Prop shapes/camber/progressive pitch - need more pull at the bottom of Vert 8

[Dennis Toth]

Guys,
I'm working though the trim out of a new sport Bipe and all is going pretty well except for the vertical 8. The ship has good speed and line tension going into the maneuver, the total wing area is 600 sq in., @44oz. prop is a narrow blade 11x5, engine is a K&B 40.

OK, all goes well through the inside loop but as it comes out of the upper outside it seems like it go from 600 sq inches of wing to 300 and musses a bit. It's fine every place else in the OTS pattern. I would like to get it to pull over the top tighter so as to maintain forward speed. I'm thinking the answer might be to work with the prop. I know going to larger diameter will help but that would likely slow the turn and may be a wash for the Bipe. I was also thinking about adding some undercamber to an 11x4, thought being the undercamber will hold speed better and re-accelerate better then the flat blade 11x5. Other option are wider blades or 3 blade.  Any thoughts?

Best,    DennisT

[Gerald Arana]

Is this an APC, FG, or wood? Personally I'd go with a "VESS" 11 X 4. The RSM 11 X 6 might also be worth trying.

Good Luck, Jerry

[Scott Richlen]

While you are playing with props, try a Zinger Pro.  Not the regular Zinger, but the Pro.  11-4 or 11-5.  The brand makes a big difference since an 11-4 is not an 11-4 is not an 11-4.  Try a bunch of different brands and see which one works best for this particular plane/engine/pilot.

Scott

[Steve Helmick]

I gotta say it. Try the 11 x 4.5 Thunder Tiger Cyclone. Tim and Tom are using them on their .46LAs.  I used one on my Magnum XLS .36, other NW Skyraider members have used them with excellent results on PA .40UL, OS .40LA, and I've been told they work very well on the Aero Tiger .36...etc. Your K&B .40 is right in that ballpark. Awesome prop, and cheap (Tower Hobby). About $2 each if you buy 'em by the bag.  Steve

[John Leidle]

  APC  11-6

[Dennis Toth]

In modeling we tend to get an idea and just try it. I was wondering if anyone has any information on the use of undercamber props on full size aircraft? Seems we can't be the originator of this type of prop design. It would be interesting to get some understanding of how much undercamber is to much and what the thrust impact might be of undercambered props. I know for stunt several fliers started sanding in undercamber to pipe setups since they had lots of power and needed a way to use it. The undercamber seemed to add pull out of the maneuver and made a few props that were "dogs" into top performers. Question is what are the limits of the performance envelop for our purpose.

Best,       DennisT

[Derek Barry]

In modeling we tend to get an idea and just try it. I was wondering if anyone has any information on the use of undercamber props on full size aircraft? Seems we can't be the originator of this type of prop design. It would be interesting to get some understanding of how much undercamber is to much and what the thrust impact might be of undercambered props. I know for stunt several fliers started sanding in undercamber to pipe setups since they had lots of power and needed a way to use it. The undercamber seemed to add pull out of the maneuver and made a few props that were "dogs" into top performers. Question is what are the limits of the performance envelop for our purpose.

Best,       DennisT

I am not sure if the undercamber does anything more than load the engine (in control line applications). For instance, I cannot run a flat back 13" 3 blade on my PA 75, the engine is too strong and I cannot get it to idle down. With the exact same prop (undercambered) it works great. I would have to use a 13.75" 3 blade if it were flat back and for me that is just too much prop.

For your issues I think undercamber will just hurt you, the engine you are using is not exactly a powerhouse. I think the same diameter and more pitch will solve the problems but you may simply have to fly that plane a little faster than you might fly a mono-wing plane. The problems you are describing sound to me like a lack of power and/or speed.

Adding diameter will tend to slow down the corner but so will changing from a 2 blade to a 3 blade. This can be fixed with a simple handle adjustment or by removing a little nose weight. 3 blades and bigger props are heavier...


Derek

[RandySmith]

Hi  The problem  is  NOT  the prop, this is typical of a BiPlane, when you are inverted, the plane has no where near the lift and normally has to establish a much greater angle of attack to maintain the same.
Does the upper wing stick farther forward than the bottom wing? If so  it will never fly as clean inverted in maneuvers as right side up.

A picture would be good :-)

Randy

[Igor Burger]

I am not sure if the undercamber does anything more than load the engine (in control line applications).

