Does prop-pitch increase thrust ?
 

Hey!

Well the title says it all :) ... assuming I have a 14X6 @ 9000 rpms prop producing X lbs of thrust, will a 14X8 spinning at the same rpms produce the same amount of thrust ?

Does prop-pitch increase thrust ?
 

IF you could spin the 14-8 at the same rpms then it would make more thrust. But if you're using the same engine then NO it won't work as it takes a lot more power to spin that extra 2 inches of pitch.

You need to balance the prop to the engine's max power point so it runs at it's best power outpu.

What most people do is trade off some diameter for more pitch if they want to go faster. The tradoff there is that the smaller size hasn't got the bite and acceleration is slower and the prop efficiency suffers. It's all about compromises and finding the "sweet spot"

Does prop-pitch increase thrust ?
 

The reason I'm asking is because one of my planes was designed to fly with a 91 four stroke and I'll be using a 100.

The 91 will swing a 14X6 at approximatley the same rpms as the 100 with a 14X8 !

BTW, this is for an aerobatic (3D) airplane.

FYI, I fly at 9000 feet ASL.

I think having a pitch of 6 would be similar to having a pitch of 4 (maybe even less) at sea level, so in theory increasing pitch will make up for the low air density ! ..then again I could be wrong :)

Does prop-pitch increase thrust ?
 

Go for more diameter before more pitch. My math indicates prop lift (thrust) varies with:

- diameter to the third power
- engine speed (rpm) to the second power
- pitch to the first power

Also, it will vary directly (to the first power) with air density i.e. your ASL.

The link below is to a thread that gives some formulas where static thrust varies with the fourth power of diameter. I think those came from a magazine:

http://www.rcuniverse.com/showthread...light=formulas

(In the absence of further thought, I don't know who is wrong.... one of us is?!?!?)

In any event, and as already been stated, for any given rpm, you'll win more thrust by increasing the diameter even if it means decreasing pitch.

The ThrustHP program gives a thrust estimate the does not vary at all with pitch. I couldn't find any info about what how thrust HP comes up with these numbers. I don't see how it could be correct..

Does prop-pitch increase thrust ?
 

There will be a combination of three things that will produce maximum thrust for a given prop. The engine has to be operating at the RPM where the power peaks. The prop has to be pitched so that it gives just the right slip for the airspeed. The prop blade width, diameter and pitch have to load the engine to the peak power engine RPM. This is complicated enough that the best way to achieve results by the trial of various props.

Does prop-pitch increase thrust ?
 

More thrust yes. But the model will work best with a specific prop that fits the flying profile.

You want more pitch to make the model fly faster and less pitch to make the prop bite better at low speeds and hovering. Where to set the pitch is up to how you fly and what prop the engine is happy with.

I'd start with a 15 or 16 x 6 and go from there. The diameter with the lower pitch will give more bite and still let the engine operate in the proper rev range (I gather 9000 is the proper range). Fine tune the sizes and rpms from there. Don't be afraid to try lots of different brands and sizes.

On the fun fly forum they were also talking about using wood props. Being lighter they let the engine spool up faster for better reactions during critical hovering maneuvers.

Yep
 

My recent experience fits this discussion.

Previously, I mostly thought about power and props in terms of maximum speed, but started doing some 3D flying last year, learning to hover, etc., I was using a YS .91FX and a prop I had heard recommended many times, a 14 X 8.

After only a few flights, a more experienced 3D flyer suggested a 15 X 6. The results were better 3D flight and better engine performance in those regimes.... interesting.

Does prop-pitch increase thrust ?
 

I had an recent experience
Im flying and Extra 300 120. size (goldberg) and I tried first an 13x6 prop at my 71 (2 strokes) engine, the plane flew very well, but when I changed to 12x8, the plane flew much better, much faster, more powerfull on vertical climbs.
very interesting this subject...

Does prop-pitch increase thrust ?
 

