Pulling smoke past the prop won't really tell you much. The airflow is far too slow for the patterns to be applicable to the operating speed. Far better to run the prop on a motor where you can check the rpms and current. Measure the watts in (volts x amps) for each run and the RPM. Keep the RPM constant by varying the voltage and current and chart the amount of watts for each mod. For this to make sense you need to use the same brand and version of prop for all testing. Try stock, dimples, trip threads and anything else you want but make only one change at a time and keep the RPM constant for all tests.

Granted this is a static test and folks will be right in stating that the results will be skewed by this but it would be hard to do this in the air. To reduce the static issue be sure to use the lowest pitch prop you can for the testing.

Hmmm --- a blank

See next post for wisdom & entertainment

A varient of this idea has been used to some effect on particular sailboats with airfoil masts. The leading edge of the mast retains a smooth finish, but aft of the point of maximum thickness (at the estimated point of flow separation), the surface is coated with coarse sand (a slurry of sand & varnish is painted on the spar). The idea was originally proposed with Rice Krispies, of all things, & the resulting delayed separation is called the "rice krispy effect". It works -- sort of -- to keep flow attached closer to the luff of the sail.

It has also been tried on the frames of racing bicycles & on the bodies of some riders -- instead of shaving their legs all around, as per usual, only the front sides were shaved -- leaving the backsides hairy [:'(]

On an aircraft wing, it might work if the surface was dimpled aft of the point of separation -- certainly not on the leading edge. It is also possible that the front face of a prop could be similarly dimpled -- particularly in the nasty root-transition zone.

Dimples or thread or sand or trim tape. It's all the same. It's there to stir up the air close to the surface and produce a turbulent boundry layer that is thin and hugs the surface. The laminar flow above will tend to stick better to this turbulent layer than it does to a perfectly smooth surface. Keeping the laminar flow stuck down longer and at higher lift coefficients produces a delayed stall and generally better performance at near stall conditions because the air tends to follow the shape longer. You pay for this with a touch high drag at the very low lift values where the air will tend to stay laminar anyway.

On the golf ball they use dimples because little bumps would soon get pounded away. It's not better to use dimples, it's just different.

So it looks like most are agreeing. I'm not putting words in anybody's mouth though.

But BMatthews, it seems you don't think would improve efficiency? I just imply that by you saying: "Its not better to use dimple..."

Ok, maybe the smoke wouldn't work, but maybe I could just measure the RPMs, among other things, to see what happens. I guess the outcome just depends on the different variables and what I talked about. I'm getting more serious, and will look into the supplies of what I will need. I think I would have to use a power drill so I could have the constant power. But if I'm going to measure RPMs, I guess I would have to put it on a engine. If I'm going to test a larger engine (in the 22-28 inch) range I don't have an engine to test that large a prop. How could this work out...


Thanks for the support everyone!!

~Michael~

The dimples is just one way to produce the turbulation. It's maybe better and maybe not. That's the point. Dimples on their own are not something magical. As I said, the dimples are just the way the golf ball makers do it as the dimples are not so easily damaged by hitting them with a big lump of metal on the end of a stick. Various models have successfully used thread and tape turbulators for years. And full sized stuff often can be seen to use "invigorators" made from little tabs of metal sticking up from the wings. Look at the wingtips of A4 Shyhawks for such an example.

A drill motor won't tell you what you need to know and they don't turn fast enough in any event. You need to run somehting like a geared electric model motor and monitor the watts inputed into the motor and vary the voltage, and thus the current as well, to achieve a constant RPM. By keeping the RPM constant and monitoring the voltage and current you're working with less variables and that makes your calculations easier and more meaningful. In any research like this you want to ensure you can keep as many things constant as possible. And to do this with your project that means either inputing a constant power and measure the RPM or running at a constant RPM and measure the power used.

How large of a prop would I need, to really show a comparison and what motor to use for that certain prop. I kinda figured the drill wouldn't be that good, but I just know I have to keep many things constant like you said, BMatthews.

