Prop pitch speeds

Prop pitch speeds
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Prop pitch speeds - 8/23/2007 2:19:48 AM

Leslie Ward

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From: Kalaheo, HI, USA
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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

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RE: Prop pitch speeds - 8/23/2007 4:37:52 AM

2fast

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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.

(in reply to Leslie Ward)

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RE: Prop pitch speeds - 8/23/2007 4:39:02 PM

Leslie Ward

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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

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RE: Prop pitch speeds - 8/23/2007 5:40:57 PM

scratchonly

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Still don't know what you're asking.

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RE: Prop pitch speeds - 8/23/2007 6:16:21 PM

Leslie Ward

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From: Kalaheo, HI, USA
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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

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RE: Prop pitch speeds - 8/24/2007 7:46:26 PM

scratchonly

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

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RE: Prop pitch speeds - 8/24/2007 8:49:27 PM

adam_one

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From: Stockholm, SWEDEN
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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.

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RE: Prop pitch speeds - 8/24/2007 10:44:55 PM

da Rock

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quote:

ORIGINAL: 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

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.

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RE: Prop pitch speeds - 8/25/2007 1:10:39 PM

adam_one

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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:

Calculate Level Flight Speed

Calculate Stall Speed

< Message edited by adam_one -- 8/25/2007 1:19:22 PM >

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RE: Prop pitch speeds - 8/26/2007 12:55:20 AM

da Rock

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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.

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RE: Prop pitch speeds - 8/26/2007 8:47:59 AM

adam_one

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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.

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RE: Prop pitch speeds - 8/27/2007 6:34:00 PM

Bosch232

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

The chart seems to provide that.

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RE: Prop pitch speeds - 8/27/2007 9:33:56 PM

adam_one

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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.

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RE: Prop pitch speeds - 8/28/2007 3:44:59 AM

Bosch232

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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.

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RE: Prop pitch speeds - 8/28/2007 6:29:37 AM

adam_one

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OK, let's quote the enquirer:

quote:

ORIGINAL: 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

Now the right answer to that question:

quote:

ORIGINAL: 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.

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.

< Message edited by adam_one -- 8/28/2007 7:54:14 AM >

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RE: Prop pitch speeds - 8/28/2007 2:10:40 PM

dick Hanson

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