Oliie, you are absolutly right, as usual, but I just would precise "in flight", as it does not seem obvious to everybody....

"The engine has to be operating at the RPM where the power peaks in flight. The prop has to be pitched so that it gives just the right slip for the airspeed. The prop blade width, diameter and pitch have to load the engine to the peak power engine RPM in flight.

I see very often engines setup at Max RPM on the ground, and the pilot wonder why flights characteristics are not that great! The only exception might be 3d, as the "speed" targeted is zero.....!

Bernard

Does prop-pitch increase thrust ?
 

May have as much to do with the engine characteristics than the prop. Some engines "breath" much better at higher rpm, and one inch less in pitch might be enough, to have your engine unloading a bit in flight, giving you better overall results. For me, the magic word about props pitch and diameter is EXPERIMENT !!! you don't have to do what everybody else do...

Bernard

Does prop-pitch increase thrust ?
 

Hi noname. First you must define "thrust." Static measurements are misleading, since a prop is stalled when airspeed is zero. Recent studies indicate that propeller "slippage" is actually much less than the ten percent rule-of-thumb that was taught when I studied aeronautical engineering.

My experience has been similar to yours. I found by experiment that a 12-10 Master Airscrew (electric) prop flies my sport-aerobatic model much better than the 12-8 indicated by the "MotoCalc" program for my Aveox 1409-2Y motor with 3.68:1 reduction gear. The increase in pitch reduced RPM by 400, while the load increased by two amps (static). Contrary to the rule that more pitch produces more speed at the expense of low-speed or vertical performance, I found that with the higher pitch I gained not only more speed but was able to complete three or four vertical rolls instead of two before losing airspeed.

Jim; I don't agree with your formulas. In my experience I have found that a change in pitch has much more effect on actual flying performance than a change in diameter. Recent full-scale propeller tests indicated that when pitch x RPM equals airspeed, the slippage measures only one to two percent!

Ideally pitch should be matched to the cruising speed of the aircraft, and that is what we do by trying different props and observing the results.

Does prop-pitch increase thrust ?
 

I certainly agree with the opinion that you have to experiment to figure out what's going to work best.

At the risk of complicating matters...

My formula was a just back of the envelope estimate. All I did was assume the prop was a wing with varying velocity across the radius.
Velocity = w*r,
Lift = Cl*(w*r)^2
integrate w.r.t. r and you get Cl*w^2*D^3

w = rotation speed (rpm)
r = radius from root to any point out to prop tip at Diameter/2
Cl = a lift coefficient representative of the pitch

I dropped all constants along the way since I didn't care about an actual number. AND, there is no accounting for air speed that will reduce the prop's angle of attack in flight (which will affect Cl), AND, of course, it says nothing about the engine.

I am curious why you say the prop is stalled at zero air speed? If it was a very high pitch prop, then I could see that it could very well be operating past the airfoil's peak lift coefficient (i.e. too high angle of attack) hence one might say it's stalled. If that is not the case, I don't see why it would be considered stalled.

By stalled, in this case, do you mean it's doing no useful work since the plane is not moving?

So, how about ThrustHP - which is so often mention here (i.e. RCU)? It's thrust estimate doesn't change *at all* with pitch, only diameter has an affect. Just seems wrong to me.

Does prop-pitch increase thrust ?
 

Hi Jim;

Good that you pointed out that a propeller operates with some of the same characteristics as a (fixed) wing. The lift/drag ratio of a prop at zero airspeed will be very poor, and even worse at negative airspeed, such as during a tail slide.

It's been mentioned twice in this interesting discussion that pitch is related to airspeed, and it was suggested that pitch should be selected to match the desired slippage. Actually, pitch should be selected to produce the least slippage at cruising speed.

This explains why in some cases an increase in pitch will produce more thrust, and in another aircraft less pitch will produce more thrust. One originally had too little pitch for its airspeed and the other not enough.

