pitching props for lap times

I am fiddling with a Prather pitch gauge ....I have not much experience with it because I use a lot of APC props and the Bolly that Randy sent me for my AeroTiger .36 works super.
I am using Bolly 13 1/2 X 4.5 3blade stuff on my PA .65. The aeroplane is an SV at 65 oz. The lap times are pretty consistent at 5.0 using a pipe length and rpm that I am happy with (and what Randy recommends!).
The gauge indicates that this prop (my baseline....) has blades pitched to 4.5, 4.0 and 3.0 at station 13. I am fairly sure that these are accurate readings.

A second, as yet unflown, prop measures 4.5, 4.5 and 3.75 at 13.

A third prop, also unflown, reads 5.0, 4.5 and 4.0.

I hate to lose my "base", but mayhaps I should twist it "base" to 4.0 at each station? And the second to 3.75?
mayhaps the third to 3.5?
My goal is to drop the lap times a bit and to have, effectively, three props...one for "thin" air and a "standard as well as one for "heavy air.
Does twisting to these numbers seem right?
Thanks

See Ya

>I hate to lose my "base", but mayhaps I should twist it
>"base" to 4.0 at each station? And the second to 3.75?
>mayhaps the third to 3.5?

I would preserve your base prop (since you like it) as long as possible. Do your initial experiments on one of the other props, and change only one prop at a time.

I would expect changing the pitch would change the RPM too, therefore changes in lap time might not be easily predictable. Perhaps the pipe helps stabilize things?

Rich- how long are your lines and are they solid or cable? I'm in the same boat, but my plane is heavier.Thanks,

Dale...pretty much full length cables...

See Ya

>I am fiddling with a Prather pitch gauge ....I have not much
>experience with it because I use a lot of APC props and the
>Bolly that Randy sent me for my AeroTiger .36 works super.
>I am using Bolly 13 1/2 X 4.5 3blade stuff on my PA .65.
>The aeroplane is an SV at 65 oz. The lap times are pretty
>consistent at 5.0 using a pipe length and rpm that I am
>happy with (and what Randy recommends!).

Bet that pulls pretty hard!

>The gauge indicates that this prop (my baseline....) has
>blades pitched to 4.5, 4.0 and 3.0 at station 13. I am
>fairly sure that these are accurate readings.
>
>A second, as yet unflown, prop measures 4.5, 4.5 and 3.75 at
>13.
>
>A third prop, also unflown, reads 5.0, 4.5 and 4.0.
>
>I hate to lose my "base", but mayhaps I should twist it
>"base" to 4.0 at each station? And the second to 3.75?
>mayhaps the third to 3.5?
>My goal is to drop the lap times a bit and to have,
>effectively, three props...one for "thin" air and a
>"standard as well as one for "heavy air.
>Does twisting to these numbers seem right?

I'm not sure I am following you, but if you mean set one prop to 4 at each station, and the other to 3.75, then yes, that's approximately right.

I would guess that going from ~4.25" with *extreme* washout to about 4 helical may not be enough of a reduction to get a reasonable lap speed, but there's only one way to find out. So a second prop set to 3.8 helical may be a good idea.

You may find a bit more poop in the corners than you had before, once you get a helical prop. Going down from 4.5 to 3 at the tips is a HUGE amount of washout.

Matt is going through the same process you are, in repitching props. I am curious as to your pipe length, because Matt's is around 17 3/4 with that prop and is where he likes it. This is with 5.3 to 5.4 second lap times. But, by all means get the blades even in pitch. Then that 4 1/2 may be pretty close. Also, although Windy says only the tips matter (or something to that effect) I agree with Brett in that you should set to helical pitch (check at least the hub where it becomes a blade, in the middle, and towards the tip). We like a little more pitch at the tip (maybe a quarter, maybe a little less). I am one who thinks the blades twist in flight (de pitch slightly) which is why this works.

If you get the prop close, then repitching just the hub may work, but I would never repitch just the tip. And, yes, we did test doing just the tip verses the whole blade. There is a tremendous difference.

>Matt is going through the same process you are, in
>repitching props. I am curious as to your pipe length,
>because Matt's is around 17 3/4 with that prop and is where
>he likes it. This is with 5.3 to 5.4 second lap times.
>But, by all means get the blades even in pitch. Then that 4
>1/2 may be pretty close.