It does, I wrote it many years ago and it is still alive on net on bottom of this page:

http://www.netax.sk/hexoft/stunt/notes.htm

it unloads the shaft when prop pulls (motor does not loose rpm uphill), so it has tendency to compensate slippage, tendency to cancel overrewing in wind and allows motor to deliver more power without overspeeding in level or downhill

[RandySmith]

It does, I wrote it many years ago and it is still alive on net on bottom of this page:

http://www.netax.sk/hexoft/stunt/notes.htm

it unloads the shaft when prop pulls (motor does not loose rpm uphill), so it has tendency to compensate slippage, tendency to cancel overrewing in wind and allows motor to deliver more power without overspeeding in level or downhill

It also  causes  , many times, wind up  in the wind, sometimes very BAD windup in the wind, especially if you have a powerful engine. Loading the motor can get critical with undercambered props, you have to get it just right.
But this still  is not  his problem, the problem is the BiPlane and their tendencies to have much less lift inverted.

Randy

[Randy Cuberly]

It also  cause  , many times, wind up  in the wind, sometimes very BAD windup in the wind, especially if you have a powerful engine.

Randy

Yes that's exactly what I've found with undercambered props.  Less so on Piped engines but still a big factor in my opinion.
I also think in most cases they actually have less drive out of a corner unless the engine is working right at the beginning of it's torque curve, which might mean less RPM and slightly more pitch would be required, to maintain the same lap times, and that is actually contrary to solving the corner drive problem which is usually fixed by less pitch and more RPM!  In my opinion they are simply more difficult to find the proper set up with, and more difficult to deal with under varying conditions, like cold morning and hot afternoons like we often have here in Tuscon.  Just about every situation I've tried to use them in failed to produce better results!

Randy Cuberly

[Igor Burger]

Yes, everything could be done wrong :- )))

[Brett Buck]

It also  causes  , many times, wind up  in the wind, sometimes very BAD windup in the wind, especially if you have a powerful engine. Loading the motor can get critical with undercambered props, you have to get it just right.

     Particularly if it is set for excessive power bursts, which is a very unfortunate tendency, since it feels so macho the rest of the time. Igor's drag bucket effect exacerbates the issue, so you need a different system response than you would with a semi-symmetrical airfoil.

    Some people just never got that and still don't, and stick to the "standard" setup regardless of the prop. Or whether it is helping or not. Pumping up the compression and the nitro until it runs in a solid 4-stroke regardless greatly reduces this tendency. Running across the break is fatal so reducing the compression to "reduce the break" just makes it more prone to the problem. Until you get to the point you have so little power in the 2-stroke part that the performance goes away.

    You need almost no power increase from the engine if the drag on the prop goes down with increasing lift. A way around it is to reduce the diameter until the prop is running in the bucket in level flight. This reduces the abrupt change in the drag and also reduced the feedback to the engine, not triggering the excess power increase. This is what I used around the time of the 2000 NATs, I came in tied for third in my first flyoff - running David's former "break-in prop"!  11.4-3.75 Eather 3B "Red", buzzed/cut down from a 12".

    The "Red" heavily-undercambered prop does this to a much greater degree than the original "purple" with minimal undercamber. The very heavy undercamber worked very well on the 46VF and still works well on the RO-Jett 61 (which runs like a giant shiny 46VF) because of the naturally "flat" response. It took a little work to get the heavy undercamber to work on the PA61, but ultimately David figured it out.

    The other factor is that you can't compare apples to apples when it comes to pitch. The undercambered airfoil exhibits MUCH more effective pitch than is shown with "back of the blade" measurements, compared to flat-bottom or semi-symmetrical airfoils (like Bolly's). More pitch = more whip-up. It also has disproportionate effect on the pitch distribution. There's an SSW post on that topic somewhere.

But this still  is not  his problem, the problem is the BiPlane and their tendencies to have much less lift inverted.

   Or just that they have poor L/D to begin with. Fiddling with the engine and prop can have startling effects, but it's not magic. This is why all the biplane ideas that look "right" (like John Miller's) have relatively high aspect ratio wings.

    Brett

[Brett Buck]

Yes that's exactly what I've found with undercambered props.  Less so on Piped engines but still a big factor in my opinion.
I also think in most cases they actually have less drive out of a corner unless the engine is working right at the beginning of it's torque curve, which might mean less RPM and slightly more pitch would be required, to maintain the same lap times, and that is actually contrary to solving the corner drive problem which is usually fixed by less pitch and more RPM!  In my opinion they are simply more difficult to find the proper set up with, and more difficult to deal with under varying conditions, like cold morning and hot afternoons like we often have here in Tuscon.  Just about every situation I've tried to use them in failed to produce better results!

  Go over to SSW and look up Igor's drag bucket observation, I think that can explain it. You have to have a system that always runs in the same state, not around the non-linearities in the L/D.

    Brett

[Randy Cuberly]

  Go over to SSW and look up Igor's drag bucket observation, I think that can explain it. You have to have a system that always runs in the same state, not around the non-linearities in the L/D.

