The propeller is such a key component of any power system either electric or glow or gas but in my observation as a seller the least understood .

a) I normally have buyers spend  a lot of time and effort selecting Motor's , ESC's batteries etc  but when it comes to the propeller the standard situation is what ever you think suits this set up please give that .

b)We all talk very easily of VPP 4 D set up  how does the plane move in the opposite direction without  the motor changing directions ?  The answer is the Prop .

c)In the case of a glow plane all know that the engine efficiency which is  as it is below 50 %  drops drastically with the wrong prop ?

d) If you see the  data sheet of a 2205 motor ( been trying to enclose a PDF file but it is too heavy  so did the best I could attached a picture in JPG ) the same motor can be used to vary the thrust from 242 gms till 570 gms just by choice of Prop .

Looking at all these scenarios I thought  why not try to see how well we regular  aero modelers  understand the prop :

Question

In the case of a 10 x 6 prop all of us know the number 10 means 10 " dia  , what do we understand by the number 6 which means the pitch  

Looking forward to answers  on this

Sai

PS : THE FACT THAT A PROPELLER IS GIVEN LEAST  IMPORTANCE  IS OBVIOUS FROM THE FACT THAT IN THE RC EQUIPMENT SECTION OF THIS  FORUM THE WORD IT SELF DOES NOT APPEAR ANYWHERE .
COULD WE FLY ANY TYPE OF POWERED AIRCRAFT OR PROPEL EVEN A BOAT FOR THAT MATTER WITHOUT THIS KEY COMPONENT .
BTW THE HELI'S ROTOR OR THE EDF'S FAN ARE BOTH BASICALLY PROPELLERS  


THE PURPOSE OF THIS THREAD IS TO BUILD A KIND OF AN ONLINE RESOURCE + TUTORIAL  IN UNDERSTANDING THIS LEAST UNDERSTOOD COMPONENT  SO ALL THOSE WHO CAN ADD VALUE PLEASE DO PITCH    IN  

Hint the answer lies in the word  "Propeller"

Anwar ,
If and once this builds up you could convert it into a Wiki

 Good Question Mr. Sai,
Its has two latin words, Pro-peller, PRO - forward and backward, PELLER - to Drive, in simple-A WAY TO MOVE AN OBJECT THROUGH TE AIR.  In simple terms, Pitch is a virtual form of automobile Gears, which is used to generate power out of the wind to control the production of power. Low pitch = good low speed acceleration and high pitch = high speed performance.
Sorry i understood this through wikipedia only.
Sandeep

Guys ,
This is key to understanding the logic of using a low pitch prop for 3 D and slo-fly applications and a high pitch prop for fast flying like jets . Why is the heli rotor quite flat with a relatively  low picth and why are EDF fans relatively high pitched.

Yes Sandeep but what  does pitch mean ? and how is its variation used in powered flight ?

The Meaning of the word mentioned by you also provides the answer to this question
Sai

Come on guys the idea is make this thread interactive .

What do you understand by pitch ??

Sai

Pitch is the "angle of attack" of the blades.  The more angle they have, the faster they throw air back/down, resulting in faster flight.  But the more the pitch, the less the thrust; so for 3D flights where thrust is more important, a lower pitch is actually desired.

Heli rotors are not flat.  They are designed to increase the angle of attack (ie pitch) as the throttle is increased (as can be seen in the video below).

Nope, nope - propeller pitch is the distance that the propeller moves along an axis in one complete turn of the propeller. Depending on the "twist" in the blades of the propeller the propeller may be low pitch - low angle of attack (if you can call it that) and high pitch. The higher the pitch the more difficult to turn the propeller I suppose and vice versa.

Hi Guys,

Something I picked off the net!!

Pitch is the measured of the length of the cylinder of air thus moved by the prop in one complete revolution, assuming 100% efficiency. It therefore gives a theoretical speed for the model, for any rpm. But not all glowfuel props have the same efficiency, some are less than 50% efficient. So for example in these cases theoretical speed = rpm x pitch x 50%

So if you have a 10 x 6 prop on a .40 engine, your model will go faster if you change the prop to a 10 x 7.
Or it will accellerate faster with an 11 x 6, but have the same top speed.

This depends on the engine having spare capacity enabling it to run at the same rpm as before, even though you have increased the load on it.

- Chan

"Nope" ?!?!?!   We are both saying the same thing, so I am not sure what the "nope" means     

Or is it because it is the "definition" versus "layman terms explanation" ? Or is it directed to some other post earlier in the thread ?

The propeller acts like a "screw" moving forward (pulling) as it turns. Some older books refer to a propeller as the "airscrew". Remember, the pitch of a screw way back in physics?? A propeller is simply that.

TG  you hit the nail on the head  here is an illustration .

Now  this explains the difference  between a slow fly and a speed prop .

For 3D and slowfly we need less distance to be traveled , so reduced speed per unit  time  and for fast flying we need more distance to be traveled per unit time .This is one of the reasons for choosing  a lower pitch for 3 D and higher pitch for pusher jets

I guess this explains  roughly the choice of the prop

In the illustration X is the prop dia and Y is what TG has mentioned .

Sai

Although I know the answer, why not do 3D with a high pitch blade running slower ?

 
You got a valid point.
Lower  input voltage will reduce total RPM which at the end of the day is determined by the KV  so you can use a higher pitched prop  .