At 17 3/4, 4.5 is about right. But Rich is at, effectively, about 4.25 and it's already too fast. I would guess from that that his pipe is significantly shorter than 17 3/4. I run 17 3/8 and I'm only turning 10100 at takeoff.

Rich,
Just to clarify. Are you saying that the Bolly prop as supplied has blades which vary by as much as 3/4" at station 13 or are these three different props?

Here is where I am at currently, and I want to thank you guys because I am "reeling it in", I think.

I pitched one prop to 4.5 at all stations from 4 out. This yielded a 5.1 second lap and plenty of tension all around.

Then I pitched the original to 4.0 and this brought 5.25 laps, about what I hoped, with a launch at 10,200.

I plan to pitch the third to 3.75 or so, or does anyone suggest 3.5? The 4.5 and 4.0 testing has been in fairly cold, dry air, at sea level. My thinking is that lap times will slow as the air becomes less dense?

Thanks

See Ya

>Here is where I am at currently, and I want to thank you
>guys because I am "reeling it in", I think.
>
>I pitched one prop to 4.5 at all stations from 4 out. This
>yielded a 5.1 second lap and plenty of tension all around.
>
>Then I pitched the original to 4.0 and this brought 5.25
>laps, about what I hoped, with a launch at 10,200.
>
>I plan to pitch the third to 3.75 or so, or does anyone
>suggest 3.5?

That's getting down there. I don't know what pipe length you are running, but I would go to 3.75 first and see. 3.5" will mean A LOT of RPM, and the engine will certainly do it, but it will likely be a little noisy.

At some point as you reduce the pitch, you will hit a point where you just can't tweak the needle enough to get it up to speed, and the engine will go "flat". That means the pipe is too long.

>The 4.5 and 4.0 testing has been in fairly
>cold, dry air, at sea level. My thinking is that lap times
>will slow as the air becomes less dense?
>

Yes. But that might still be OK. I can consistently fly slower at the NATs in the hot, humid air, than I can around here.

I can consistently fly
>slower at the NATs in the hot, humid air, than I can around
>here.
>
>
>Brett
>

Isn't this contrary to the thinking on density altitude and lift-making? I understand why the plane might fly slower as a result of power making and reduced prop efficiency. And given renewed power through increased Nitro and optimized prop, I would think the plane might fly a small increment faster in thinner air given the same power, but why is it that it is able to "fly" (I assume up to your same critical standard) slower at higher density alt.?

Thanks

Captcurt

Probably from the resulting loss of engine power and propeller efficiency. Of course you can only let the plane slow down to a point. Then you need to do someting to keep the speed where it needs to be. I use Prop pitch, venturi size, or compression ration, sometimes more than one at a time. Of course being on the east coast where the humidity is higher, I don't have to make too much of an adjustment in mine except where it's cold! You can change pipe length too, but that's a last resort since it's so time consuming.

Hi Jim:

Yes I understand that part--it WILL fly slower--but Brett used the term that he was "Able" to fly it slower--like it was something he was very deliberate about. I also have heard him say they generally bump the nitro some in Muncie to get the engine performance back.

It was the way he said it that got me curious.

Thanks

>Yes I understand that part--it WILL fly slower--but Brett
>used the term that he was "Able" to fly it slower--like it
>was something he was very deliberate about. I also have
>heard him say they generally bump the nitro some in Muncie
>to get the engine performance back.

No mystery, really. It's not like I was running out of lift, even with 3000 feet more density altitude. But for whatever reason, I seem to get more line tension than I do at sea level, for a given level flight lap time. Around here, I typically fly around 5.2-5.3 second laps. In Muncie, I typically can fly a little slower, like maybe 5.3-5.35 or thereabouts, with similar control feel and sufficient line tension. Don't know why for sure, probably the thin air softens the corner entrances and exits (but not the overall radius) and thus bleeds off less right at the entrance points. I also tend to fly smaller at Muncie, just because I'm paying more attention to it, but being slower sort of leads that way, anyway.

Engines tend to run "better"/smoother in those conditions, too. It's like the thin air acts like a filter to knock the harsh edges off a bit. They are obviously way down on power with the same fuel, but that's trivial to fix with more nitro. I've flown with engine setups that seemed to work fine for everyone back East, but was so incredibly harsh I almost couldn't fly them here. Just completely different.