    Brett

Hi Brett,
I tried what you suggested but couldn't find what you were talking about...I never seem to have the right "magic words" to find anything on SSW.  Any suggestions?

Randy Cuberly

[RandySmith]

Most Biplanes are  rather  "draggy" and actually need to have the engine hit harder than a normal single wing stunt ship , you will  need to tune your setup for the Bipe... and normally you will need a more than average powerful engine.. unless you just want it to stall and flop around like a Slob an a FOX 35
You will find a higher RPM with a lower pitch prop will be your friend with the BIPE setup. Also you may try a little faster airspeed with longer lines will also be a benefit

Randy

[Dennis Toth]

The Bipe is called the YAK-YAK which I built originally in 1992 ish for a Biplane event we were having at the GSCB contest that year. Every one was building 38 Specials (which is a very good flying Bipe), what fun was that so I built mine from two Yak-9 wings (I know flys twice as bad). Only difference I added 1/16" sheeting and cap strips so it is about 1/8" thicker wing then the standard Yak-9.

On the vertical 8, I think part of the problem was my approach to the outside part of the maneuver. I tried to open up the top and should have pulled it at the intersection and let it go further back overhead, if it didn't complete the outside with enough air just continued back around to the top. I waited at the intersection and it got way back and then wound up kinda side ways which caused it to be lower on the outside and required a tighter pullout. OK will fix that.

I did make up an 11x4 with undercamber added so I'll give it a try and see how it works.  The motor is very well behaved and runs a very constant rpm with quit a bit room to increase rpm if needed. We'll see.

Best,       DennisT

[Brett Buck]

Hi Brett,
I tried what you suggested but couldn't find what you were talking about...I never seem to have the right "magic words" to find anything on SSW.  Any suggestions?

   Use different magic words?

   http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=48203&mesg_id=48203&listing_type=search#48214

   Brett

[Randy Cuberly]

Thanks Brett,
That really is interesting and an excellent explanation of some of the problems I've seen with undercamber props.
The mention of being sensitive to changing conditions agrees with what I've seen here in Tucso where we sometimes experience density altitude differences of 3000 ft or more between morning and afternoon at 105 degrees!  It explains why adding nitro helps but is not a panacea! 
I'm going to try a couple of things this weekend with undercambered props on my PA65.  Previously I had dismissed them as no improvement and more difficult to tune...I may still be right but...I like to fiddle anyway!
I guess I really need to "library" a bit and learn more aerodynamic facts.  Typically I've just left them to others but I think I'm developing a little more interest now!

Thanks again...also to Igor, of course!

Randy Cuberly

[Dennis Toth]

Guys,
One other issue that I have addressed is the CG location. Originally I had a 2 1/2oz muffler that I changed out for a 1/2oz tongue muffler. The engine didn't care but this moved the CG back maybe 3/8". I then changed out the old 1/2oz fiberglass pushrod with a 3/16"CF rod that is 1/4oz. This moved the CG back forward 1/4" so it was still ~1/8" back from were it was originally with the heavy muffler. Flying it last weekend it was flyable but both upright and inverted it wanted to fly at 15ft in level flight, I could fly it down to 6ft but any little bump in the air and it would bounce (basically it was work). I added a 1oz nose washer and it was better but still work. Luckily this ship had at one time had an OS 40FS in it which required the mount holes to be moved forward about 1/4" to clear the old CC plastic spinners. The holes all line up so I've moved the K&B forward and will try it this weekend. Odd that the 1/4" movement of the 10oz engine didn't move the CG much but I will start with the 1oz washer in place and then try it without to see if it is needed.

Question, I know the basic CG trim procedure but never messed with it to much (lazy) to see how far back one should go before turn performance falls off. I assume that if it is to far aft it will tend to tighten loops and way far aft it will hedge hop. How far do you go? Should it be a little bit uncomfortable in level flight?

Best,   DennisT

[john e. holliday]

Re-adjust your handle for level flight.   I have to do it as I seem to hold some up on the handle.  Yes, I have had to re-adjust the handle on my planes after the first flight to keep from fighting it to keep it what is called normal level flight. 

[Randy Cuberly]

As the CG is moved back for better response you will also probably find it necessary to reduce the line spread at the handle...also if you're using a cable handle the added response will be more difficult to control accurately due to response delay in handle input.  (handle input and control response get out of phase and you have hunting.)
This is one of the main improvement factors in hardpoint handles.  Fine tune the CG and the line spread at the handle together.

Randy Cuberly

[Dave_Trible]

    Particularly if it is set for excessive power bursts, which is a very unfortunate tendency, since it feels so macho the rest of the time. Igor's drag bucket effect exacerbates the issue, so you need a different system response than you would with a semi-symmetrical airfoil.

