Airscrew, Propellor, Propeller Tuning

[Head in the clouds]

I used the three words in the title for the benefit of the search function...

 

The following are quotes from a previous thread which we wrongly hijacked, apologies again Rick, but which have merit for discussion.

 

In essence Maj has described a situation where he discovered that he got more thrust when he reduced throttle to a lower rpm. It was assumed this was because the engine was now at its best torque, but since the airscrew is spinning more slowly how can it be producing more thrust?

 

In this case it would seem likely that it is because the airscrew (OK I'll call it a prop to keep everyone happy) prop blades are stalled or progressively stalling as the the rpm increases.

 

In the last post Maj says the prop has been cut down and that produced a quiet 'ah huh' from me - the discussion continues in the next posting...

 

Don't know about the F30, but the F23 was quite selective where it wanted to give you best power. It pays to study the power/torque curve graphs on these engines. For instance with the F23 you would go full power for take off, the tail would come up, but you were going nowwhere. Pull the throttle back, yes I said back, and away you went !...best power/torque on that engine was around 85 percent RPM.Now I can tell you it is a very starange feeling to pull the throttle back, for takeoff, but that's what you had to do !...

 

It all made sense of course the first time you dropped the nose for level cruise, and then you pulled the throttle back even more and just took off !... lower RPMs=lower fuel burn.

 

The F23 was also very prop and exhaust sensitive, and if the prop wasn't right things would not happen either.. get everything right, and away you go. The Hirths were probabily the smoothest engines I've ever flown with by far, almost like a little turbine or rotary. The crankshaft is aerobatic quality chromeolly, cylinders are Nikasil coated (before the 912), and pistons are Cima Marhle forged alum same as in the 912. Quality piece of gear for sure.

 

If you can Rick, get an old butchers scale and rig it up in a rope tied to the tail. Do some ground runs and measure the thrust that the engine/prop is putting out, and find where you want that throttle for take off.....................Cheers Maj...

 

This argument comes up quite regularly on fora but something's a bit wrong with it...An engine may well produce best torque at a particular rpm and it may well be beneficial to set up an adjustable propeller's pitch so that the engine can only rev to that rpm so that you're getting max torque at full throttle. If you really wanted to operate at max torque when torque is most needed then you'd set your prop to a pitch which gave you that max torque rpm while at climb speed, not as a static rpm. However there's sometimes too much attention paid to torque. Sure for aircraft purposes it's better to have an engine that produces high torque at lower rpm but it's power output that counts at the end of the day so props should really be pitched for best power rpm at climb speed rather than best torque rpm, and then care should be taken to not allow over-revving in level flight.

 

I'm not saying that Maj didn't experience what he describes but it wasn't a matter of reducing the throttle to get the prop operating at the best torque rpm, except by co-incidence perhaps. To reduce throttle to best torque rpm, and therefore reduce the rpm of the propeller and expect the propeller to be producing more thrust isn't right. The propeller produces more thrust the faster it spins - with a couple of caveats. We'll assume that it wasn't experiencing a transonic airflow condition at full throttle/rpm so compressibility wasn't a factor, therefore the most likely cause was the prop being stalled, or partially stalled, and that can increase/propagate toward the tips as rpm (actually the airspeed experienced by the prop blades) increases, particularly if the airfoil of the prop isn't ideal. I'd be interested to know the type, diameter and pitch of the prop and the reduction ratio that was being used.

 

The butcher's scale is, as mentioned, an invaluable tool to help you to correctly pitch your prop at static rpm and if you find that your thrust is dropping off at higher rpm it's not because you've passed the best torque rpm but because your prop is stalling due to excessive pitch or perhaps the upper surface condition (dirty, rough) is allowing the laminar flow to break down. Some very high speed aircraft with non pitch adjustable coarse airscrews do experience this stalling phenomenon and don't achieve laminar flow and hence good thrust until quite high airspeeds have been reached, that speed adjusting the angle of attack on the blades sufficiently to un-stall them.

 

Head in the clouds, I have read through what you have said with some interest. I still have the prop that I used on the F23, so I'd be happy to provide it's length and pitch. The poly-V redrive was a straight 2 to 1 . The torque figures and curves should also be available for the F23. I'll also take a photo of the prop and txt it to you if you want to PM me your Mob no. We were in the habit back then of customizing our props a bit, and I believe this one was cut down a bit lengthwise. I've also got other props in the house, each with a story or two.The particular prop and F23 did provide some quite adequet and very enjoyable flying as I remember, once I got used to how things worked best, especially at takeoff !.................................................Maj...