Slippage is defined as the difference between the forward distance actually traveled by the airplane and the theoretical forward distance that the prop would achieve if were screwing through jello instead of air. Remembering that the prop is an airscrew, one can calculate the correct pitch for a given airspeed using the simple formula; Pitch = Airspeed/rpm. Units must be matched, of course. If you measure pitch in inches, you will need to convert airspeed into inches per minute. For example, if your airplane flys at 50 mph, what would be the correct pitch? 50 * 5280 * 12 /60 = 52,800 inches per minute. My 12-inch prop turns at 6,500 rpm, so the correct pitch for 50 mph airspeed would be 52,800 / 6,500, or 8.123.

A prop with the pitch matched to airspeed will have noticeably more thrust than one that's trying to bite too much air (or too little) for a given airspeed.

Does prop-pitch increase thrust ?
 

Most props use a cambered airfoil. Cambered airfoils produce zero lift at a small negative angle of attack. The "pitch of most props is measured to the flat of the back of the blade for convenience. Using this "pitch" as a reference puts the airfoil at an angle of attack somewhere in the middle of its useful lift range. The angle of attack for best lift to drag is nearer the high end of the lift range and is at a higher angle of attack than that associated with the "pitch" measurement. A few degrees of slippage can put the prop at its best lift to drag ratio. Then there is the matter of induced angle of attack. Low aspect ratio blades will have a higher induced angle of attack than high aspect ratio blades and the low aspect ratio blades will require a little more slippage for best results.

Props....
 

Ollie..Gentlemen......I remember several yrs back someone did some testing of a number of different prop manufacturers (can't remember what magazine it was in) and their props and found that there were a number of props that weren't made as "Advertised".....I.E a 22x10 prop that was actually closer to 22x9, etc....These abnormalities could explain why some props "appear" to work better than some others....I've found for example a 22x10 Menz prop works better than a 22x10 Zinger......The Menz props hub is much thicker than the Zingers hub and I've found that the Menz prop doesn't flex like the Zinger prop..so performance is better..also maybe the "advertised" pitch is more accurate on the Menz......These abnormalities might also just be one more reason to "test" different props to get the best results......of course keeping in mind what it is I'm trying to achieve........Bill.... :D :D

Does prop-pitch increase thrust ?
 

tHE PROPS WITH THICKER HUBS , TYPICALLY HAVE A PITCH WHICH IS CLOSER TO BEING "CORRECT"-THAT IS , If you used a protractor and measured the back side of the blade at -say 10 points along it's length, you would find, typically, that each of these stations equaled the same FORWARD prop movement in one revolution.
Each prop manufacturer has his own ideas as to what makes up the best combinations of angles.
Some are really quite removed from the theoretically perfectly pitched prop.
Some of the reasoning is pretty good - some - well not that great.
Props which flex , willy nilly are pretty much -worthless.
Props designed to flex and make a specific change as they load or unload -can be great --but on our models -I have not seen any of these --
In running larger engines -at lower revs--we are now typically running engines in the 4000-6000 rpm range, we find that the prop types which work best , are different than the small props we used on models which were revving along in the 8000 rpm range.
It all makes sense tho -at least to me - that these props -which have more pitch, yet really not more relative blade width, work well.
The main reason, appears to be the fact that the models we are using are relatively lighter --and can change speed -with the prop speed -- much more easily.
Bottom line - you really have to try a variety of blades to see where your particular engine/model combo works the best --for you.
All the calculating in the world won't find the "sweet spot" you may be searching for.

Does prop-pitch increase thrust ?
 

[P]

Nicely summarized, Dick. I know that you have tried many different props at DH Models.[P]

However, I think the simple calculation I suggested above is worth while, as a starting point, especially for a new engine or motor. The "right" pitch is directly proportional to the speed of the model and the rpm of the propeller, and therefore the correct pitch would be just about the same whether noname uses a 0.91 or 1.0 cubic inch engine. The bigger engine could probably swing a prop of an inch greater diameter, and I would try that first instead of more pitch.

