HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Nov 25, 2012, 04:33 PM
Registered User
yomgui's Avatar
France, RA, Cormaranche-en-Bugey
Joined Feb 2011
545 Posts
Discussion
Propeller modification - leading edge notches

Hi !

Anyone has read this one:

http://soar.wichita.edu/dspace/bitst...pdf?sequence=1

I was very interested by the leading edge notches mod.

So I've tried it today in my wind tunnel, on a APC 9x6E.

Here's a sum up of the experiment :

http://aerotrash.over-blog.com/artic...112827140.html

I've noticed no difference at all...

Does any one have a idea why ?

BTW I'm not sure how to understand the results of this mod as shown in the thesis (page 29 for the detail of the mod - results figures 40,41,42 and 64, 65).

Guillaume
yomgui is offline Find More Posts by yomgui
Reply With Quote
Sign up now
to remove ads between posts
Old Nov 25, 2012, 11:56 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,365 Posts
Well, for a "fix" of this sort to work it needs to be doing something that fixes an existing problem. It also mentioned that it worked best on higher values of "advance ratio".

Higher advance ratios on a prop used in a wind tunnel setup suggests higher angles of attack for the blades and that suggests that perhaps the blades were at a high enough angle that the forward surfaces were suffering from some amount of separation bubble formation or even to the point where the blade was stalled. A stalled blade will still produce thrust but it does so with a high drag penalty. The notches act as a turbulator to generate a turbulent boundry layer which could be working to reduce the size of the separation bubble or even to delay the final full stall condition. Not to say that ANY of this is the answer for you. It's all just possibilities since we simply can't see what the blades were doing in his testing and what the airflow was like.

So the notches didn't help on the APC prop. Likely because the prop was not suffering from poor airflow issues in your case. So adding the notches did nothing since there was nothing that the effect of the notches "fixed".

Your blog also mentions that the prop was operating in a rather high ambient airspeed. So likely the angle of attack at the blades was low. Again a recipe for a smooth airflow over the blades which had nothing that needed "fixing" by the notch effect.
BMatthews is online now Find More Posts by BMatthews
Reply With Quote
Old Nov 26, 2012, 06:11 AM
I am actually really slow
SlowBarless's Avatar
Brisbane
Joined Jun 2008
961 Posts
Notches need to be bigger

SlowBarless is offline Find More Posts by SlowBarless
RCG Plus Member
Latest blog entry: Carbon Fiber up close
Reply With Quote
Old Nov 26, 2012, 09:35 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,263 Posts
This one has popped, (propped ? ). up before -

Serrated prop leading edges
eflightray is offline Find More Posts by eflightray
Reply With Quote
Old Nov 26, 2012, 01:42 PM
Registered User
yomgui's Avatar
France, RA, Cormaranche-en-Bugey
Joined Feb 2011
545 Posts
Maybe I should explain a bit more. The tested prop is not the wind tunnel propeller, and it was tested at various airspeed, from static to zero thrust airspeed, just like the propeller from this thesis. So the angle of attack of the blades was diminishing as airspeed and advance ratio increased (rpm were left to increase a bit as power required was diminishing).

From the thesis we can see a large increase of thrust at any advance ratio, so at any angle of attack from highest in static to lowest at max airspeed. The modified propeller show a Ct curve with an equivalent slope as the original one, but moved forward in advance ratio, the advance ratio for Ct=0 was higher, which may suggest a higher effective pitch. We can guess the Cp curve to show a similar behavior (from fig. 40). Also the huge increase in efficiency at higher advance ratio is typical of a higher pitch propeller.

But the peak efficiency is much higher than what one could have expected from a higher pitch.

My experiment shows neither change in static or low advance ratio, nor at the higher advance ratio. Unfortunately I couldn't have anything in between... but there is no reason to think there will be any change also.

The notches are small, almost the same size as the 14x6 from the thesis but on a 9x6, so comparatively bigger.

I can't explain myself the results with the 14x6, which seems to have nothing to do with higher max lift coefficient.