Thanks Brett and Jim:

I've been watching alot of this type input here since I'll be bringing my first piped ship to Muncie this year. Hope to learn alot.

See ya'll there.

Captcurt,

One other thing I forgot. I know Brett doesn't like to do it, but it seems to help my portly plane out in July's heat in Muncie. I'll add about .1 or .2 on the prop tips at the last station. Seems to get that last little bit of vertical performance I'm looking for. Since I have swithced to an RO Jett .61 from the PA .51 I was using that may not be necessary. I guess I'll find out on the first hot afternoon practice flight at Muncie.

>Captcurt,
>
>One other thing I forgot. I know Brett doesn't like to do
>it, but it seems to help my portly plane out in July's heat
>in Muncie. I'll add about .1 or .2 on the prop tips at the
>last station. Seems to get that last little bit of vertical
>performance I'm looking for. Since I have swithced to an RO
>Jett .61 from the PA .51 I was using that may not be
>necessary. I guess I'll find out on the first hot afternoon
>practice flight at Muncie.

I don't mind you doing it, as long as it works. It's a useful technique - just not one that I have found to be necessary most of the time.

Beside, don't you fly in nearly the same conditions *all the time*?

Brett,

Similar conditions yes, but there are some variations from place to place in the east. For example. What works well in the deep south like Alabama usually works at Muncie too, but is not necessary in the same abundance. The air is a little dryer in Muncie than it is in the deep south. Virginia seems to be quite similar to Muncie, except, I think that it can get a little hotter in Muncie since it is pretty much centrally located. However, sometimes those simi Alberta clipper fronts can come through Muncie and really wreak havoc as you (and everyone else) found out last year. Hopefully, no fronts will come through this year! Are you going early and pit for Ted at the WC's?

>Similar conditions yes, but there are some variations from
>place to place in the east. For example. What works well
>in the deep south like Alabama usually works at Muncie too,
>but is not necessary in the same abundance. The air is a
>little dryer in Muncie than it is in the deep south.
>Virginia seems to be quite similar to Muncie, except, I
>think that it can get a little hotter in Muncie since it is
>pretty much centrally located.

You don't understand - the difference between Alabama and Muncie, within a reasonable variation of weather, is *very very small* compared to the difference between Northern California and Muncie. I would consider it close to *negligible*. I used to live there and went to contests all over the Midwest - the difference from place to place is something you could come close to *ignoring completely*.

You are reacting to microscopically smaller variations that I usually discuss. Those midwest setups will react *completely differently* once you get past the Sierras, typically, FAR more aggressive and far too sensitive to vsriation over the day. You know what it's like - you'll go to a contest in the Central Valley and it will be 55 and cloudy at 8:30 in the morning, and 100+ with humidity at <10% by 1 PM. If afternoon conditions from Alabama to Muncie change your engine run noticably, I submit you setup is drastically too senstive to variations.

It's not mandatory to fix if that's what all you do. But we have to handle both with minimal changes and minimal test flying once we get there. We have it licked on the PA - use David's setup, Eather props, and then leave everything *exactly the same* and go from 10% to 15% nitro (in fact, the test to see if it's working is that *you don't have to touch the needle from home when you go from 10% to 15% nitro*). But I never have to change anything at all to accomodate the variations in Muncie weather conditions over the years. or at least anything that seemed to help.

The VF - same thing, run it exactly the same, and you don't even need to change fuel. RO-Jett - too early to tell, but the 15% thing seemed to work reasonably well last year, and at the TT. 4-strokes? Don't ask me!

All seem to be close to 4.5 pitch at stations 4 and 8. I believe that I am pretty much concerned with the tip pitch here?

See Ya

>All seem to be close to 4.5 pitch at stations 4 and 8. I
>believe that I am pretty much concerned with the tip pitch
>here?
>
>See Ya
>
>Peabody

Were these props that somebody repitched at some time? I think I would bring them all back to 4.5 at the tips to match the rest of the blade, and then if they are producing lap speeds that are too fast, I would heat them at the root and twist the whole blade. If you depitch at the root, however, the net effect will be to have less pitch at the tip than at the root. So if you remove very much, you will need to go back and increase the tips just slightly.

Leonard is quite right although if you are going to do the whole blade I wouldn't bother re-pitching the tips initially as you'll only have to do them again later.

I suggest that before you do anything you balance the prop accurately.