[facthunter]

There's an old saying about prop length. Keep the prop as long as possible for as long as possible Nev

 

 

[skyfox1]

Tell me if l am wrong but this is what l have been doing for years when l set up my aircraft mostly rotax 503s l always use adjustable pitch propellers .

 

Because max revs for a 503 is 6800 l set static revs at 6600 so at full power on take off it shouldn't exceed 6800 rpm,it does varies a bit depending on air density.

 

l have never set for max torque only rpm.

 

cheers Geoff.

 

 

[RickH]

As Nev says the saying goes re prop length. However this seems to very much gone out the window these days and is a subject of some interest to me. Example :- friend has X-Air 65hp rotax 68 inch prop. friend No. 2 has Hanuman 80hp Jab 62 inch prop I have seen many examples of this disparity and have yet to figure out why. and Yes to Geoff I recall having read of this technique many times as the best way to set up your prop.

 

RickH

 

 

[pylon500]

There is possibly two fronts to look at this.

 

When you talk torque curves for engines, the reason they drop off near the top is usually caused by the engines inability to continue breathing and porting at higher RPMs, unless it's turbo'ed, and eventually even that ratio will give up.

 

The other factor is as mentioned, that the prop is stalling near the root (while static) because of the high pitch inboard.

 

Really high pitch props can stall, cavitate, FLUTTER while static, if you can get enough torque 'up' them!

 

I've heard flutter a few times over the years, typically on VW and Jabiru engines running small diameter, high pitch props.

 

I've also heard it when not so clever people have put props on backwards!! yes it happens.

 

RickH, the prop size thing is to do with the operating RPM's of the different engine/reduction systems.

 

A 503 at 6800 with a 3:1 C box has the prop turning at 2266 RPM, a Jab (which is direct drive) needs to run to 3300 RPM to put out any power, so the prop diameter needs to come down to stay sub-sonic.

 

 

[Head in the clouds]

I'll write this up in a series of posts because it will be a rather lengthy description.

 

PART ONE

 

OK, so Maj said the prop had been cut down, and that's where we'll look for the culprit.

 

The usual reason for 'tipping' a prop is because the engine doesn't have the horsepower to get a prop of that diameter to the revs that we want to run it i.e. max permitted rpm in climb or max power output rpm in climb. So we cut 50mm or whatever off the tips and now it'll spin up to our desired rpm, but what we've done to the prop isn't so immediately obvious as we might think.

 

Remember we're dealing with wooden fixed pitch props in this case (could be metal of course but let's just talk about wooden ones), so because it's a carved wood prop we can't just change the pitch - and ground adjustable props can wait for a later post.

 

Prop shaping is a hyper-complex science if you want an efficient prop. Sure you can carve something that looks like a prop and it will produce some thrust but if you want to get up around 95-98% efficiency, which is what we have come to expect, then your prop needs to conform to a lot of specific requirements and these days very sophisticated software is used to develop the sections, size and solidity (total amount of blade area divided by total amount of disk area) to ensure high efficiency for the mission profile (high speed or STOL for example).

 

Before we go too far do bear in mind that I'm not a prop specialist but have read up and discussed a lot on the subject, so I've just been a data miner and am simply passing on what I've learned. If anyone wants to go a lot deeper into the subject I can recommend some exceptionally knowledgeable folks on other fora that you can talk with.

 

Let's consider a simple fixed pitch prop that someone clever in the art has developed for use in a typical application for an LSA or ultralight. The figures I'll use for the sake of the discussion are only approximates... Let's say it's a Thruster Gemini or similar (like Pudeston's) and it's powered by a Rotax 582. We want a good compromise between large diameter - because the plane is pretty draggy and so we want good static thrust for climb and also for cruise - and as coarse pitch as possible to give a reasonable cruise speed, but we accept that we can't have too much pitch with our large diameter because then it would consume too much power and the engine won't get to the ideal rpm. So we'll go for a largeish diameter and a fineish pitch, and it's carved timber so we'll have two blades - we'll choose a 72" diameter with a 44 inch pitch.