And, of course, I'm quite disappointed to fail having such results with the 9x6 .
yomgui is offline Find More Posts by yomgui
Reply With Quote
Old Nov 26, 2012, 02:17 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,365 Posts
Well, again things like notches, sawteeth and other add ones are typically functioning as turbulators. And turbulators do not in themselves increase anything. They are there to "fix" airflow separation problems and to some extent delay the stall.

So it might just be that your APC prop did not have anything that needed "fixing" over the range of operation that you used. Perhaps try buying a Master 9x6 and run the tests again without notches and then try notching the blades similarly to that in this report. Perhaps then you'll see some improvement. The next question up is will the MA prop with improvements due to the notches exceed the performance and efficiency of the stock APC? It may be that all this "fixing" only allows the MA to come up to or nearly to the performance provided by the stock APC.

When a buddy and I ran some static tests on props and motors back some 20 years ago we found that it was pretty tough to beat the thrust to power ratio achieved by the APC props. The MA props similar to that used in that report were consistently down in performance compared to the APC's. So it may be that the MA shape has issues which were "fixed" by the addition of the turbulators in the form of the notches where the APC has nothing that needs fixing. That would explain your results.
BMatthews is online now Find More Posts by BMatthews
Reply With Quote
Old Nov 26, 2012, 03:23 PM
Don't take your guns to town
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,580 Posts
Just because some kid slaps together a report for school does not mean that his theories, methods, or conclusions have any merit. Not only did he use a Master Airscrew prop which is little more than a twisted bar of plastic with no aerodynamic merit to begin with, but some (all?) of the things he tried to test were so ridiculously arbitrary that I fear for his safety. All he did was glue a bunch of random trash randomly all over the prop and test it. There was no science, no method, and thus no meaningful result. Any apparent benefits from the test configurations can be assumed to be simple measurement errors.

This is not to say that notches or teeth or scraps of sandpaper, etc. don't have potential. Just that you should ignore any of the methods or conclusions in that paper.
vespa is offline Find More Posts by vespa
Reply With Quote
Old Nov 27, 2012, 12:22 PM
Registered User
yomgui's Avatar
France, RA, Cormaranche-en-Bugey
Joined Feb 2011
545 Posts
Well I guess you may be right... it's true that this paper don't give the smallest explanation about the results.
yomgui is offline Find More Posts by yomgui
Reply With Quote
Old Nov 28, 2012, 12:43 PM
FS One
MSelig's Avatar
United States, IL, Champaign
Joined Sep 2005
500 Posts
Yomgui, I have not read this thread completely, but I did download the thesis you referenced. The tests you're talking about (the thesis) were done on a Master Airscrew propeller. Our prop measurements showed that these propellers generally are not that great good (low efficiency relative to many other brands that we tested). I think the real test of something novel like this (saw notches) needs to start with a baseline that is already very good. Then the challenge is -- can it be improved? I think your own tests w/ an already good prop show that saw-notching the leading edge does not help. FWIW - We will not be trying these same tests because I think we'd come to the same conclusion.

Michael
MSelig is offline Find More Posts by MSelig
Reply With Quote
Old Nov 29, 2012, 09:18 AM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
6,921 Posts
Quote:
Originally Posted by MSelig View Post
Yomgui, I have not read this thread completely, but I did download the thesis you referenced. The tests you're talking about (the thesis) were done on a Master Airscrew propeller. Our prop measurements showed that these propellers generally are not that great good (low efficiency relative to many other brands that we tested). I think the real test of something novel like this (saw notches) needs to start with a baseline that is already very good. Then the challenge is -- can it be improved? I think your own tests w/ an already good prop show that saw-notching the leading edge does not help. FWIW - We will not be trying these same tests because I think we'd come to the same conclusion.

