Relationship between pitch and thrust?
 

This mostly concerns 3d/IMAC type flying styles. I have not found much information in the forums or on the web concerning this. If it is a dead horse, my apologies.

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I have been wondering for quite some time how much thrust pitch generates in a propeller. Generally, static thrust will increase if you increase the diameter and reduce the pitch (to maintain a specific RPM) of a propeller. However, I would imagine that there comes a point where pitch provides more power, especially in a dynamic environment, ie., not sitting still on the ground.

For instance, in a vertical situation (coming out of a hover, doing a 45* upline etc.), having a propeller with more pitch (or, the same pitch, but less diameter than another prop, spinning at higher RPM) may pull the plane up faster (due to more pitch or RPM) than a propeller with more diameter and less pitch (or more diameter and the same pitch). Therefore, it may not always be better to have more diameter. When does more pitch or more RPM provide better vertical pullout/speed than more thrust? Is there any way to calculate this without having to switch props constantly?

The reason I ask is I have been using an APC 15x4w on a Saito .82 that can spin it at around 9500 RPM. Spool up, however, is not great, and while it pulls a 6lbs. plane out of hovers nicely, it doesn't move it quite as quickly as I'd like (especially in a vertical attitude). I have been wondering if switching to an ACP 14x4w, which will spin at around 10600 RPM, will provide better vertical performance due to more speed (higher RPM at the same pitch) even though it has a fair bit less thrust. Also, this begs the question: what about the 14x6, or the 13x8?

I broke my last 14x4w and don't have another to try this with. Besides, this question has been in my mind for quite some time, as I am planning how I wish to build and equip my 27% Extra 300 this summer. At the very least, I'm sure this question will provide for some good conversation.

RE: Relationship between pitch and thrust?
 

Boy you openned a can of worms. When dealing in dynamic relationships such as these, everything effects everything else so you basically get in the ballpark and then do some experimenting to get the best result for your application. Generally you don't want to run a .82 4 stroke over 10,000 rpm on the ground due to the chance of breaking something when it unloads in the air. So prop your Plane to run in the high 9's ready to fly. You need set the pitch first and the diam of the prop second to keep ypur rpm in the right place. A 4 pitch prop would only be used on aircraft needing only very slow speeds to perform, this is based mainly on wing loading and application. a 6 pitch prop would be used on an airplane of moderate speed, 8 pitch for fairly fast plane, 10 pitch for really fast plane etc. This is of course based on engine rpm etc. The other thing to consider is aircraft momentum. A fast plane, with high wing loading will carry alot of momentum therefore in using a high pitch prop, it will allow it to get up to speed where it performs best. A light airplane with lite wing loading will generate very little momentum so you will need to pull airplane through manuvers more with just the engine and therefore less pitch would be helpful. There are to many factors at work here to be able get the best prop without doing some experimenting.

RE: Relationship between pitch and thrust?
 

I think you meant something other that what you said. If you maintain the same RPM more pitch is more thrust. But would take more HP to do that. But when adding the engine power to the equasion then more pitch is more load, so much the engine slows down and has less thrust. So for our purpose more pitch is less thrust. This is differant from say a full scale plane where ground clearance is a problem so a smaller diameter prop is used and given a slightly higher pitch to regain some of the lost thrust. Because the engine has a redline speed the higher pitched prop gives more thrust at that redline speed.

RE: Relationship between pitch and thrust?
 

That 10,000 RPM limit comes from earlier engines which had that limit. I don't think there is one made now with a limit less than 11,000. Many have a RPM limit of 12,500.

RE: Relationship between pitch and thrust?
 

A large diameter, low pitch prop will give the highest static thrust, due to a lower disk loading.

When the conditions are other than static, this will no longer be true.

While 3-D models maneuver in basically static conditions, F3A models maneuver at speed...

12.25x3.75W to 16x4W props are common on 3-D models. Current F3A models fly with 17x13.5 props...

RE: Relationship between pitch and thrust?
 

This is true, however he said that the low pitch prop has the most thrust "at the same RPM". If the RPM is constant the higher pitched prop with have more thrust both static and dynamic every time. Or almost every time.

RE: Relationship between pitch and thrust?
 

Hugh,


This is inaccurate.

At equal levels of power absorption, a large diameter, low pitch prop will have more thrust than a small diameter, high pitch prop.

However, if the pitch is 0", the diameter can be large enough for the power absorption to be the same, the disk loading will be still lower, but no thrust will result...


If you take two props of the same diameter, that are spun at the same RPM, the higher pitch prop will give more thrust; even static.
It will, however, absorb more power as well.

...So the higher pitch prop will have to be spun by a bigger engine...

RE: Relationship between pitch and thrust?
 

Here are some examples.

A 20x8 prop @ 7200 RPM vs. a 19x8 prop @ 8200 RPM. Less thrust from the 19x8, but more speed. Would it pull the plane vertically faster, even though it has less thrust? If it did, then the speed would be more important than the thrust.

A 20x8 prop @ 7200 RPM vs a 19x10 @7200 RPM. Again, would the speed pull it vertically faster than the 20x8?

A 21x6 prop @7200 RPM vs a 20x8 prop @7200 RPM. Same concept as the last one.

In other words, how does vertical speed relate to thrust? When does having more potential speed from a propeller/engine combo outweight the benefits of having more thrust? How would one determine this mathematically?

Sorry if the original post was not very clear. I was very distracted at the time.

RE: Relationship between pitch and thrust?
 

This is not somthing that can be set up as a math. eqaution. You have to test different props.
In general you can say higher pitch=higher speed.

Higher thrust is not the same as better vertical performance. It's just more acceleration at very low speed.

RE: Relationship between pitch and thrust?
 