 

Our prop spec sheet tells us that this prop needs to spin at 2220 static rpm for best efficiency, our 582 has a 2.62:1 re-drive so we need it to run at 5800rpm under standard ISA conditions. We have an ideal day and run the engine up but it won't rev beyond 5300rpm and our butcher's scales tell us we're way under the static thrust that we want. What to do? Well we ordered the wrong prop, we should have bought a 42" pitch as advised.... Anyway, can't send it back and we all know that diameter affects rpm far more than pitch, so we'll cut a bit off the tips, it can't make much difference overall can it...?

 

So we keep trimming it until we get the rpm we wanted and then re-measure and find we now have a 68"x44" prop but it's not giving us the thrust we expected, in fact the scales tell us we get more thrust at 5600rpm than at 5800rpm... WTF? Surely the faster it spins the more thrust we get? The tips can't be going sonic because the rpm is what we originally wanted and the diameter is less than we started with so the tips are now slower than they would have been at 5800rpm if we hadn't trimmed it, so what's going on?

 

One of the things that makes prop design so complex is that they're not like a wing at all except very superficially. Without getting pedantic about a turning aircraft etc when a wing flies all of the wing has the same speed of air flowing over it, the prop has a very low airspeed at the root (hub) and an exceptionally high airspeed at the tip (about 500mph ish), so as we progress further out along the prop blade the thickness, shape and angle of incidence changes so that the section at any one point is optimised for the airspeed which that region of the prop experiences in operation, and the tip is shaped to minimise the propagation of tip vortices which could create large amounts of drag.

 

Continued in PART TWO...

 

 

[pylon500]

Just for interest;

 

<https://plus.google.com/photos/113292981019876413104/albums/5573889605855816225?banner=pwa>

 

 

[facthunter]

A couple of other considerations. If your prop doesn't enable your engine to reach the revs stated by the engine manufacturer as being the revs where the max power is achieved, during the take-off roll you haven't got the rated power of the engine, available. It will be something less.

 

This might happen if you fit what is loosely called a cruise prop, IF it is overpitched.

 

A spring balance will measure engine static thrust which gives you a measure of how well the plane will accelerate during the take -off roll, (at least initially.) If the pitch was too fine it would be like being stuck in low gear in a car, and it may not work well in cruise. In fact you may not even get to a climb speed.

 

The pitch has to be related to the desired cruise speed so that RPM (you want) xGeometric Pitch( less slip)= cruise speed.

 

The size of the prop( Diameter) number of blades and blade width decide whether the prop can use the engine power.

 

The diameter is dictated by ground ( and other ) clearance available, and tip speed ( direct drive engines are more limited here) and compressibility due local near sonic speeds being achieved causing loss of efficiency and noise. Nev

 

 

[JimG]

So Nev/Alan/Pylon, in reading your posts above, is there a calculation or some way to determine what typical 'slip' would be for a given prop .

 

What has always fascinated me is all the prop pitch calculators that i have found are just a mathematical computation that has no provision for slip.

 

Thoughts please.

 

JimG

 

 

[facthunter]

Jim, Pitch calculations seeem to be all over the place. Its easy enough to take a distance from the centre and measure the angle on the back face and draw a right angled triangle from your measurements . the base is pye x 2r and the vertical becomes the distance it moves forward with no slip. Of course the aerofoil shape etc effect the slip and the type of plane (Draggy or clean) as well. I could only guess at it but would start at about 15%? for a 90 kt plane tractor. Thoughts? Nev

 

 

[Head in the clouds]

So Nev/Alan/Pylon, in reading your posts above, is there a calculation or some way to determine what typical 'slip' would be for a given prop .What has always fascinated me is all the prop pitch calculators that i have found are just a mathematical computation that has no provision for slip.

Thoughts please.

 

JimG

Jim, Pitch calculations seeem to be all over the place. Its easy enough to take a distance from the centre and measure the angle on the back face and draw a right angled triangle from your measurements . the base is pye x 2r and the vertical becomes the distance it moves forward with no slip. Of course the aerofoil shape etc effect the slip and the type of plane (Draggy or clean) as well. I could only guess at it but would start at about 15%? for a 90 kt plane tractor. Thoughts? Nev

Good guess Nev, 15% is the usual starting point when calculating it, for simple and conventional props and airframes, from there it's a matter of testing and collecting data.