I then suggest that starting at station 3 or 4, you mark across the blades with a marker pen at about three more stations (eg 6,10,13), then, doing one blade at a time, set the pitch at successive stations working from the root to the tip, dipping the blade in the boiling water a little past the mark you are working on.

For a bit more vertical performance without increasing lap times significantly, you might want to experiment by tweaking in a little more pitch at the tips. You can get rid of it again quite easily by simply dipping the tips a little at a time without twisting them as the pitch will usually release.

Heat twisted props do tend to have a memory however, especially if you leave them uncovered in the sun so you need to check them from time to time and keep them covered.

You can get excellent results with the above proceedure so my next parragraph is not intended to detract from that.

An alternative though is to buy custom props which have the pitches you specify filed (not twisted) into the prop. Eather props are the only ones I know of like this. The balance and pitch will be dead accurate and they will hold their pitch.

More could be said on twisting technique but a little practice and experience is worth paragraphs of description.

Good luck. It's not as daunting as it may sound.

>All seem to be close to 4.5 pitch at stations 4 and 8. I
>believe that I am pretty much concerned with the tip pitch
>here?
>

Until you really, really know what you are doing, I would strongly recommend setting them to the same at every station. Just changing the tips (effectively, changing the wash-in or wash-out) can be a useful technique in some cases, but it's a different effect than changing the pitch overall.

It's been my experience that the effects of wash-out or wash-in really can work like gangbusters on relatively high-pitch props and 4-2 break motors. I have tried it several different times on TP motors and it always had far too much effect for my tastes, or, put another way, it was too hard to consistently control the effects. You can make similar things happen other ways (like fiddling with the engine) that are much more consistent and not too touchy.

I currently run straight helical -as measured- pitch distributions, and allow the wash-in of the prop camber effects to be whatever it want to be for a particular prop.

And yes, that implies there's not only an iceberg of which you only see the tip, but there's an entire ice field which you can spend many years exploring...

Is this anything like the famous "Fox .15 toss"? So you PA dudes stand around and pitch props for distance to a mark and then bet on lap times and winner takes the pot is that it???

Jake

>Brett,
>
>Only very mildly noticable. I usually compensate with minor
>changes in nitro content, base is 5% nitro for morning
>flying and 6.5 to 7% for hot afternoon flying in Muncie. In
>'Bama a hot afternoon would need 10% nitro. Of course it's
>probably not necessary but I still do it anyway. I have
>managed to get to a point where I can tell the difference in
>airplane/engine performance with small amounts of nitro
>content change. It may not be necessary to change the nitro
>content, but I still notice a difference when I do! Of
>course your variation needs to be considerably different,
>ergo, using the 15% nitro at Muncie from the 10% locally in
>the SF Bay area climate.

It's not just the overall power and runtime - it's also the operating point (i.e. where it is relative to the break point). The key is getting it to stay in a 4-stroke most if not all the time, particularly in the outside part of the square 8. That way you can run all the compression you want and you don't have to care how much break you get.

>I try to keep the amount of nitro in the fuel fairly low due
>to some length of run concerns I have with a 6.5 ounce tank.
> Rich tells me that he uses a 6.5 ounce tank too in his .61,
>but I'm not sure of his nitro content.

If you want to run our setup, better go find a 7.6 oz tank! I run around 7.2-7.4 oz of fuel in the PA, and around 6.6-6.7 on the RO-Jett. Fuel consumption is the same in Muncie - in fact, that's another way to know you have enough nitro. Match the fuel consumption and it pretty much runs the same.

I guess I had better stick with Rich's setup since a 6.5 ounce tanks is all that I can squeeze into the nose.

>I guess I had better stick with Rich's setup since a 6.5
>ounce tanks is all that I can squeeze into the nose.

Is that because it's only 1" thick? I have no problem putting in a 8.5 oz tank in mine, and I can't imagine that your nose is tremendously shorter than mine. But my tank is 1.25" thick.

Yes, it is 1" thick. I havn't looked into getting a 1.25 made for it, but I suppose I could.

>Yes, it is 1" thick. I havn't looked into getting a 1.25
>made for it, but I suppose I could.

They're easy to make yourself. I can send you a template.

Brett,

O.K. I'll take the template.
Please send it to my wife's
e-mail address at home. That
would be [email protected]

Thanks a million.