    Some people just never got that and still don't, and stick to the "standard" setup regardless of the prop. Or whether it is helping or not. Pumping up the compression and the nitro until it runs in a solid 4-stroke regardless greatly reduces this tendency. Running across the break is fatal so reducing the compression to "reduce the break" just makes it more prone to the problem. Until you get to the point you have so little power in the 2-stroke part that the performance goes away.

    You need almost no power increase from the engine if the drag on the prop goes down with increasing lift. A way around it is to reduce the diameter until the prop is running in the bucket in level flight. This reduces the abrupt change in the drag and also reduced the feedback to the engine, not triggering the excess power increase. This is what I used around the time of the 2000 NATs, I came in tied for third in my first flyoff - running David's former "break-in prop"!  11.4-3.75 Eather 3B "Red", buzzed/cut down from a 12".

    The "Red" heavily-undercambered prop does this to a much greater degree than the original "purple" with minimal undercamber. The very heavy undercamber worked very well on the 46VF and still works well on the RO-Jett 61 (which runs like a giant shiny 46VF) because of the naturally "flat" response. It took a little work to get the heavy undercamber to work on the PA61, but ultimately David figured it out.

    The other factor is that you can't compare apples to apples when it comes to pitch. The undercambered airfoil exhibits MUCH more effective pitch than is shown with "back of the blade" measurements, compared to flat-bottom or semi-symmetrical airfoils (like Bolly's). More pitch = more whip-up. It also has disproportionate effect on the pitch distribution. There's an SSW post on that topic somewhere.

   Or just that they have poor L/D to begin with. Fiddling with the engine and prop can have startling effects, but it's not magic. This is why all the biplane ideas that look "right" (like John Miller's) have relatively high aspect ratio wings.

    Brett

A great discussion,  thanks Brett.  Though I haven't worked with my under cambered props very much ( didn't like them very much so they stayed in the prop box) it explains some of the issues.  I noticed an occasional vibration under load on the .76 with one of these I attribute to tip flutter.  I think they aren't as ridgid as the flat back props.  Maybe I'll play with them some more...later.....much later...

Dave

[Brett Buck]

I'm going to try a couple of things this weekend with undercambered props on my PA65.  Previously I had dismissed them as no improvement and more difficult to tune...I may still be right but...I like to fiddle anyway!

  I don't think the 65 is any more tricky to deal with than the 61 or 75. But you may have to deviate from the basic setup with undercambered props because you have some version of Igor's effect* enhancing the pronounced power change you might otherwise get. That's why the U/C props  works such a treat on the 46VF. That's where they were developed, Ted more-or-less figured out what to do with Brian's props and the 46VF with Randy's AAC system. The 46VF has *much less* tendency to change power based on load than the PA40/61/65 per the standard setup, so you dob;t have both effects working at the same time. The PA51 was a completely different breed of cat and ran utterly differently, presumably due to the intake timing difference, and it was killer with David's system  - and still has the highest static thrust of any competitive system we have run, for what good that gets you. It also explains why the electric props with heavy undercamber seem to have far better airspeed control than you might expect.

   Most of this had been figured out before we ever saw Igor's observation, and we had more-or-less no idea why what we were doing worked, it was entirely cut-and-try. But seeing it was a real revelation, it explained most of what we had been seeing.

   This is why we have always run a different setup on the 61, it started with David dealing with the power changes while using heavily undercambered props. Ted figured out how to most effectively use them and David figured out how to set the PA40 and 61 up to best use them. The solution on the 40 was to reduce the diameter so much that it was on the 'positive slope' side of the curve all the time, which also had the beneficial effect of reducing the feedback to the engine. I never found a PA40 system that I was satisfied with, and eventually went back to the 40VF (between flights at the 97 NATs) and ran that until the PA61 got figured out - which solved 95% of the issues I had with the 40, mostly by being so much larger that it could run in a 4-stroke all the time, thus avoiding the power change associated with the 4-2 break.


On the topic of density altitude, while I have no valid  mechanism or explanation for it, the power-change effects seems to follow the *humidity* far more than the density altitude. We get the same density altitude changes you do (2500-3000 feet from morning to afternoon) and they are far more than going from here to the midwest. And of course I have flown real airplanes at Tucson for comparison. Other than the absolute altitude changes, my engine runs about the same in Tucson as it does here. The temperature/density altitude changes have minimum effect and it seems very predictable.

     It runs completely differently in Muncie (and Lawrenceville, back in the day). The density altitude range is much less, and is usually right in the middle of what we get here. The only thing that is genuinely different is the humidity, which is of course much higher. In these conditions, the power variation is *far less* and some of the things we consider problems are greatly moderated, or seem to go away completely. Once you get the nitro set correctly, it tends to run much more steadily than it ever does here. In particular, the little irritating items like breaking in a 2-stroke on outsides particularly the outsides in the square 8, tends to be tremendously moderated compared to home air. The opposite it also true, MANY people have perfectly good systems in the southeast and find them to be far too aggressive when run here in 20% humidity.