 

 

[Yenn]

If you have a prop which will not allow the engine to reach desired revs, you cut it down to get to the desired revs and as far as I can see you have lost the most efficient area of the prop, ie the outer few inches. You have increased revs, but originally the prop was absorbing all the power, now it is reduced in diameter, but the power absorption is less. Will you be getting more thrust? Bear in mind that as you level off at altitude you will be able to increase revs with the original prop by dropping the nose, but you will probably not lose altitude.

 

 

[geoffreywh]

I am SO impressed with the hand made prop. Congratulations.....I feel encouraged to try it myself. Tasmaniam oak would be fine I think......I used to fly Wakefields which had VERY specialized props. Very efficient. The pitch distrubution was everything , almost zero at the tips.... ..very technical subject. How do you decide the planform? which if I remember was as important as the pitch distrubution. More please.....

 

 

[pylon500]

some way to determine what typical 'slip'

OK, I'll throw another spanner in the calculations and mention 'Reynolds number'

Too deep to go into and more relevant to making model props....

 

most efficient area of the prop, ie the outer few inches

Sounds like the case at first glance, but reality is that the tips work best while static (I'm talking fixed pitch here),

Once you start to get up any speed, the tips tend to go towards zero relative pitch, thence less power absorbtion and increased RPM.

 

Result is, shorter prop gives less takeoff performance but a faster cruise for the same RPM.

 

Naturally this also appears on a graph curve, so it still needs to be matched to torque, airspeed, aircraft drag, etc, etc.

 

 

[Head in the clouds]

OK - in PART ONE we'd decided to 'tip' the prop to get the revs up, we started with a 72x44 and could only get 5300rpm and ended up with a 68x44 which gave us our desired 5800rpm, so now we're 'right' or are we?

 

PART TWO

 

To illustrate a point it's often best to exaggerate the situation, so with your indulgence... let's say instead of cutting a little bit off each blade, instead we cut half of each blade off...

 

With that in mind we'll go back to doing some simple calculations. You'll remember the tip speed I mentioned in part one - about 500mph. Our 72" prop was supposed to run at 2220rpm which is 5800 engine rpm with a 2.62 reduction gearbox. There are 5280 x 12 inches in a mile = 63360 inches. The tip of a 72" diameter prop travels (pi x diameter x rpm) 3.142 x 72 x 2220 inches per minute = 502217. Multiply by 60 for the number of inches travelled per hour 502217 x 60 = 30133036 inches/hr. Now we divide by the number of inches in a mile and we have 30133036/63360 = 475 mph. So our 500mph tip speed was quite close.

 

But - we cut a total of 4" off the diameter so our tip speed is now 68/72 x 475 = 448mph which is a bit slow for a prop of this solidity (blade area/disk area), so at that tip speed we need a prop blade shape that has more area than the one we chose. So now we've discovered the first loss of efficiency we caused by cutting the tips of the prop.

 

Now we'll go back to the exaggerated example where we cut half the blade length off. Here we have three things to consider. First - we cut off the extreme tips of the blade, which as mentioned in part one, have a special job to do and that is to resist the generation of tip vortices as far as possible, as the tips are washed out (locally reduced in pitch) more than the rest of the blade, so we lost a second load of efficiency just by messing with the tip at all.

 

Next - we have a prop that is only half the diameter that we had before so the tip only has half the distance to travel per rev, than it had before, so at 5800 rpm it will only be travelling at 475/2 = 237 mph and that's not all - before we had (pi x radius^2) of disk area = 3.142 x (3 x 3) = 28 sqft of disk area, now we have 3.142 x (1.5 x 1.5) = 7 sqft of disk area, so we only have a quarter as much disk area which means, without going too deeply into it, that even if the engine could rev happily to double the rpm = 11,600 rpm, to get the tip speed back to 475 mph, we'd still have the problem of not getting any useable thrust because, even at that rpm we'd need twice as much disk area as we have if all other things were equal (but they're not and that's the next issue). So the loss of disk area is the third loss of efficiency, even though it is on a smaller scale in the real rather than exaggerated example, but the problem is that efficiency losses multiply upon each other rather than just adding up.

