Ok, a little background first. The delta in the pictures below has been flown many times with the Norvel Start Up .061. Non-throttled, cheapest engine on the shelf engine. I've been flying it with a Master Airscrew 6x3. It flew very well. Slow and stable like a trainer. I had my black box in it for a while and it showed 40mph in a straight line. (no dive for speed first) Wednesday I decided to experiment with the prop. I had an old cox thimble drone 6x3 prop sitting around, so I took the scissors to it and cut it down to 5x3. Of course, the black box was not in and it performed spectacular. Just eyeballing it I would claim 75+mph without a dive for speed. The engine was spinning over 20,000rpm after it unloaded in the air. (again, just guessing.) That doesn't seem safe to do with the old plastic props.

Q. Does the increased tip pitch on a cut down prop increase speed and thrust?

Q. Are there safer props to use for this?

Well you have 2 good things happening at once with the cut down Cox I think.Number one the engine must be happier with the 5x3 and number two the Cox has alot more blade area than the Master I'm Screwed props. A Norvel just does not like to lug a bigger prop like a cox TD can. Just my 2 cents. Awsome looking delta you have there!

Later,
Tim

In my tests and all I've seen in magazines cutting down the prop will reduce thrust but increase speed. With a slippery airframe you may not need the thrust just the flying speed over the controls. Although the thrust may be greater with the larger prop the plane may feel mushy because of the slow speed. Most props efficency is best at a tip speed of 325-375 mph. Narrow tips will not provide the same thrust a wide blades but do usually spin faster.
Make sure that when you cut down a prop you shorten both blades PRECISELY the same and then balance the prop.

Ptuey:
First a quick note. If you start with a 14x6 prop and cut it to 6" diameter it's still a 6" pitch, provided the blades were shaped accurately you end up with a 6x6 prop. When you decrease the blade area you have decreased the engine load, and rpm will go up with the same power to drive it. In theory, if you increase the rpm by 50% you increase the speed by the same amount. In the real world it doesn't work that way. The larger diameter you can run, the more efficient it will be. This is a generalization ignoring speed, sometimes you can get more speed by decreasing diameter even though the prop is not as efficient as the longer one might be.

I'd be leery of the old Cox props, they were just plastic. For normal rpm ranges they are fine, going to 30K and higher I'd rather use wood or the FRP ones like the MAS and APC.

Now let's get down and dirty. How fast do you want to go, and what rpm do you think your engine will pull? Let's try 80 mph and 40K rpm. Your 3" pitch prop, at 100% efficiency will give you almost 114 mph, real world efficiency of 70% puts you right at 80. Now take that 5x3 and start trimming the blades until you get about 35K rpm static. What? It blew up at 27K? OK, then we need to pitch for 80 at 25K rpm. Believe it or not, just by going to 3 1/2" pitch at 70% we get a little over 80 mph at 25K. Now start trimming the prop again, this time stop about 20K static. If there's enough prop left to get the plane off the ground the engine will unload in the air, and if you do get that 70% efficiency you'll get your 80 mph. But more likely it wont fly at all. Just not enough prop left.

I'd suggest the best way, really, is to get a 5.5x3.5 prop and fly it. Cut it down a little bit at a time between flights, balance it, of course, when you don't get any further improvement either leave it alone, or cut another to the size you had on the flight before. Props that size are cheap enough to ruin one or two.

But I really don't think that Norvel "Start-up" is going to have enough power to get a lot of speed. But I have been wrong before.

Bill.

That's a very nice looking plane! I bet you 90 cents that if you cut down a 6-4 COX or MAS to about 4.5" or 4.25" it will work. If it doesn't, I owe you about 90 cents, but save the prop for a AME 061 .

I know Dickeybird likes the cox 5x3 props. He must have 20 of them hung on a wire on his toolbox.