Michael
A logic test shows that the best props move thru the air without changing the flow PATH- ( ideal just really not possible)
So notches may help control the path but I don't see how -from a practical standpoint
Every ding dong prop mod I have ever seen , just reduces prop efficiency.
The swept tip being the common (hot trick) approach-
the revs go up and that may/may not be a good thing.
richard hanson is offline Find More Posts by richard hanson
Reply With Quote
Old Nov 29, 2012, 01:43 PM
Don't take your guns to town
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,580 Posts
The idea of notches is to allow air to leak from the bottom of the blade to the top, re-energizing the boundary layer near the top LE just as slats do for a wing. And just like slats, they should increase max thrust at the expense of efficiency. You should only see the effect when the prop is near stall. The sawtooth blades use the same strategy but with delta-style LE vortices to re-energize the boundary layer rather than slot/slat leakage.
vespa is offline Find More Posts by vespa
Reply With Quote
Old Nov 29, 2012, 03:44 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
6,921 Posts
You are serious?
if this idea really worked , there would be saw tooth props everywhere .
richard hanson is offline Find More Posts by richard hanson
Reply With Quote
Old Nov 29, 2012, 06:24 PM
Don't take your guns to town
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,580 Posts
Everywhere? Not quite. My post above explains why sawtooth/slotted props have virtually no useful benefits for any application -- except where efficiency can be traded for max thrust. This is a very rare occurrence as efficiency is almost always the ultimate goal of any design. For example, to increase max thrust of a cargo plane one would simply increase the propeller efficiency.

One example of an unusual situation where this tradeoff may be valuable is F5B where the propeller is stalled and the motor is cold at launch. So for that first second of flight, you can afford to heat the motor at a higher rate (low efficiency) in exchange for the much greater thrust provided by a non-stalled prop. So it's conceivable that a prop of this type could provide enough benefit during launch to overcome the drawbacks during the rest of the flight. Due to the very extreme aerodynamics involved in semi-stalled near-transonic propellers this is nearly impossible to analyze computationally and even more difficult to test so we may never know if it has any merit or not, but it's conceivable.
vespa is offline Find More Posts by vespa
Reply With Quote
Old Nov 30, 2012, 10:54 AM
Registered User
yomgui's Avatar
France, RA, Cormaranche-en-Bugey
Joined Feb 2011
545 Posts
Thanks for the answers !

So it appears that:
- maybe the APC prop as a baseline is good enough so it don't see any improvement,
- maybe the thesis' test is suffering of some misleading errors, whatever it is (the improvement don't look like what is expected from the modification).
- surely I'm disappointed...

MSelig, you know my wind tunnel can't be compared to what you have, or what they have at Wichita ! same for my knowledge...
I look at the UIUC website twice a week to see if there is something new ! propeller tests are really interesting, maybe a good drag built up method is now lacking .

Thanks
yomgui is offline Find More Posts by yomgui
Reply With Quote
Old Nov 30, 2012, 11:47 AM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
6,921 Posts
I guess this is all just a "theoretical thing".

Props are interesting devices -
do anything to em and the engine/motor will change rpm.
IF you select some arbitrary rpm and use it as the fixed referrence point - you will see what I mean.
If you are looking at the best efficiency at non changing rpm - (fixed)?
selecting best efficiency involves airframe , engine and the prop ,TOGETHER
it's a 3 legged stool.
I have piles of props of various sizes n types - depending on the application - one always proves best .
Never saw a trick feature which really improved anything over current designs
Some of the really efficient ones I have tried have nasty problems of bending under loads such that harmonics occur at various rpm points - it's not the powerplant - Iuse only electric outrunners now which are pretty smooth through the entire operating band.
Structural integrity is as important as any shape. When you have 3000 watts churning a prop - you want smooth predictible operation
richard hanson is offline Find More Posts by richard hanson
Last edited by richard hanson; Nov 30, 2012 at 11:56 AM.
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Fixed leading edge slats, examples RCB1 Fuel Plane Talk 27 Feb 17, 2014 02:13 PM
Discussion Propeller modifications Overworked Racing Boats - Electric 5 Mar 28, 2012 10:12 AM
Help! Sanding leading edge on a Lil Bird 2M kurtenstein Sailplane Talk 3 Mar 27, 2012 11:20 PM
Discussion Leading Edge Leading Edge? gorgeflyer Slope 27 Sep 22, 2006 09:17 PM
Discussion Notched leading edges Azarr 3D Flying 31 Mar 22, 2006 03:32 PM