 

And now comes the real crunch - where we cut the prop blade off at 'half span' for the sake of our example, the pitch of the blade is far greater than its pitch is close to the tip, so if we did actually spin the prop up to 11,600 rpm it wouldn't be doing what we hoped for anyway because the 'new tip' is at a pitch angle that is far too high for that speed so it would be completely stalled until the plane reached, say, 100 mph and it wouldn't get there in the first place without any static thrust. So the wrong pitch at the new tip is the fourth loss of efficiency

 

So you can see that cutting anything at all off the blade tips is a step toward cutting half the blade off and so is accompanied by a large loss of efficiency and increase in drag through the change of tip pitch and the loss of the original tip's non-linear shaping.

 

However it is quite possible to reduce the diameter of a prop without losing any efficiency at all, I'll discuss that in PART THREE and further down the track we need to introduce the effects on non-chordwise airflow. By all means folks please do throw in your thoughts and experiences. And have a stab at working out how to reduce a prop's diameter and then get it back to the original efficiency level - P500 knows this one of course - anyone else?

 

Cheers, Alan

 

 

[Head in the clouds]

I am SO impressed with the hand made prop. Congratulations.....I feel encouraged to try it myself. Tasmaniam oak would be fine I think......I used to fly Wakefields which had VERY specialized props. Very efficient. The pitch distrubution was everything , almost zero at the tips.... ..very technical subject. How do you decide the planform? which if I remember was as important as the pitch distrubution. More please.....

Geoff, I should think Tas oak might be a reasonable timber for props as long as you can find it with straight enough grain. The bigger problem is to get it quarter or rift sawn, and in planks thin enough for laminating...

 

There are many books on the subject of prop design and several Youtube videos about making them. Try Amazon for the design books. If you want a very knowledgeable contact and info about design software send me a PM and I'll put you in touch with him.

 

Sander Veenstra, a friend of mine years ago used to make what some people regarded as the best handmade props in Oz at the time and he always used mountain ash which is a very hard but light wood. It was so hard that he could make the edges extremely thin - and they were positively dangerous for hand starting even a small engine. Also Pylon500 would probably point you in the right direction and it might be worth speaking with Dick Sweetapple who has always been very helpful even though he sells his very nice props as a business. I know he has supplied a few people with pre-laminated blanks to get them started.

 

IIRC you are friends with Werner Bekker? I'm pretty sure he used to make his own props too.

 

 

 

[geoffreywh]

Thanks for the feedback....Yes I am great friends with Werner, unfortunately he is pretty well buggered now...25 years as a para. does that to a person....I already have a Sweetapple prop, 69 x 54. It has a little too much pitch. My best WOT is 2550rpm. I should like a couple of hundred RPM more. I was reading the Bolly website and they are quite clear that the planform is very important plus the tips should be rounded , not square like mine....I shall read further. I have a milling machine that would eliminate that chain saw work!!!!

 

 

[Head in the clouds]

Thanks for the feedback....Yes I am great friends with Werner, unfortunately he is pretty well buggered now...25 years as a para. does that to a person....I already have a Sweetapple prop, 69 x 54. It has a little too much pitch. My best WOT is 2550rpm. I should like a couple of hundred RPM more. I was reading the Bolly website and they are quite clear that the planform is very important plus the tips should be rounded , not square like mine....I shall read further. I have a milling machine that would eliminate that chain saw work!!!!

I must give Werner a call, it's been way too long. Yes the planform is important but not as much so as you might imagine until you start to play with swept scimitar style blades. And straight cut tips work just fine, as do rounded ones. I'm sitting looking at my new Bolly - with it's straight cut tips... (!)

 

Part Three will show you how to easily get your couple of hundred extra revs from your Sweetapple prop, so don't start cutting off the tips just yet... ;-)

 

If you're adept with a chainsaw it's ten times quicker than any other method. Most folks use a hand held circular saw, your milling machine would take you forever because each chordwise cut is at a different angle so your setups would be the time killer. On the other hand if you cut a set of wedges first and hold them in place with your T bolts the mill would be a reliable way to cut your slots, you then need to chisel the remaining lumps out and get to work with the grinder. I use flap disks, I saw that Arthur uses sanding disks - horses for courses.