Let me confuse the issue a little further. When the pitch is determined for a prop it is determined for a whole prop, right? So, if you cut a prop down the tip has a greater angle than it otherwise would have. Plus it has a much wider tip. Would that change the effective pitch? I'm looking for an explanation for the speed increase that occured just by cutting 1" off the prop I was using. I know I'm getting more rpm and that accounts for some increase, but I'm not getting double the rpm.(I am getting double the speed)
Since nobody said "TAKE IT OFF BEFORE IT KILLS YOU" I'm going to fly it this sunday with the cox.

Bill,
I'm not really looking for ultimate speed. In redneck "I liketa pooped my pants when it happened!"(must be said with tobacco in mouth) It was alot of fun to see the jaws drop when I dived and flew it down the runway. I'll look at getting some "real" props.
CP,
Where are you buying props for 90 cent?! Tower wants almost $3 each for the cox props. The LHS is a little cheaper, but the 1 1/2hr drive is a little expensive. Keep trying for the speed record. I've got to practice flying a fast 1/2a for a little while. I've had two of these deltas with 15LA's on them. They flew completely different. (both RIP)

Ptuey:

Pitch and diameter are totally independent of each other. But the blade angle depends on both.

Pitch is the distance the prop will screw itself through the air in one turn. At a 10" diameter the tip is traveling a much greater distance with each turn than a 5" diameter tip, because the 10" diameter has a lot greater circumference. The 10" tip travels 31.4" per turn static, and the 5" has only 15.7" for one turn static. I specify static because as the prop travels forward the tip distance per turn will increase, but we don't need to get that complicated here.

A 10x5 propellor will have the tip at an angle of a hair over 9 degrees, while the 5" prop, to get the same pitch, has a tip angle just a little under 18 degrees. But on the 10" prop, if you check the blade at the 5" diameter point guess what you will find? Right. The blade angle is 17.66 degrees, the same as the tip angle on the 5" prop.

The propellor blade is a wing, all considerations for wing shapes apply to props. The most efficient wing shape is an ellipse, but it's also the most expensive to build, Because of the cost, very few production aircraft have been built with them. The Republic P-47 was one of the more well known, another was the Supermarine Spitfire, at least in most marks. There were some Spits that weren't elliptical. Because of the way prop blades are made, there's no great difference in price from one shape to another, and the ellipse shape being the most efficient, is also the most common.

Wing tip plates, Hoerner tips, and winglets are all ways to make a relatively square tip approach the efficiency of the elliptical wing. Most simply put, the inefficiency comes from the tendency of the air to go around the tip from the lower surface to the upper instead of following the airfoil shape. The added tip thingies are attempts to stop or limit this flow around the tip. However, as air speed increases, the air has less "Time" to leak around the tip, so tip shape becomes less important with increasing air speed. The propellor tip will always be traveling at a much higher airspeed than the wing tip. I wont go into the mechanics of it here, but give it a little thought and you'll see why.

So back to prop tips. Again, no explanation, but as the tip speed goes up you reach the point where a blunt tip, as on your cut down prop, approaches the aerodynamic efficiency of the elliptical tip. And when the larger blade area is factored in, the blunt tip can actually be more efficient than the ellipse shape. Again, explanation would take too much time for this discussion, but it works.

Or, to put it in the simplest way, when you cut the diameter of a prop running at model plane rpm, rounding the tip is more trouble than it's worth. Just be sure both tips are cut the same, and the prop is balanced.

Bill.

Ahhhh.... (digesting slowly)

The increased rpm over the larger prop keeps tip efficiency up and the broader tip actually increases it. So I can cut props till I find what I like. Finally...The cut down props would be equal to an uncut prop of a different size and pitch, but it would be difficult to determine which one without just trying different props. (and the actual prop may not be made..which is why I would need to keep cutting?)

I'm not putting words in your mouth, just summarizing what I think I understand.

Thanks for the well thought out explanation.

Ptuey:

You have the idea down well enough to work with it.

After you use cut and try to get the best performance, though, you might find a commercial prop of the same diameter with a little more pitch works better, because while the cut prop works well, and we aren't losing that much eficiency we are losing some, and the factory prop could be better. But at the same time, your cut prop could be the absolute best match to your plane and engine.