Eg 8 x 4 prop will move 4 " of the air column of 8" Dia hence if you use a 7.4V cell and a 11.1V cell  on a 1000 KV Motor you  the RPM will be around around 6500 in case of the 7.4 and 10500 in case of 11.1V assuming efficiency loss . Hence theoretically for 1 rotation you move 4 " hence you can calculate the  difference in air flow in  both cases .
Now increase the pitch ie distance of air travel as you say to an 6" or 8" prop and there you can see the diff. yourself

This precisely the reason the same motor with different props gives such a wide range of thrust.

I am not a technical guy like many of you so I may be wrong , please correct me if I am wrong


Sai

TG,

You nailed it down. The pitch figures that you see in propellers is the "distance" travelled by the propeller in one complete rotation, when you spin it in a perfectly ideal medium (like cheese or butter).

Now for Anwar's question. Yes you can get the same amount of thrust with high pitch props spinning slower but then any variation in prop speed will be amplified by the larger prop pitch. IOW, throttle reponse would be coarse. For more finer throttle control you would require lower pitched props. So, in 3D you almost always go for lower pitch props.

If you are looking for faster moving plane then go for higher pitched props. The reason is quite self explainatory.

If you need more thrust from your prop, increase the diameter of the prop - NOT the pitch of the prop. I am sure there would be some questions on this. Anyway, for 3D you obviously require good thrust and finer throttle control and so you choose a larger dia prop with a lower pitch.

-Ismail

To further  what Ismail says, we have to also consider that a lower pitch prop has it highest efficiency at a lower speed than a high pitched prop. We have to keep in mind that during 3D, the thrust of the prop directly balances the auw of the plane and therefore has a higher slip...

Much like a fine pitch screw thread to a coarse pitch screw thread

Saju

I was considering the case where the diameter remains the same, as we were discussing pitch only. 

It is easier for any power-system to turn a lower pitch prop than a higher pitch one of the same dia (for electrics, that means lower amps draw).

Anwar ,
Just to clarify we are not talking only about pitch what I was trying to get people to understand the prop as such and especially the pitch  in particular as I have  found that this is major grey area for most modelers .

Most take all the trouble in choosing the airframe , power system , servos , radio etc but when it comes to the prop which is  a key component of propulsion they simply don't take any effort what so ever in understanding why they are using a specific prop in most cases it is  trail and error in arriving at the prop or some thumb rule experience of someone  else .

So I thought it would be worthwhile to get people to understand the prop better .

I kept pointing the pitch earlier as that is  one of the least understood funda's .Many will quote figures like 10 x 6 , 13 x 8 etc  but I have found there are very few who actually know what the second figure actually means , I would hazard a guess that around 60- 70 % of the aero modelers don't know what it is  and why they choose a particular PROP.

Sai

I do not think people take props lightly on purpose.  Some factors that contribute to this are :

1. They feel they can experiment with it, and settle on one later on.

2. They are cheap, unlike other stuff (so it helps with [1] above).

3. There aren't any real hard rules, even motor manuals come with ranges (as it sort of depends on type of flying).

What you say may be  true  but knowing  the fundamentals of how it works  helps one  better arrive at an informed  guess   

Surprising that the angle of attack of high pitch props and prop stall hasn't been brought into the discussion yet. If you run a higher pitch prop slower to move the same air column, beyond some pitch the prop will stall. This will just waste energy in opposing the air rather than moving it.

The relative angle of attack changes when the prop is moving. The effective pitch reduces as the prop moves faster. This is why high pitch props are used in faster planes and EDFs. Where as low pitch props just stop producing thrust once the plane moves closer to the pitch speed.

This is also the reason why static thrust measurements of high pitch props don't give real values. As the prop unloads with speed, the motor load decreases. If the pitch was high enough for the prop to stall, you'll also see the actual thrust increasing more than the static thrust once the plane is moving. This is easily observable in EDFs and turbines.

Keep it coming .
Rajesh , could this angle be explained in a bit more details preferably with an example as  some of the points mentioned by you went over my head . a bit of layman's explanation would help guys like me   . I am surely learning here
eg what do you mean by prop stalls

sai

Sorry had to write it short since I was short of time.

The thrust from a prop is not just from the angle of the blade, but also from the airfoil shape. Once you start thinking of a blade as an airfoil, the stall will make sense. It exactly same as stalling of the wing that we are all familiar with. As the angle of attack increases the wing or the prop in this case stalls and it is no longer generating the lift/thrust.

You asked for example so do this silly experiment (I'm sure you'll understand this without doing it). Take a flat plate (a flat plate is also an airfoil) in your hand and spin around holding it horizontal. Now increase the angle of attack slowly. You will feel the lift. But as the angle of attack increases beyond a point, you'll feel the lift decreasing, but you are spending a lot of energy just opposing the air. This is what happens with a high pitch prop on a slow plane. As the plane moves faster, the effective angle of attack decreases and the prop will start generating thrust again.

Here is an explanation on props: pitch and dia and all: quite a long video but very enlightening for a beginner from around 2.30 mins onwards. dave rocks!!

The Video is cool and explains in a simple way  what this thread was started of to do in the first place .
Thanks that is valuable.
In fact I liked he said  that a Heli blade as such has a low pitch  and an EDF or Boat prop are the other extreme I think this has been mentioned  some where in the thread earlier.

It also hits quite clearly that the Prop in his opinion is the most important part of the power system unlike what I have seen in my experience that everything else  takes a priority  and the prop is trial and error.
Most interesting is that he designs his planes around the prop
sai

Most people (I guess everyone who do not "design" themselves) tend to experiment with props, so this is not surprising.

He mentioned 2200 KV for a 6x4, but what about the KV rating for a 10x4 ?  This information would to be useful to someone