    If you can get it to work here, it seems to be better everywhere else (once you correct for the absolute altitude with nitro).

   Brett







 (note that the real airfoil isn't a Gottingen 342, so the data posted is not necessarily quantitatively accurate - but certainly seems to be reasonably representative)

[Dennis Toth]

Got out to the field today and got a couple flights. The balance seems pretty good and on the first flight I went in on the needle which brought the lap time to about 4.7 ish (a little quick for me). Anyway with the CG in corrected the ship felt solid on the lines. There may be a tad to much tip weight as it seems to hinge a little I'm working on it. I did the first flight with the 11x5 that I had been using to get a base flight. All went well then we got to the vertical 8. The first one was a little slanted then I decided I was just going to go for it and it was fine, had lots of smooth turn and no mush.

I then changed to the undercambered 11x4. I opened up the needle a quarter turn and went a little to far  as the lap time drop to 5.4 ish and it was pretty soft. The wind came up and I didn't get a second flight with a better setting, I will give it another try next weekend.

One thing I want to look at is the leadouts. I originally built it with solid leadouts and I'm thinking they may be causing a bit of stickiness and a little hedge hop in level flight. I'm thinking that since the bellcrank and leads are open it would be easy to change them to cable which would have more flexibility. Any thoughts?

Best,          DennisT

[Randy Cuberly]

One thing I want to look at is the leadouts. I originally built it with solid leadouts and I'm thinking they may be causing a bit of stickiness and a little hedge hop in level flight. I'm thinking that since the bellcrank and leads are open it would be easy to change them to cable which would have more flexibility. Any thoughts?

Best,          DennisT

Hi Dennis,
I have solid leadouts in the Whitely Shoestring I'm flying right now, and they can cause a little stickyness sometimes.   This is a 12 year old airplane Resurrected by Bob for me to fly after I lost my GeoXL.  I have discovered that a very small squirt of Rem Oil (very light gun oil marketed by Remingto Arms Co) on each leadout where it passes through the leadout guide gives instantly slick movement of the leadouts and controls.  This oil also contains a active solvent so it disolves any crud picked up over time.  I of course would stress light application from an aerosol can (it comes packaged that way) through the fine plastic tube supplied with the can.  Obviously you want to minimize spraying oil onto the balsa in the wing.
At any rate this does work like a miracle and might prove easier than replacing the leadouts.  It will also instantly tell you if the friction is there at the leadout guide or somewhere else in the system.

I'm in the process of covering and painting a new Whitely Shoestring right now...It's really an excellent flying airplane, with no bad habits and is very easy to trim.  No this one doesn't have solid leadouts...the Rem Oil will go back to the gun bench!

I really like the look of it too!

Randy Cuberly

[Randy Cuberly]

 I don't think the 65 is any more tricky to deal with than the 61 or 75. But you may have to deviate from the basic setup with undercambered props because you have some version of Igor's effect* enhancing the pronounced power change you might otherwise get. That's why the U/C props  works such a treat on the 46VF. That's where they were developed, Ted more-or-less figured out what to do with Brian's props and the 46VF with Randy's AAC system. The 46VF has *much less* tendency to change power based on load than the PA40/61/65 per the standard setup, so you dob;t have both effects working at the same time. The PA51 was a completely different breed of cat and ran utterly differently, presumably due to the intake timing difference, and it was killer with David's system  - and still has the highest static thrust of any competitive system we have run, for what good that gets you. It also explains why the electric props with heavy undercamber seem to have far better airspeed control than you might expect.

   Most of this had been figured out before we ever saw Igor's observation, and we had more-or-less no idea why what we were doing worked, it was entirely cut-and-try. But seeing it was a real revelation, it explained most of what we had been seeing.

   This is why we have always run a different setup on the 61, it started with David dealing with the power changes while using heavily undercambered props. Ted figured out how to most effectively use them and David figured out how to set the PA40 and 61 up to best use them. The solution on the 40 was to reduce the diameter so much that it was on the 'positive slope' side of the curve all the time, which also had the beneficial effect of reducing the feedback to the engine. I never found a PA40 system that I was satisfied with, and eventually went back to the 40VF (between flights at the 97 NATs) and ran that until the PA61 got figured out - which solved 95% of the issues I had with the 40, mostly by being so much larger that it could run in a 4-stroke all the time, thus avoiding the power change associated with the 4-2 break.