 

 

[geoffreywh]

Hmmmm.... yes I see you would only get one cut per setting. I had visions of using a adjustable angle milling vice that I have.....Please tell me how to reduce the pitch on my Sweetapple!!I can hardly wait.....Re: Werner !He loves calls from old friends. 58562154.... He has written an autobiography, very entertaining, with much about early days of ultralighting..................regards, Geoff

 

 

[Head in the clouds]

Got caught up with work and almost forgot this thread.... Still busy so this will have to be brief -

 

PART THREE

 

In parts one and two we discussed the evils of cutting the tips off your prop, for whatever reason you might have wanted to do it, probably in an attempt to get more engine revs at WOT. So - how to get the engine revs up without cutting the tips off your fixed pitch prop? First we need to establish what the pitch of the prop is and then we need to work out how much less pitch we need and then we need to change the prop's pitch to that new setting. So this Part Three is about determining what the existing pitch is.

 

Metal (aly) props can be pitch adjusted if you're careful. First you need to determine the section of the prop at about 2/3 of the blade span then you simply cut a profile of your prop's section out of a piece of 12mm steel plate (laser or waterjet cutting is best), weld the plate to a 2m long piece of 25mm roundbar, pad the blade with leather, slip the 'wrench' onto the blade and twist it slightly in the direction which reduces the pitch. Have some kind of apparatus set up so that you can note where you pulled the lever to, swing the prop to the other blade and adjust it the same amount.

 

Now - don't do that... it is how it's done but it's a job for specialists and easy to bend the blade or damage it, I just described it because that's what you do to your timber prop, but not by forcibly twisting it.

 

First you need to establish what the pitch of your blade is, (we'll do all this in inches because props are usually measured in inches but metric works just the same) so you make a chordwise pencil or crayon (not ballpoint) line on the back (flat side) of your blade and then select a central portion of the line and make two marks, say 4" apart (or whatever distance covers about 80% of the chord). Now measure from the centre of the prop hub to the chordwise line. Lets say it's 16" (if the prop diameter is 64"). Then we determine the distance that midspan part of the blade would travel in one revolution. The formula for a perimeter distance is pi x 2r where pi is 3.142 and r is the radius (16" in this case). So the distance of one rev of the half span position is 3.142 x 2 x 16 = 100" in round figures.

 

Now we measure the actual pitch over the 4" marked on our chordwise line. Lay the prop flat side down on a flat surface and use a pair of calipers or dividers to obtain the distance up from the flat surface to each of the marks that you made which were 4" apart. Let's say one measured 1.7" and the other measured 2.9". So we know that the difference between them is 1.2" and that is over a distance of 4". The 4" is just a small portion of the total distance of the travel of the blade at that location, in fact it is 4/100 = 1/25 of the one rev distance. Therefore the amount of pitch of the blade that we measured (1.2") is 1/25 of the total pitch of the blade. So if we multiply 1.2" by 25 we have the blade pitch = 30". So we now know that we have a 64"x30" prop.

 

 

[pylon500]

HitC is going to hate me for this but, when I was describing the prop I made in;

 

https://plus.google.com/photos/113292981019876413104/albums/5573889605855816225?banner=pwa>

 

Once I started running it I found it was over pitched.

 

Initially I trimmed it down in diameter, about 1½" off each tip (picture 14).

 

I was now getting closer to planned static RPM, but during strip runs, it stayed just below, so I needed to 'depitch' a bit.

 

This is done in picture 15 & 16, where I used the old carpenters trick of holding a pencil between thumb and forefinger, while 'gauging' the edge distance with the third (social?) finger, to mark a tapering line on the underside if the trailing edge, starting with nothing at the root, widening to about 3/16" at the tip.

 

I then shaved this rear section away with a small wood plane, leaving a flat edge along the trailing edge.

 

Using a coarse half round file, working from the trailing edge forward, I filed away the underside towards the leading edge, until the trailing edge was nearly sharp again.

 

Once you get the hang of this sort of thing, you can do it with some pretty coarse machinery (big angle grinders), then finish with your choice of varnish again.

 

Trying to add pitch is a little trickier as, shaping the top surface to a trailing edge cut or shaving the underside to a Leading edge cut, will coarsen the 'helical pitch' of the blade.

