Leslie Ward

I am wondering how to go about dtermining what the right prop pitch speed should be for a given airspeed.
In this case an Easy Glider Elet. I am guessing it might fly at 20/25 kts. Is there a "general rule of thumb" ?
Aloha , Les

049flyer

My Feedback: (18)

Here is a good rule of thumb for approximate prop speed:

RPM/1000 * pitch = speed in MPH

Example: 10 x 6 prop turns 10,000 RPM -----> 10,000(RPM)/1000 = 10 ---> 10 x 6 (pitch) = 60 mph

Usually works out pretty close. Remember the engine will unload a little in the air so your RPM in flight might be slightly higher.

Leslie Ward

Thanks 2fast,

Well as usual I did not ask what I really wanted to know. OK, we have established what our pitch speed is for a given set of parameters and we come up with lets say 30 mph. So now we need to know if if that is the approiate pitch speed for our aircraft.

Lots of things to consider here;

Prop slipage, Aircraft drag, Are we doing 3d, or just putting around, Towing, Pylon racing, etc. Power/climb or cruiseProp?
Other than a box full of various props, gears, and the old trial & error method, is there a formula for getting close with the right prop on the first or second try? $$$

Obviously the proof of the pudding is in the tasting /flying.


Aloha, Les

scratchonly

Still don't know what you're asking.

Leslie Ward

OK!
Let us say:

I want my ezglider to fly at 25 mph while under power, so that I can make it to the field/landing zone, in 20 mph head winds ( not negotiatable ).
What pitch speed should I try to attain to accomplish 25 mph? OR what is the relationship/ratio between, Actual air speed and prop pitch speed?

Example, would prop pitch speed of let's say 40 mph, give me an actual, of lets say, 30mph.?

That's my question however theoretical it may sound.

Aloha, Les

scratchonly

pitch x rpm divided by 1054=speed in MPH; Work backwards to find out what I THINK you want

adam_one

Maybe explanation of some definitions might be of interest...

Geometric Pitch is the distance an element of a prop should
advance in one revolution if there was no slip.

Mean Geometric Pitch is the mean of the geometric pitches
of the several elements.

Slip is the difference between the prop's Mean Geometric Pitch
and the actual pitch, which is called Effective Pitch.

Virtual Pitch is the distance a propeller would have to advance
in one revolution in order that might be no thrust.

Pitch Speed is the Mean Geometric Pitch times RPM, which
means the speed the aircraft would make if there was no slip.

Virtual Pitch Speed is usually 20 to 30% higher than the
Pitch Speed.

Unless it's a glider, the adequate static pitch speed should be
greater than 2.5 times the plane's stall speed.

da Rock

There really isn't simple enough theory to do easily what you want to do.

What do experienced modelers do if for some reason they wanted to prop the engine that'd suit an ezglider flying at 25mph and have the ezglider then fly at 25mph? They'd find another model the size and weight and basic configuration of an ezglider that is motorized. They'd start with that engine and make some decisions from that one.

adam_one

That's an interesting chart.
I've seen a similar one in Fly RC Magazine (issue 10, Sept. 04) by Andy Lennon.

But for those who like typing figures on a computer, may try following:

[link=http://adamone.rchomepage.com/calc_speed.htm]Calculate Level Flight Speed[/link]

[link=http://adamone.rchomepage.com/calc_stallspeed.htm]Calculate Stall Speed [/link]

da Rock

Keep in mind that computing the speed based solely on the pitch of the prop and the rpm gives a speed that means nothing unless you find an engine with sufficient power to turn a prop with sufficient diameter and efficiency to pull the airplane at that speed.

An .049 on the front of a quarter scale airplane for example, will probably turn 15,000 rpm with a 6" pitch prop. It will still be on the front of an airplane that won't even move while that little engine is howling away.

The degree of adequacy of the chosen engine and propeller to move the load also affects how close the computed airspeed will be to the actual airspeed.

adam_one

Yes, powered model aircraft performance may also be estimated by calculating the
weight/power ratio, also known as power loading.