On the topic of density altitude, while I have no valid  mechanism or explanation for it, the power-change effects seems to follow the *humidity* far more than the density altitude. We get the same density altitude changes you do (2500-3000 feet from morning to afternoon) and they are far more than going from here to the midwest. And of course I have flown real airplanes at Tucson for comparison. Other than the absolute altitude changes, my engine runs about the same in Tucson as it does here. The temperature/density altitude changes have minimum effect and it seems very predictable.

     It runs completely differently in Muncie (and Lawrenceville, back in the day). The density altitude range is much less, and is usually right in the middle of what we get here. The only thing that is genuinely different is the humidity, which is of course much higher. In these conditions, the power variation is *far less* and some of the things we consider problems are greatly moderated, or seem to go away completely. Once you get the nitro set correctly, it tends to run much more steadily than it ever does here. In particular, the little irritating items like breaking in a 2-stroke on outsides particularly the outsides in the square 8, tends to be tremendously moderated compared to home air. The opposite it also true, MANY people have perfectly good systems in the southeast and find them to be far too aggressive when run here in 20% humidity.

    If you can get it to work here, it seems to be better everywhere else (once you correct for the absolute altitude with nitro).

   Brett


 (note that the real airfoil isn't a Gottingen 342, so the data posted is not necessarily quantitatively accurate - but certainly seems to be reasonably representative)

Thanks Brett,
The set up I'm running now on the PA65 runs a constant fast 4 cycle and does not break with a fairly long pipe setting and a lot of nitro (15 to 25 % depending on the temp etc) and a relatively large venturi...it is very steady running and has lots of power and never breaks and regulates so well that wind up is minimal or non-existent...

The prop I use most is a Bolly 12.5 - 4.5 three blade...but it seems to run nearly identically on a Bolly 13 - 4.25 Three Blade with about the same lap times with a slightly leaner needle setting but the same RPM range.

I have several undercambered props in that same general size and pitch range...what do you think?

Randy Cuberly

[Dennis Toth]

Randy,
Thanks for the gun oil suggestion. I will clean them again with the Scotch Brite pad then add the gun oil. It is worth the try as I too have used solids for many years and never noticed a problem. Could be I've just learned a thing or too and recognize things that I missed in the past. Worth a try.

Best,        DennisT

[Dennis Toth]

Has anyone tried different blade and tip shapes on undercambered props? Seems that long narrow UC blades with either fairly pointy tips or the swept back or angle tips might influence how the UC works. It was pointed out earlier that some props were/are available with either a deep or shallow UC. How much are we talking about for deep vs. shallow?

Best,     DennisT

[RandySmith]

Has anyone tried different blade and tip shapes on undercambered props? Seems that long narrow UC blades with either fairly pointy tips or the swept back or angle tips might influence how the UC works. It was pointed out earlier that some props were/are available with either a deep or shallow UC. How much are we talking about for deep vs. shallow?

Best,     DennisT

Some  Under camber  goes from 3/16 inch deep  to  just about 1/32 inch in both wide and N blade, and  YES  I have tried most every shape, aspect ration, and scimitar tips, swept back tips , rounded, rounded scimitar, square  etc....... not to mention the other hundreds of things that can be changed on a prop to affect performance, and make the flight envelope large  or very small, like  progressive or regressive pitch , bend blades curved blades true pitch all along the blade  yada yada.....

Randy

[Steve Fitton]

... The PA51 was a completely different breed of cat and ran utterly differently, presumably due to the intake timing difference, ...

   Brett

Interesting, I never knew the 51 was a different cat, at least as far as timing.  It sure is crazy strong though, I was flight testing one a few days ago and it seemed every bit as strong as my 61 setup, even to the extent of using a prop (Bolly 12x4.25 3bld) that I have used on the 61.  I uses even more fuel that the 61 as well.

[Randy Cuberly]

Interesting, I never knew the 51 was a different cat, at least as far as timing.  It sure is crazy strong though, I was flight testing one a few days ago and it seemed every bit as strong as my 61 setup, even to the extent of using a prop (Bolly 12x4.25 3bld) that I have used on the 61.  I uses even more fuel that the 61 as well.

Having used a couple of PA51's and a PA65 in the same very heavy (72 oz) Trivial Pursuit for many years, I can attest to the power of the 51.  I believe the 51 to be about as powerful as the 65, however it does so over a narrower range of settings and is less forgiving to "state of tune", needle valve setting, pipe length, prop, etc..  As mentioned before with similar power output the 51 used about 1/2 oz more fuel than the 65 and needs a little less prop, and demanded more nitro for a given condition (at least here in Tucson heat).  
The 65 on the other hand is much more even tempered and runs very well with a much broader range of settings on the needle, pipe, prop, and fuel.  It seems to be a fiddle proof engine.
I would never criticize the 51, but for an all out competition setup the 65 is a better choice in my humble opinion...if there is such a thing as a bullet proof engine, it's the PA65.