Too much pitch for the speed of the airplane results in less thrust because the high angle of attack seen by the propeller blades in this condition results in a lower lift/drag ratio.

Does prop-pitch increase thrust ?
 

Hi Ralph,

I agree with the above, but this question has a different meaning to me ...

I fly at 9000 feet above sea level ... air density (and oxygen) at this altitude is much less than at sea level (at least 30% less). With this in mind and given the fact that I'm a 3D flyer, I have noticed that larger diameter props improve vertical ascents, but don't produce enough air over the control surfaces for several other maneuvers (I.E. I can hover but I can't do a waterfall nor bring the plane down in a hover attitude ... this is not pilot error as I can easily do these in lower altitudes).

Being a little more specific to my case, a 14X6 @ 9500 rpms provides a little less thrust than what I'd like, so assuming the 14X8 produces more thrust (and more air) in theoretical terms it should be the right choice of prop. Chances are the thrust generated by a 15X6 are greater than a 14X8, but I *think* using a 15X6 at this altitude would be equivalent to using a 15X3.6 at sea level.

Also, going back to the pitch matter, since there is less air density than at sea level, I should be able to get away with a couple of degrees more in pitch without excessively overloading the engine so in theory things should work out smoothly (this holds true with helicopter blades) :)

Does anyone see any flaws in my pitch/thrust/density assumptions ?

Does prop-pitch increase thrust ?
 

BTW, why is it that most of our models in the 25% - 40% range use really high pitch props ? ... I.E. a 35% gas model usually swings a 30X10 while a 40% uses a 32X12 prop ... most of these models are used for 3D so wouldn't using less pitch and a larger diameter improve thrust ?

Does prop-pitch increase thrust ?
 

RPM- counts in all of this - I tried to show that it was part of the formula , in my previous post.
To really fix the problem of operating in thin air -the really good fixes are : A larger prop with more pitch and a more powerful engine , to turn this prop.
The big models turn slower but the tip speeds are still fairly high- due to diameter.
My personal approach tho --
rather than build a model and try to power it correctly, I find the power to weight engine I like best ----then --- build a model to fit .
Old timers may remember, that is the approach used by the best fighter aircraft designers in WW 2.
A classical example is the Japenese fighter ,called the Zero.
The designer had a specific engine for use.
His job was to make a craft based on that engine.
As Bob the Builder says "we can do it!"

.

9000 ft above sea level
 

WoW, 9000 ft ASL. You definitely need the bigger engine, and I'd try an inch more pitch than the manufacturer recommends, to start with.



Keep in mind, tho, pitch determines the angle of attack of the propeller blades, and too much is likely to be worse than too little. In the interest of simplicity, the formula I proposed above to determine optimum pitch does not take air density into account. The slippage I described would be greater at high altitude, since your jello is pretty thin up there. ; p



Theoretically, less pitch and greater diameter would be better for hovering, but as you noted large-scale models using powerful gasoline engines and relatively slow-turning propellers use high pitch props. The reason they do this is because they can! With an abundance of power, they can turn those props effectively no matter what the airspeed is, even zero! Note that pattern airplanes also use high-pitch props, although most of the maneuvers have vertical components.



Some full-scale aircraft have adjustable-pitch propellers, so the pitch can be adjusted to best suit the speed of the aircraft, and the speed of the engine(s). It's only a matter of time before this technology will be available for larger models.



In the meantime, noname, I would go for the pitch!

Does prop-pitch increase thrust ?
 

I have run a Saito 91 at 10,00 feet + density altitude. I normally fly at 4200 feet but a couple times a year at a 7200 foot altitude field, usually during the heat of summer. I started with a 14 x 6 and the plane did not fly fast enough. The air is pretty thin and engine power is down. The plane had unlimited vertical with the 14 x 6 prop. I tried a 13 x 10 and the forward flight was better, but the prop loaded down the engine and the vertical suffered. I stepped down to a 13 x 8 prop and the plane maintained it's unlimited vertical (not as well as the 14 x 6) but it had good forward speed. I have found that to be the best overall prop for the plane-motor combination.