 

 

[facthunter]

If your prop is overpitched it is easy to do something about it. "tipping" it is the most crude. You can reduce the pitch easily by removing some of the "wood" from the trailing edge on the back and the leading edge on the front, particularly towards the tips of the blades. this has the effect of reducing the actual size of the blades as well as reducing the geometric pitch. . Nev

 

 

[pylon500]

Sort of what I said, but if you haven't changed the angle of the under surface of the blade, then you have only cut down area (which can help) but not pitch.

 

 

[Head in the clouds]

HitC is going to hate me for this.......

No, not at all, you saved me half an hour of writing up part four, and excellently described - thanks!

 

Just a few small things I would add to wrap it up -

 

Don't forget that as you trim away the trailing edge to provide 'meat' to remove for reducing the pitch, you are also reducing the blade area (disk solidity) and that will also have an effect on increasing your revs, so since you are in effect making two load reducing adjustments at the same time you need to be extra careful not to overdo it, so just do a little at a time. If you do overdo it and have to go back the other way i.e. trim away material at the trailing edge so as to have meat on the upper surface to remove to be able to increase the pitch again, then you will be increasing the pitch but once more reducing the solidity so it takes a lot more to load the engine up again, than it did to unload it, that's the big trap for new players...

 

The method P500 described is exactly how to go about it if you have a good feel for how much to reduce the pitch but if you don't, and therefore need to be working to numbers you need to know how much to trim away from the trailing edge. So to use the example we started in Part Three, we have a 64x30 prop and we know (or have calculated) that we need a 64x28. Over a 4" distance at the half span we had a deck height difference of 1.2", and 4" was 1/25 of the distance that the half span point travels in one rev. And those numbers gave us 30" of pitch. See PART THREE in post #20 if you're lost so far..

 

So we want the deck height difference to be 28/30 x 1.2" = 1.12" to reduce the pitch to 28" from its original 30", and so we need to reduce the difference, over that 4" portion of the blade width by 1.2 - 1.12 = 0.08". Now, if the total blade width at the half span point is 5.4" (remember we only used a flat 4" portion of the blade width to take our measurements) then over the whole blade width we need to reduce the deck height by 5.4"/4" x 0.08" = 0.108" (0.1" will be close enough). So now we proceed as described by P500 but instead of trimming (planing) away the trailing edge to an arbitrary line we draw the line for reference but we plane away the edge until we have a squared off trailing edge with a thickness of 0.1" at the half span point. Then carve away the back surface until the trailing edge is sharp again as P500 described.

 

The tips of the prop. Wind tunnel tests and engine torque tests/thrust tests indicate there is no appreciable drag difference whether the tips are rounded or square cut. Square cut is generally employed because they are stronger and less prone to damage, and if they get damaged you can always round them off, whereas if they're already rounded you can't reshape them or go back to square without reducing the diameter. Rounding the leading edge of a square cut tip to produce a scimitar tip possibly provides a very slight improvement to efficiency and seems to be a bit quieter. But - the important thing with whatever the planform shape of the tip is, the bottom edges must be sharp i.e imagine any air trying to roll around the tip from the flat back side of the prop to the curved front side, that first edge that the air has to roll over must be sharp. If it is that alone will prevent any tip vortex becoming locally strong enough to create significant drag. The air on the flat back side of the prop is not flowing straight from leading edge to trailing edge, it is heavily influenced by surface drag and that causes it to be thrown toward the tip so the flow at the tip is both chordwise and radial (spanwise flow) so nothing more than a sharp edge is required to encourage the flow to continue outward rather than curl over the tip into a local vortex. A vortex will form of course, but it will be a percentage of the span beyond the tip and no variation of tip shape will prevent that.

 

Q tips (like winglets) were trialled a couple of decades ago but didn't catch on so presumably there wasn't any significant advantage.

 

While planing away the trailing edge to reduce the pitch a slight bit of extra reduction of the trailing edge at the tip (not enough to round the tip off completely) will result in a bit of washout at the tip and that will also reduce the strength of any tip vortex that may form but it will be a very small overall effect.

 

 

[geoffreywh]

I can see that. In the series of pictures (15 +16) the blade has been recut to a smaller airfoil shape within the outline of the original airfoil . Is this the only way to de pitch a wood prop? I really need to know. .....................................oops my post was a little late. the information required has just been posted. The main problem I have is that my prop is shaped like an EVRA prop. Very tapered toward the tip ( tip width about 2 inches) so I may end up with a very small tip!