There may be better regulating electric systems around than a PA65, and I'm ready to believe that the very top guys may have some, but quite honestly of the ones I've seen they are more trouble and do not work any better or as well in regulation for stunt than a PA65 on a pipe!  When the wind blows it's like magic!

Maybe I'm just a dogmatic old fool but I have been doing this for a while (since 1951) and while I don't claim to be world championship material, I do fly fairly well, and have been doing this long enough to know what works and what doesn't and has to be crutched with talent.   That's my opinion for what it's worth...maybe not much.

The only thing wrong with a PA65 is availability!  Randy S. needs to make some more!   
Yeah I pretty much understand all the problems with that!

Randy Cuberly

[Scott Richlen]

I've got a side exhaust PA-51 in my Foghorn-Leghorn Legacy and it is one powerful engine.  It seems to have more power than my PA-61 rear exhaust and that engine is no slouch!  All you really need is a couple of LA-46s for the cheap seats and a couple of PA-whatevers for the box seats and you got the best of both worlds!

Scott

[Brett Buck]

Has anyone tried different blade and tip shapes on undercambered props? Seems that long narrow UC blades with either fairly pointy tips or the swept back or angle tips might influence how the UC works. It was pointed out earlier that some props were/are available with either a deep or shallow UC. How much are we talking about for deep vs. shallow?

   Quite a bit, the shallow "purple" were about 1/3 the depth of the later "red" types. I think Brian sanded the undercamber into purple type by hand. The red was made that way to start with. I never ran the purple myself, but I did hand-sand the UC into a flat-back prop to reproduce the shallow type at the 94 NATs, after Ted whacked the last of the Brian ones nosing over on launch. There is probably still black dust coming out of the white couch cushions at the Residence Inn, Lubbock.

    David figured out how to make best use of the deep undercamber on the 61. They were a bit much for the 46VF.

     To get the same effective pitch, the purple needed to be about .25" or so less than a flat-back, and maybe 3/8" less than a Bolly. The red was more like 1/2" lower, but it also acted like it had way more pitch at the tips. That's also what it measured after I took the camber into account. A 12" Bolly that measured 4.25 acted like it had about 5" of pitch at the tips. The red heavy undercamber that measures 4.0 acts like it has about 6+" of pitch at the tips. We really didn't figure out what was going on until Igor posted his observation, and I used Joe Supercool's camber/AoA relationship to come to the realization that the effective pitch distribution was wildly skewed towards the tips.

 
    Brett

[Dennis Toth]

Randy'
You indicated that you did try all these different shapes, widths and cambers. So what ones worked and what didn't?

Brett,

You have explained the impact on pitch that the different prop have and that is good information for those using them. Could you tell us what the depth of the undercamber is on the different props - 1/64; 1/32; 1/16. How much did you sand in on the one made up?

The ones I have made up have 1/64" max depth sanded in.

Best,     DennisT

[RandySmith]

Randy'
You indicated that you did try all these different shapes, widths and cambers. So what ones worked and what didn't?

Brett,

You have explained the impact on pitch that the different prop have and that is good information for those using them. Could you tell us what the depth of the undercamber is on the different props - 1/64; 1/32; 1/16. How much did you sand in on the one made up?

The ones I have made up have 1/64" max depth sanded in.

Best,     DennisT

Dennis  the  more important question was which ones worked best, and were easier to setup, and which ones had the largest "envelope" that it would fly in, nothing is a simple answer, but the slight U/C props worked best all over the country , with differant seasons and weather. They had the widest "envelope " of any of them, and performed with the best of them. that said you REALLY  need to match the prop and the style of the prop to the motor, for example, the wide heavy under cambered 13 x 4 worked wonderful on the PA 65,(near 1/8 inch of U/C cut into it ) the load was just right, and that U/C prop was very draggy and requires a lot of power to turn, and cycle. it worked best in a very strong 4 cycle, with just beeps of 2 at the top. The 61s had a very hard time pulling that prop, so it needed to be trimmed down quite a bit, at least an inch.
The 3 blade slight under camber worked best on the 51 and 61 at 12 inches, These blades are much higher A/R than the wide 2 blade (between a 64th to a 32nd in cut into it) .
The same profile U/C 3 blade in a 13.75 x 4.5 or 14 x 4.5 worked very very well on the 75... and still does.
The slight U/C 15 x 4.8  2 blade ( 1/32nd to near 3/32 U/C cut into it) is killer on the 75, however you must have a plane that can handle that prop, or you need to trim it to 14 inches.
The piped 40s that were strong, worked very very well on a Narrow 3 blade  at 11 inches  (maybe 1/64 in UC), very consistent and very constant speed. especially in the high winds.  The other prop that was almost universal was the Bolly slight under camber 12.25 x 4.25  trimmed to between 11.2 to 11.7. that depends on what motor you were using. You need to really be careful  NOT to overload the 40s with prop, you will find it harder to needle and will get richer as you take off, again making it difficult to hit the needle setting just right , also many found it would wind up in the winds if over propped and would be very touchy to get a break right. with too much of a break coming at times, trimming the prop down closer to 11 inches worked much better for the less stout motors and would result in a much more consistent runs, with almost every motor These ran in the 11,000 or slightly over range, and would also work very well in a solid 4 cycle, or 4 with just a beep of 2 cycle at the tops.
Tip shapes vary wildly, I found a fat scimitar tip worked very well for my use, it seem that it had the best characteristics of  the slightly longer prop, with the less load of the smaller cut down prop , a swept back slight rounded tip, sort of like the VECTOR  wing tips worked well, and like the fat scimitar shape, was quieter .
I will try to write more  later, this could take many hours  to tell most of this.