And what was the absolute best prop in the morning might not work very well in the afternoon, with the lower air density.

But as I said, you have the idea, now it's play time. Get out your cutter and files. Start chopping.

Bill.

If you are an AMA member, go to the AMA archive and do a search on propellers, you will find a wealth of information. 3 or 4 articles on modifying them for speed and 3 or 4 again for making your own propellers from scratch(fiberglass or wood), and tailoring a prop for the application. Problem is, the majority of it is written for .40 size racing engines, a little for giant scale, and another little for electric...but nothing specifically for 1/2a, although it is all more or less applicable. The best articles in my opinion are the ones written by Hal Debolt, if you search for his name in the author heading, you will find them pretty quickly.

Austin

That BILL is a pretty sharp guy when he lays off the SKOTCH. I have a more simplistic, "NEANDERTHAL type" understanding of what's going on, I think the phrase,"cut and try" was coined by a curious modeler. The LHS just raised the price of the MAS twin pack of 6-3 and 6-4 to $3.00, but they were $1.80 for ages around here. I haven't seen a NORVEL AME respond to anything under a 4.2-4 on my deltas, but it sure is a great little trick to make them run with the big dogs..

Piggy:

Referring to my wonderful Glenfiddich Black label as "Scotch" is like calling you "Just another Chuck." I'm sure you will admit to being the one and only "CombatPigg," so return the favor and say "Ambrosia," or "Nectar of the Gods" when referring to Glenfiddich. please. Thank you.

Haw.

Bill.

PS: Thanks for the compliment. wr.

Speaking of Neanderthal... my first experience with cutting down a prop was extremely crude.
I had a brand new Fox Q500, with the tuned muffler, back in about 1986-7, on a design of my own.
Couldn't get it to really come up on the pipe, but the plane was flying nicely. One of those
guys who shows up all the time, but doesn't have his plane ready right now tells me that
my prop was too long (which I knew, but I'd never cut one before) and that the way they used
to do it was to run the engine up, then alternately tip the plane forward into the concrete or asphalt,
shaving the prop down... then back, to see if it would come up on the pipe. Rinse, lather repeat.

To this day I can't believe I did it, but I did. I finally got the thing up on the pipe, too... but the prop
must've been BAD out of balance... because I lost the crystal, and of course, the plane, the receiver,
and the brand new Fox which was probably only about half broken in. (Along with my 18Yr old wallet)

Sure did scream like a **** for about a minute and a half though
I don't cut props down these days, without rebalancing... and I learned to tape my crystals in place, too.

I have one of those,"DUAL RESPONSE" type sessions to do here at the keyboard. First of all, BILL ROBISON, I would like to round all of you," HIGH PRICED SKOTCH SNOBS" up someday and administer a blindfold test! It would be fun to see how well the palette can discriminate between the industrial swill, and the good stuff.

C WATKINS, I have seen NEANDERTHAL TECHNOLOGY applied to a FOX .36 COMBAT SPECIAL that was running the TF 8.5-6.5 pylon prop. Instead of grinding on the sidewalk, the local guru was using a pair of "dykes" to trim it to a length that would keep the motor happy through the high G manuevers. It worked well enough for the R&D session, and I have used the same technique myself. that is a pretty funny [ and barbaric] way to find the winning combo, but in the service we would call it a, "field expediency". What looks ridiculous, but works very well is a 5-3 COX chopped down to 3 7/8" on a 1/2A C/L COMBAT plane. A VA will kick "A" with one of these $1 wonders.

I just spent the last couple days flying with Larry Driskill at the Little Rock S.M.A.L.L. fly-in and noticed that he & I both run nothing but black nylon 5x3 Cox props! We were talking about how much magic Bill Atwood designed into that prop way back in the early 60's.