I have run the small engines at higher altitudes for some time. I have the best success cutting an inch off of the pitch or diameter. I would recommend against putting a larger (one that would load the motor more) prop on the engine. From experimentation, the loss in power has a much greater affect than the decreased load on the prop due to the lower air density. Infact, I have seen engines overheat because they are running on the upper end of the manufacturer's recomendations. The ones I have seen is the OS 25 FX with a 10 x 6 prop and a 46 FX with a 12 x 7 at altitude in the heat. The prop was replaced with a more reasonable one and the problems went away.

As an example. I have a modified Quickee Jet 50. It is available on special order. The motor needs to run at 19,000+ static RPM to be in the tuned range and unload in the air. I flew at a field in the spring time down around 3000 feet when the temp was in the 60's. I flew with an 8.79 x 9 prop and got 20,500 RPM static. The plane was incredibly fast. I would say flat and level around 160-170 MPH. I then took the plane home and flew it in the hot summers. I had to step down to an 8 x 9 prop to run at the same RPM. The plane is probably around 25 MPH slower.

I think you would do better with a 120 on that plane at that altitude if you need any nose weight. There is a big weight jump between the 100 and the 120. I'm a speed freek and sometimes go 1.5 to 2 times the recommended displacement (2 times is on the high side. I wouldn't recommend it to everyone, sometimes the planes need to be reinforced and throttle management needs to be used). I have had fun landing the models in the thin air and I realize you will want to keep the weight down. I would also recommend a .90 2 stroke engine, but it sounds like you will be doing the 4 stroker. The 91 FX would give you more perfomance than you will get with either the 91 or 100 Saito.

Props are cheap.......I love to experiment. I have a prop collection of around 50. Every time I go the the field with a new plane I take a couple of props and see which one suits the engine-plane combo the best.

OK, OK I'll stop typing.......I'm sure I'vesaid something that will be disagrred upon. Good night.

Does prop-pitch increase thrust ?
 

Hi!

This would be a different topic but for kicks & giggles I'll post my plane's specs.

Plane: Doghouse Extreme (*very* minor structural mods)
Weight: 4.59 lbs (dry w/ 12 oz fuel tank)
Wingspan: 56"
Wing Loading: 13.85 oz/sq ft
Wing Volume Loading: 5.99
Wing Area: 778 sq inches

As you can see, even at this altitude this plane should be as close to a rocket as it gets with the Saito 100 :D :D

BTW, the plane is built with contest grade balsa and *many* fiberglass, carbon fiber and kevlar reinforcements, so I don't think it'll blow up that easily.

I'll fly her maiden flight on Sunday and I'll post the prop results in this thread and the flying characteristics in the 3D / Aerobatics forum.

.... I wonder what this plane will be like at sea level !!!!

Does prop-pitch increase thrust ?
 

Yep. 120 would be too much.

Re: Does prop-pitch increase thrust ?
 

Going back to Noname's original question, the simple answer is yes, the prop with more pitch will produce more thrust than one of the same diameter, when both are turning at the same rpm.

However, the 14 x 6 may produce more thrust at lower airspeeds (e.g., vertical or hovering), while the 14 x 8 will produce more thrust at higher airspeeds (e.g., level flight)

This was indicated by AQ-500's experience, above.

Then Noname added that he flew at 9,000 ft above sea level. This is certainly a factor of great influence, mostly because an engine won't produce nearly as much power at that altitude, and also because the prop will be about 1/3 less efficient.

At your altitude, the best combination may be the larger engine with the smaller prop. I have no experience with high-altitude model flying, except for Costa Rica (3,200 ft ASL), where I believe only minor changes in needle-valve settings were required.

We'll be looking forward to your report after the first several flights. Take a few different props along.