Randy

[Brett Buck]

Randy'
You indicated that you did try all these different shapes, widths and cambers. So what ones worked and what didn't?

Brett,

You have explained the impact on pitch that the different prop have and that is good information for those using them. Could you tell us what the depth of the undercamber is on the different props - 1/64; 1/32; 1/16. How much did you sand in on the one made up?

The ones I have made up have 1/64" max depth sanded in.

Best,     DennisT

     Where on the prop are you talking about?

   Brett

[Dennis Toth]

Randy,
Thanks for sharing that information, I think lots of people will find this type of information interesting and useful. If you can go into some information for non-piped set-ups that also be useful. Thanks again.

Brett,
I measured this at about the 70% out from the hub on the blade. My undercamber is just a two and half inch dowel with 100 grit paper on it. Sanding down the middle of the blade so it is simple constant radius to the bottom of the blade.

Best,      DennisT

[Brett Buck]

Randy,
Thanks for sharing that information, I think lots of people will find this type of information interesting and useful. If you can go into some information for non-piped set-ups that also be useful. Thanks again.

Brett,
I measured this at about the 70% out from the hub on the blade. My undercamber is just a two and half inch dowel with 100 grit paper on it. Sanding down the middle of the blade so it is simple constant radius to the bottom of the blade.

  When I get a chance I will take some readings at various points along the blade on a "red" 12-4 3B. I don't have a "purple", Brian stopped making them since they required a lot of hand work, and replaced them with the red where it was undercambered in the mold. Ted may have the last one (which is the one I made). You'll just have to take my word for it.

   When the Eastern Block guys were making Brian Eather copies, I requested that they make a copy of the shallow type, but I didn't have one for them to use for a mold.

   When I did it by hand, I used 80 grit on a bit of cut-off header tubing, got a groove along the LE and TE, then dug out the middle. It turned out WAY better than I expected, it looked and measured the same as the original, done by the Master himself (Brian). I thought about using something of larger radius but I was concerned about what would happen if I got is skewed a little. The way I did it, I at least had some control over where it when, and could easily keep it away from the edges by using my finger as a guide along the edge. To open it up in the middle, I skewed the tubing sideways but stayed well away from the LE and TE until it was very close to done.

    Brett

[RandySmith]

Randy,
Thanks for sharing that information, I think lots of people will find this type of information interesting and useful. If you can go into some information for non-piped set-ups that also be useful. Thanks again.

Brett,
I measured this at about the 70% out from the hub on the blade. My undercamber is just a two and half inch dowel with 100 grit paper on it. Sanding down the middle of the blade so it is simple constant radius to the bottom of the blade.

Best,      DennisT

Hi Dennis

Much of what I wrote was about engines running on a muffler, doesn't seem to matter much pipe or muffler, it still applies, example the Aero Tiger runs a 11.25 x 4 standard slight U/C prop at about 11,000 RPMs in a 4 cycle, it works really nice, the same Aero Tiger runs a very Wide heavy Under camber 10.6 x 4.9 2 blade and works again really well , This is one people call the Werwage , or Phelps prop.
Both of these props are being run by 100s of pilots all over the world. The AT 36 also works very well with a 11 x 4 N (narrow blade) 3 blade prop with slight U/C, Deeper uc on the same prop and the motor doesn't do well. too much drag, I have not worked past 10 3/4 but I suspect If I had to run that prop it would need to go down to 10 inches length.
Same applies to the side exhaust muffler PA 51s 61s 65, 40s, They can turn the same higher RPMs that the piped motor turns, although most people run 1/2 more pitch with them on mufflers.

Randy