I run mine at full diameter on the modified sport Tee Dees and Larry clips his down as needed to get those howlin' VA's of his into the stratospheric rpm range that makes him happy. No fancy work on the tips as far as I could tell....just a clean, square cut. He put on a few demo flights of his u/c combat Litehawks with a good VA on the front and yes, that little prop do be hookin' up!! Most of the sport r/c flyers there had never seen anything like it. UNBELIEVABLE rpm, speed and maneuverability![X(] His r/c converted Litehawks were AWESOME as well.

Digger was there with his Mig RCU design contest entry and couldn't get a successful hand launch on it with it's Norvel .061 and APC 5.7x3. It kept digging furrows of dirt every time due to poor low speed thrust. We talked about it and I gave him a (guess what) Cox black 5x3 and voila! Easy hand launch, no sweat. He said the top speed was down but the hand launches were tons better.

Glad to see you made it home in one piece. Did I miss much by leaving at noon on Sat? I was sure glad I didn't have a 3 hour drive staring me in the face this morning when I got out of my OWN bed.
Sure enjoyed the talking time we managed. Small Steps is so much like a family reunion that I never get to talk to everyone as much as I would like nor ask as many questions as I want to.
Dunno if I told you but after watching Larrys combat demo with that shrieking VA I simply told him, "That's a d**m lie!". Hard to believe anyone can keep up with something that fast on those short lines, much less look around at the scenery. Darn it Larry! That's embarrassing to incompetents like myself. Tom

Yep, the black Cox 5X3 is, as Dickie said, truely magic. It works so much better than it looks that it does not get its full due. Years ago, Dan Rutherford took to calling them Rubber Ducky's. It fits.

We have used them in 1/2A C/L Combat for years. Now, most serious C/L Combaters have gone to carbon and glass props, but a Rubber Ducky won CL 1/2A Combat at the Nats again last year.

To get the VAs turning where they need to I cut the tips off. I usually make up a 1/2 dozen props from 4" to 4&1/4" before a contest. You need the engine turning in the 30's in order to not bog in continuous tight loops (at S.M.A.L.L. someone asked what the VA on the CL plane was turning, so I dug out the tach and it said. . . 31,400).

I start the cutting process with a pair of side cutters to get the length close and finish with a single edge razor blade to square the tips and balance the blades.

I have never thrown a Rubber Ducky blade. We abuse them with lawn dart landings, midairs, and such and they "wear out", but seldom do you break a blade off.

Magic!

Thanks for the pics Larry. I use the same balancing method for small props but as I am an accident waiting to happen I kept my eyes open 'til I found a small, clear plastic box just the right size to glue the razor blades into opposite sides and still be able to close the cover to prevent 'oopses' and leakage of vital fluids. The cover is about a third of the total depth of the box. Had I been thinking I would have glued the blades into the cover giving more room for the prop to teeter-totter without having to grind a relief into the sides of the box. A piece of music wire or drill stem of the proper diameter to fit the prop hole tightly and slot into the box diagonally for storage completes the assemblage.
I really need to find a couple more boxes and build new ones with the blades in the short side of the box. As it is when I need it in the shop it's always in the flight box and when I need it at the field it's always in the shop. I think there's a natural law about such things. HTH, Tom

Chris,
Nice story about using asphalt for sandpaper. It made me feel a little better about listening to the guy that told me to put a tap in the muffer and crankcase. Supposedly if you put a line between the two it would supercharge the engine. ??? It might have been the same guy you talked to.

Here's my 1/2a prop balancer. Magnets came from happy meal toys and the rod is 1/8 music wire.(I think) To make the points on the end I put the rod in a drill and stuck it to a running bench grinder. The rod seems to be balanced very well. Anyway, it does get my props close enough to eliminate vibration problems.

Dickeybird,
I ordered some 5x3 cox props after getting those low rpm numbers with the 6x3 MAS props. They are not cheap @ 2.69 each from Tower. The new .049 and .061 Norvels should like them too.
I guess my plane was so heavy the last time I tried them, it was never able to get up to speed. I have learned not to use AA alkaline batteries

Larry, I'll have to try that razorblade balance, I have the balancer that you hold between your index finger and thumb and the razor blades will work with that too. Thanks for the picture.

Lynn