thrust calculator

[210solo]

Hi i have a problem i carnt solve

using a thrust calculator i found on this web site

some figers i put in for the raket 120 engine

Static Thrust Calculator

thrust 76 kg

203 m/s

7.4 horsepower needed

17 miles per hour

standard prop

51 in prop 2 blade

pitch of 6 in

3000 rpm

this shows that the radne has lots of thrust what am i not understanding

i found a prop site that said the radne prop had a pitch 0f 5-6 also top 80.

other info found said said paramotor take off speed should be around 10-

12 miles per hour

thrust needed to maintain flight mutch less than takeoff as the air reaches the prop at a different speed when it is flying.

tip speed of prop must be around .6 mark moor and it looses power

if you increse pitch to 12in flying speed increses to 34 mph power needed increses to 14.8 horsepower thrust stayes the same

so there must be some thing i dot understand as most to 80 15 hp have thrust of around 55 hp

and radne the same 50-55 hp

but i have found some claming to have radne thrust up to 65 kg with right choise of prop and reduction ratio

The big question what factor or relationship does horsepower have on thrust

as the thrust claculator said pitch effects flying speed and power needed and paramotors fly slow anyway

[ptwizz]

Unfortunately, your post has a number of mixed units and little clue as to how they relate to one another.

Could you provide a link to the online calculator you used?

Before you trust any online calculator, it's worth verifying it by running some known values. If the calculator doesn't give the same answers as real world measurements, then something is wrong with the calculator.

[210solo]

This is the link to the calculator

http://personal.osi.hu/fuzesisz/strc_eng/index.htm

I think most other info came from the Para motoring bible

[210solo]

The main question is what does horsepower have to do with thrust what is the relationship between them.

so then we can work out what horsepower motor is needed for weight of pilot that will have enough power to get off the ground with ease.

[ptwizz]

The calculator has a field titled CF (under Propellor Type).

CF is an effeciency coeffecient, specific to a particular prop design at a particular rpm and airspeed.

To use the calculator effectively, you would need an accurate figure for your prop's CF. To get that figure, you would need to measure thrust and RPM in flight, rendering the calculator virtually useless.

Our best source of data for thrust / power / weight / wing type etc. is existing motors / props / wings and pilots.

[210solo]

Thanks there had to be some thing that was missing because i could get that calculator to say that lots of thrust could be produced and well under 14 horsepower of power needed but most paramotor engines today have betwen 20 and 30 horsepower to get thrust up in the high 60 plus area as i am 95 kg and need plenty of power .

[Christoffel]

Hi i have a problem i carnt solve

using a thrust calculator i found on this web site

some figers i put in for the raket 120 engine

Static Thrust Calculator

thrust 76 kg

203 m/s

7.4 horsepower needed

17 miles per hour

standard prop

51 in prop 2 blade

pitch of 6 in

3000 rpm

this shows that the radne has lots of thrust what am i not understanding

i found a prop site that said the radne prop had a pitch 0f 5-6 also top 80.

other info found said said paramotor take off speed should be around 10-

12 miles per hour

thrust needed to maintain flight mutch less than takeoff as the air reaches the prop at a different speed when it is flying.

tip speed of prop must be around .6 mark moor and it looses power

if you increse pitch to 12in flying speed increses to 34 mph power needed increses to 14.8 horsepower thrust stayes the same

so there must be some thing i dot understand as most to 80 15 hp have thrust of around 55 hp

and radne the same 50-55 hp

but i have found some claming to have radne thrust up to 65 kg with right choise of prop and reduction ratio

I think your data input was not quite right.

Raket 120 spec says it reaches max power at 9000 RPM.

with gear ratio 3.6 the prop will spin at about 2500 RPM (remember that this is the spec from the factory, so ussually it is for ideal condition).

this value will also give you tip speed around 0.5 mach, which is a very good compromised value for prop efficiency and prop noise.

Propeller pitch is defined as the distance the air will travel with one full turn of propeller.

unfortunately most paramotor prop manufacturer didnt explicitly put this number on their product (unlike the way model airplane prop manufacturer does).

Normal paramotor pitch would usually be at 20 to 35 inch, it is about 0.5 in aspect ratio (with the diameter),

so lets just assume for 51 inch diameter, the pitch number will lie around 25"

run the calculator again, and you will get an acceptable result.

The big question what factor or relationship does horsepower have on thrust

as the thrust claculator said pitch effects flying speed and power needed and paramotors fly slow anyway

lets have a look at an example on certain engine that has max power of 16HP at 9000 rpm.

it means that 16HP will only appear on 9000 RPM, not below, not above 9000 rpm. this is the character of engine in general.

so we use reduction ratio to obtain the same power but at different RPM.

reduction gear ratio will increase the torque, but does not increase power.

on the other hand, prop efficiency also varies with RPM. this is why they usually use 0.5 to 0.6 mach on propeller tip speed.

so if you choose 51" on diameter, you will be stuck on 2500 to 3000 RPM due to the tip speed rule.

from this point, you can vary the pitch value to get the highest static thrust that you need. this is your optimal pitch value. (with proper reduction ratio to get the right RPM).

if you put higher pitch value, you will overload the engine and that will decrease the rpm.

if you put lower pitch value, you will underload the engine and that will make the rpm increase.

If you lower the prop diameter, with the same method above, you will get more RPM and higher airspeed.

this is like when you drive a manual transmision car with high gear. the thrust maybe smaller, but it would last longer as you gain speed.

static thrust is the thrust when you are on static condition. when you move forward, the thrust will decrease too and will finally reach zero at some forward speed.

thats why you can get big static thrust on the calculator (with quite a small horse power), but in lower max air speed (the thrust would quickly disapear when you gain speed) that might not enough for taking off.

The calculator was made for calculating model airplane thrust.

with paramotor we have pretty big drag compared to airplane, so you must divide the estimated flyng speed result there by two (roughly).

just for comparison, I have made a paratrike with 16HP engine and 6500 RPM (no reduction gear), and 32x10 carbon prop. with this configuration I can only get 32 Kgf on static thrust, but 100 kmph in air speed.

[Guest]

I just had to google something about prop speeds and this came up

Thanks for the help guys!

SW