I'm thinking if I can really prove something with this, I might make it my senior exit project for next year. I have to do the paper this year and product next year, but am considering doing this if I'm able to produce something worth while.

Wonder how much I would have to invest to get this thing going: with the prop, plexiglass (or whatever I'm going to need for protection), engine/motor.

Couldn't I use an engine, like a 46 that I have laying around. I might be able to borrow a larger engine from some flying buddies if I can convince them what I'm doing is good.

Here's the things I could measure for an engine: -RPMs
-Fuel Consumption
-Decibels

for a motor: -power in
-power out
-RPMs and Decibels

Anything else that I might want to check for?

Thanks for the support!!

~Michael~

I just came across this thread and would like to add my experiance. I painted a master airscrew 18-10 prop with a hammered metalic silver paint and the end result was a loss in rpm. when I flew the plane I noticed a real lack of performance. it was a year ago so I don't remember the rpm loss but it was a lot. the engine was a g38 on a dynaflight supercub

Hmmm...a thought comes to my mind...What is hammered mean for a paint? It is like textured paint or something. I've never heard of this!?

Well, I guess it all depends on what you paint it with will make the end result different.

~Michael~

Hammered paint looks like it's rougher than it is. There is some roughness but it also adds a lot of thickness.

If you're looking at making this into an official paper then I would suggest that you stick with the electric motor and only use one motor for all the experimental series and one type of prop for each series with varying turbulators.

Also the need for turbulation depends on what the airflow around the prop is like. If the airflow is clean with no separation then adding turbulation in the form of dimples or trip threads is going to hurt and not help. Adding dimples on their own does NOT reduce drag. It's the effect they MAY have on encouraging the air to stay in contact with the surfaces longer rather than pull away in a drag producing separation bubble.

It's the same with the golf ball. The ball tries to make the air flow around a very short curve. At some point the air can't remain stuck and it pulls away and forms a turbulent wake. This is the separation point. The dimples act to provide a source of turbulation to the boundry layer that encourages the air to remain conformal to the ball shape for a longer time. Not to the point where it joins at the rear but significantly longer than a smooth ball would allow. With the dimples a much smaller cross section turbulent wake is the result. It is this smaller wake that provides the reduced drag. Even on the golf ball the dimples ADD DRAG but the drag they help to eliminate more than makes up for the direct dimple drag.

So be sure you're doing this for the right reasons and do not be surprised at all when a lot of the lower pitch props produce no gain and in fact show a loss with the addition of dimples or threads or yak hair stuck on the blades. But there will be one prop that showed itself to have a much higher need for power where the pitch increased just beyond what the air could stick to in a static situation. THAT prop will gain in the static testing by having a turbulator of some form added to it. But that same prop in the air would have seen a reduction in angle of attack as the speed came up and may have gone back to no separated operation.

I saw and heard this when I tested a mitt full of props on electric motors a bunch of years ago. Low pitch props wound up like crazy and drew little current and had a sizzle to the sound. High pitch props didn't produce a lot of rpm, drew mondo current and sounded "fluffly". But some of these "fluffly" props on a higher speed sport model proved to work well. You could hear then unload in the air and the revs jump.

I still am not sure what I want to do. I, as of right now, am doing my senior exit on alternative fuels, but figured if I could get enough information, and something actually worked, then I might get somewhere, and would be able to make a good argument for my paper (which has to be 7-10 pages long). My argument would be that modifying the prop with dimple like holes on XXX part of the prop would imporve effeciency. Then I would go on to talk about how much this might save and what the aerospace industry could gain from this.

For one type of prop, you mean like one brand. I guess I would probably go with APC. Maybe even MA because they would be cheaper. Would it be worthwile to try different props to see what the difference is.

For the dimples on the ball, you're saying that the dimples reduce enough drag to make up for the little bit that they cause.

Thanks for all the help, I will take everything into thought!

~Michael~

Exactly.

