means the variable changes with a change in time. For example, we can write Newton's second law as either

F = d(mv)/dt or F = (mv)dot "

Factoring in free stream velocity, exit velocity, and net change of pressure in the flow, we get the General Thrust Equation:

F = (m dot * V)e - (m dot * V)0 + (pe - p0) * Ae

We'll need to know propeller disk area (and air density) to back calculate velocity.

In Ken's examples, note how thust (mass flow) is essentially constant even though velocity changes and how velocity is dependent on propeller area (roughly propeller diameter). Larger prop diameter "moves" more air per rev and for a given mass flow, will turn slower than a smaller diameter prop.
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Old 06-14-2013, 11:00 PM
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Default Thrust vs speed vs volumn

Hi guys, I hope your still on line in this thread. I need to post a question about other than AERODYNAMICS and more toward HYDRODYAMICS. I am speaking about a trolling motor in water. I know the differences between the two( air vs water), but my question is mainly about the amount ( volumn) of air/water that gets pushed ( or pulled ) through a prop. I am NOW positive that the angle of the blade, and the size ( not diameter) of the blades will also determine the speed of the vehicle being pushed. I just used three tools to test two electric trolling motors One being the RPM tester ( very expensive tool). The RPM on both motors was 675 RPMS. I used a wind meter ( U. S. Army acquired ) which produced approx 7.5 MPH from both motors with different size blades. But here is where the difference is. I used an ordinary wind sock. It proved that the larger blade pushed MORE wind through the wind sock at any distance from the blade. This volume of wind would equate to the volumn of water being pushed which would equate to higher speed being that the vehicle,( boat in this instance) really never gets up to the 7.5 MPH being pushed by test of the windspeed.
My final question is. AM I RIGHT?
I hope you guys are still here.
RON W, Reading Pennsylvania and can you PLEASE reply directly to
the above screen name at AOL.
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Old 06-15-2013, 01:27 AM
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id guess its either a very heavy boat or very Draggy. you wl probably need more motor to over come this.
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Old 06-16-2013, 05:52 AM
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Originally Posted by Buck Rogers View Post
Strangely I was thinking about this just a couple of days ago. I cant see why you shouldn't be able to come up with a formula that gives a good estimate. I will have a go at working it out when I gat abit of spare time.
Each model whether air, surface or water has drag in many forms, main being aerodynamic and parasitic ...
The surface area presented to force through the medium, with drag, will counter the prop thrust ... till there is a net zero .. where drag / resistance to move matches thrust making it move.

So you can have same power set-up but different models - and achieve vastly different results.

I have a spreadsheet that has a % factor column so I can alter final speed calcs ... the factor is created by flight tests and actual speed checks with various props and power packs on same model. The model design characteristics make a big difference to the results... and a general result cannot be determined for all models. Only for specific models that you test on various set-ups.

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