It can be calculated. It's done by aerospace engineers countless times for every aircraft. It gets very complex. For modelers testing the props and observing the results is the easiest way to go.

http://www.mh-aerotools.de/airfoils/propuls1.htm

To estimate prop performance you can use his program.

http://www.mh-aerotools.de/airfoils/javaprop.htm

RE: Relationship between pitch and thrust?
 

If you [link=http://www.mvvs.nl/prop-power-calculator.xls]Use my calculator[/link], enter the 15x4 in engine one with 9500 rpm. I do not know the prop form factor for APC-W, so enter 1.2, which should be close. Juggle the prop factor until the thrust output matches your experience.

Do the same in engine 2, but this time use the 14x4 at 10600 rpm. Use the same prop factor, and check against experience. Juggle both engines's prop factor until you are closest to your own experience.

Now you can try any APC nnxnnW in that diameter range and play with the figures. Move the sheet to the right and down a bit to check prop thrust against speed (graphs). This only is graphed up to pitch speed. Above that speed, the thrust reduction is not linear anymore. Up to pitch speed, linearity is good enough.

Do not forget to Post the prop factor you found in this thread.

RE: Relationship between pitch and thrust?
 

Hi!
The answer to your question is easy...You have to try it out in practice!
No computor program will tell you exactly what prop is best for what airplane at what airspeed ect, etc.
That's why you have us...Ask questions and you will hopefully get a better, more reliable answer, from us than from a computor pogram ...because we have been there, and done that;)

RE: Relationship between pitch and thrust?
 

Good computer programs are a tool, and not a law. They help reduce the cut-n-try by about 2/3rd, and thus reduce trial times and cost. It does not eliminate the interaction choice between prop and airframe completely, but it is getting closer.
If I have a new engine to try, I first calculate the prop to slap on. I never am far off. Fine tuning starts from there, but here also the program helps to eliminate unwanted selections.

RE: Relationship between pitch and thrust?
 

I allow a 1500 to 2000 rpm pad on my engines so when it unloads in the air especially in a dive, I don't over rev. Engines turn alot faster in the air then they do on the ground. Nobody runs a 4 stroke much over 10,000 on the ground except for a few exceptions such as some YS motors.

RE: Relationship between pitch and thrust?
 

Martin Hepperle, a great authority in aerodynamic matters, published his research on several calculations in use, especially [link=http://www.mh-aerotools.de/airfoils/prpstati.htm]under static conditions[/link].
However, he uses advanve ratio and blade angle for the prop, which is less than easy to use for our purpose. This can be translated to pitch and rpm quite easily, if we accept small errors.
If one further observes the fact that a flat-bottom airfoil as used in most props has a lift coeffiency (Ca) of about 0.5 when at zero angle of attack (at pitch speed), it is easy to understand, that there still is a substantial static thrust available even with pitch zero. A prop can be seen as a disk, which has a pressure difference in the front and the rear part. This is thrust that causes the air to flow, even if the disk has no speed by itself yet. Depending on the forward speed of the disk (advance ratio). the blade angle of the prop must be adapted to account both for both the induced airflow, and the disk forward speed. This will eventually reach equillibrium with the resistance of the plane and available power.
Static thrust are the conditions we have when we hover a plane, or near the top of vetical uplines, so it's calculation is important enough to us.

RE: Relationship between pitch and thrust?
 

Pe

I have two versions of your prop calculator spreadsheet, and get slightly different results between the two. Is this latest version more accurate?

Thanks

RE: Relationship between pitch and thrust?
 

later versions take fuselage drag into account, so you get less effective thrust.

RE: Relationship between pitch and thrust?
 

RPM, the same RPM Dar! I said nothing about the same power absorbtion. If you pull a higher pitched prop at the same RPM as a lower pitched prop it will absorb more power, and it will have more thrust.

RE: Relationship between pitch and thrust?
 

Thanks Pe.

RE: Relationship between pitch and thrust?
 

Sure, Hugh.


But isn't it all about power absorption???

A thread like this is started by someone who wants to put the most appropriate propeller on his aircraft and the engine is a given value; not one that this person has not selected yet...

If the question was 'would a 10x10 prop give more thrust than a 10x6, spinning at the same RPM?', what you wrote would have clearly been correct.

But the 10x10 prop would require a .60 engine and the 10x6 requires a .40, to be spun at the same RPM... Not the same.

RE: Relationship between pitch and thrust?
 

Dar,
I agreed with your first post, but noted he made the following incorrect statement.

This is incorrect and I even stated that he much have meant at the same power.

RE: Relationship between pitch and thrust?
 

Hi!
As I said earlier ...Tell us what airplane you have and what engine you intend to use ...and we can tell you what prop is best! That simple ...because we/I have been there, done that! Simple ?! And no mumbo jambo calculations!

RE: Relationship between pitch and thrust?
 

If I use my calculator on a 14x6 prop at 9000 rpm, I get 1.08 hp and 8.23 lb thrust
then switch to a 15x4.35 at 9000 rpm I get 1.08 hp and 8.62 lb thrust
The advantage of the larger prop is due to the smaller disk load
At 60 km/h (low flight speed) however, the thrust is 2.3 and 1.37 lbs respectively, and the smaller prop wins

RE: Relationship between pitch and thrust?
 

Ok, if you increase the diameter and decrease the pitch by approximately the same amount you do get more thrust at the same RPM. But this is only because the diameter increases the area to the square. Increasing the diameter at the same speed increases thrust, and decreasing pitch decreases thrust, at the same RPM. However at static and with the same power then to increase diameter and decrease pitch does indeed increase thrust.

RE: Relationship between pitch and thrust?
 

Google aircraft proving grounds-propeller selector; safe download, may help.