By using a consistent set of props that was what I meant. Take something like Master Airscrew 10 inch props in 4, 5, 6, 7, 8 and 9 inch pitch and run your tests on these. Chances are the lower pitch props won't benifit and may even loose by adding turbulation. At some point around the 5, 6 or 7 inch range I suspect you'll find that dimples, thread, sanded in roughness or some other form of turbulation will produce a reduction in drag in this static situation. But in the air at speed it may not help and may go back to hurting. You'd need some telemetry to test this or a way to move air past your props at about 40 mph to simulate flight operation.

I know turbulation works as I've used it with excellent success on two of my gliders. I also know "it depends" based on trying them on another one with no luck at all.

Ok, thanks for the clear up. That's what I thought you meant. I will go more in depth with my dad and see what we can come up with.

Thanks for the help though

~Michael~

Not to be discouraging, but given the ease of making molds with different dimple patterns with a CNC setup, and given there are folks with same giving prop efficiency some thought, I wonder why, if dimples, or trippers, or invigorators, or whatever will work, why is such a prop not on the market?

Maybe they have just never gotten around to it.

There was a "dimple tape" on the market 6-8 years ago for full sized aircraft. It boldly came and quietly left the market a very long time ago. At the time it was proposed for struts and props and the like. I think the article was in "Sport Aviation".

Kinda off the subject, but if you are looking for more efficiency, check out the cross section of a dragonfly wing - looks nothing like a conventional airfoil - but was tested to give high lift for a given glide path( I thnk I remember 15% better by one study) try this http://jeb.biologists.org/cgi/content/full/207/24/4299 or http://search.yahoo.com/search?fr=sb...ics&btn=Search Granted they're wings also employ sculling and independant movement, but we must remember, insects have been flying, and therefore refined by nature, for far longer than any other class of animal(bees and flys don't count, they fly differently - like a hummingbird at hover). Also keep in mind, that bird wings(the ones man has roughly copied) were originally used not to fly, but to increase ground traction for running.

Those are some very interesting sites. I have seen that concept about the dragon flies. I haven't personally seen an actualy video or anything, but it certainly is intriguing.

My personal reason why prop manufactures won't put anything out because it would be too different. Most modelers just want a prop that will get the job done and fly around their plane. I think it would be so different that nobody would want to try them out, seeing how they flew with the different prop.

I know with CNC, you can do anything, so it wouldn't take anything for a APC, or MA to just punch in some numbers into the computer and let the machine do the rest.

~Michael~

This site has great info on golf ball dimples and history.

http://puttingzone.com/Science/balldimples.html

Holy cow, that site is amazing. It'll take forever to read all that. I just skimmed over it, but I know there's alot of good stuff.

Thanks so much. I wish I had a laptop so I could read that on the Jon!!

~Michael~

Last spring, mother nature dimpled my car for me when she sent forth golf ball sized hail to smite my car. I haven't noticed an improvement in performance though.[]

Oh man, sorry to hear about that. I was hoping something might happen!

There's another thought, wonder how they would do on cars. I know I've seen that Ford made a dimpled car, where it was dimpled on the bottom, but nothing other than that.

~Michael~

There was some research about dimpling baseball bats to allow players faster swings. Same for golf club shafts. They all work and have been duly outlawed by their governing bodies.

I haven't read all the posts, but if dimpling propellers and leading edges actually improved efficiency, wouldn't manufacturers already be doing it??

Surely Boeing and Airbus would be dimpling leading edges if they thought it would improve aerodynamic efficiency by even a fraction of a percent; that's all winglets improve by, but when you're talking 10,000 kgs of fuel burnt in an hour (B747), then a fraction of a percent makes a huge difference over the life of the aircraft.

Outlawing the dimpling of the bats and golf clubs. Hmmm....interesting.

I wondef Boeing or Airbus or whoever HAS done something like this. See, just because someone does something and they think it's right doesn't mean it's right, they were just doing it wrong all these years. I say that they could have experiements with the dimpling, but they didn't use the right compnents or something they used made the outcome not to their liking, so they thought the idea wouldn't work and dropped the idea.

I'm still a believer. I just have to get something together to have my own experimentation.

~Michael~

Oops -- see next post