A slow and low wing loading plane (for a beginner) with weight/power ratio of 440 to 500g/c.c. (270 to 300oz/c.in.) might be good enough, whereas an aerobatics would need about 340g/c.c. (200oz/c.in.) to achieve good performance.
This is assuming 2-stroke engines and that the power of different types is proportional to their displacements, (which isn't too far off).

There are also tables with recommended engine size vs wing area.

Bosch232

My Feedback: (1)

Everyone always wants to over analyze an answer.
The original question requested a "Generarl Rule Of Thumb".

The chart seems to provide that.

adam_one

No, because it is not the prop pitch speed alone that makes the plane to move forward.
It is a combination of pitch speed and thrust, which together means power.

Bosch232

My Feedback: (1)

And I quote myself (which I do a fair amount) "General Rule Of Thumb"

He knows his plane. He's just looking for a base line.

adam_one

OK, let's quote the enquirer:
Now the right answer to that question:
A “General Rule of Thumb� might only be useful for those who are aware of its limitations.
That’s what the previous answers try to explain.

rmh

Which prop makes it go fastest?
The answer is really , how much hp can you produce.
There is a new BARF called the Sundowner and questions pop up now about which prop will make it go fastest?-with a particular small engine
The answer is -- it really makes little difference--it is all based on how much power you have to work with and a "secret"prop formula simply does not exist - what works at your place may not work at mine
With the advent of electric motors -we can quickly sort out what is really happening as we can accurately measure watts used by the motor
as an example - I was looking for correlation of thrust and power -and found that X amount of watts consumed -always produced a corresponding thrust -irrespective of props selected . I tried multi blade short -long high low pitched -same answers

I wa s watching a wattmeter and a thrust readout from force applied. so -- no guesswork.

bbrought

It sounds like you only did static tests (i.e. in stationary air on a test stand outside a windtunnel and without taking advance ratio into account). That means it would be impossible to get any indication of prop efficiency versus airspeed. The tests would only be valid while you are hovering or possibly at the very beginning of the take-off run. It would not be relevant at all to maximum speed. You need a prop that will peak in efficiency at high flight speed while the motor is turning as close as possible to the rpm where the power peaks.

It does not make "little difference" which prop you use, as you claim, because the question is how much of the power produced by the motor is actually converted to thrust, and how much of that power gets lost --- at the flight speed you are interested in. If you base your prop selection on static thrust, you will end up with a low pitched prop that although it will give you good take-off performance and probably ok climb, will be very inefficient at high speed. Furthermore, a well designed prop with proper airfoil shapes, thin but stiff blades, etc., can easily give you an extra few percent in peak efficiency which translates into a few mph at high speed. Probably not that noticeable if you are flying by yourself, but if you are in a pylon competition it could make the difference between winning and losing.

da Rock

Actually, if it made little difference what prop was used, CL speed flyers would have quit spending so much time carving their own. R/C Pylon racers would have been using 11x5s or 12x4s to get around the pylons with their 40s.

The prop really does need to be matched to the engine and airplane. And trying to balance the rpm to the engine and the pitch to the drag/speed-envelope-desired requires some degree of effort. And does make a difference.

For speed in the air, the prop has to let the engine operate in the engines best output rpm range. And on an electric, the prop has to match the design rpm of the motor to the air load and do it without buring up.

I guess I don't understand what was actually meant about "little difference", but on the few electrics I've screwed around with, the prop made a huge difference. Pitch maybe didn't matter much by itself, but it did matter that it worked with whatever diameter was loading the engine. It didn't look to me, like say for example, an 11x4 would have flown as fast as a 9x8 even if they did pull the same watts. For one thing, it appeared to me that the electric motors design rpm got in the way of simply slapping any pitch prop on the specific motor I was screwing with.

rmh

I stand by my test-- (not in front of it )
Of course the most efficient prop will improve speed
HOWEVER----------
That prop can only be determined by flying the model in question as the load (the drag of the model ) is simply impossible to calculate except for a finite condition.
Getting that last mph or reducing elaped time by .1 second is far tougher than it sounds and cutting n trying is the only workable method
everyone uses wind tunnels computers etc on- n -on -to get best calculated SWAG ( scientific wild as sed guess) THEN you start the real work - setting up things for the task of the moment .
You can tunnel yerself to death but basically power - is what makes the real difference .
My test simply showed that in researching thrust -(of course it was a static test ) was simply a matter of power applied -using watts on electric motor reduces the unknowns very quickly - I used different pitches, lengths ,number of blades -actually stacked small two blade props - same answers .
Once I got clear on the idea that power is king - I decided that any given speed model -would be fastest when max HP was at play. so the prop whittlin would be based on getting engine to that rpm range at speed - tricky- .
BTW electric motors have max power (torque) at max power draw (watts ) . different ballgame than IC engines which have max torque when they are breathing in and exploding the biggest charge of fuel/air. which is understandable as more fuel used is more power. The max HP is a different figure and thank WATT for that confusing bit of info---he could have made a HP a watt -but -noooo he had to throw time in there to confuse the masses .

Bax

My Feedback: (11)

[GEEK WARNING!]

Watts and Hp measure the same thing. Just different units. 1 Hp. = 745.7 W.

...and for those who wish to know:

Power = work per unit of time
Work = force x distance moved
Force = mass x acceleration
Acceleration = change in velocity per unit of time
Velocity = change in distance moved per unit of time

Like an airplane or a building, it all comes together from the basic, simple units or parts.

[geek moment now over]

rmh

nope:
Power -having it means you don't have to work
Work - opposite of above
Force - may it be with you -always
Acceleration- what days do as you get older
Velocity- opposite of dribble --
Prop- (cane/walker )

scratchonly

"Aero design propeller selector" from google demonstrates/ supports in watts or HP static and at speed for most of what you are saying (I think). Safe download.

adam_one

However, propellers of all types are referred to as screws, though those on aircraft are usually referred to as Airscrews...

Further:

The engine power delivered to the propeller = Torque * RPM

While the Prop's Output Power = Thrust * Pitch Speed

So, assuming a given output power, the more thrust you have, the less the top speed.
Large diameter & less pitch = more thrust, less top speed (like the low gear in a car).
Small diameter & more pitch = less thrust, more top speed (like the high gear in a car).

However, don't get misled by the electric motor Power consumtion (P = V*I).
For example, an electric motor consumes the max power when its shaft is held so that it cannot move (stalled), the prop Output Power at this point is zero.
So, an increasing of Input Power doesn't necessarily mean increasing Prop's Output Power.

Also, every electric motor type has an ideal voltage, current and RPM at which the motor's max efficiency is obtained.
That gives the max flight duration but not the max Power Output.

rmh

Ok -but efficiency and max power for max speed are not the same things
More power input is more power output till you blow the thing up
Been there--You keep loading the prop till something gives .
Electrics are THE way to go for max speed (this will kill some old diehard IC fans) because there are NEW battery chemistries which produce more power at elevated temps and the output per pound is simply mind boggling .
Different subject of course - BUT these cells allow us model flyers to prop for max speed and stil have a prop which will get us into the air and up to speed - I use A123 cells now
On racing watercraft - -one of the problems (of many) was setting up the prop position to get the boat moving then -when up to speed -somehow take advantage of the fact that the boat then has less drag and IF the prop could somehow become more efficient - -the rig would go faster . The really quick stuff use really high pitch props which literally come up out of the water at speed and the props are running along in the surface
On really fast planes the engine/motor prop are never all working at peak till power is at max and prop is at max speed efficiency- tricky-
If you don't blow one up - you don't know if you are trying hard enough.

Jim Thomerson

As I understand it you are limited to 8 inch props. Go buy an assortment of available 8 inch props and find out if there is one which will work. That's what you are going to end up doing anyway.