Nov 25, 2012, 04:33 PM Suspended Account Joined Feb 2011 639 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
 Nov 26, 2012, 06:11 AM I am actually really slow Brisbane Joined Jun 2008 1,003 Posts Notches need to be bigger
 Nov 26, 2012, 09:35 AM An itch?. Scratch build. South Wales U.K. Joined Mar 2003 17,007 Posts This one has popped, (propped ? ). up before - Serrated prop leading edges
 Nov 26, 2012, 01:42 PM Suspended Account Joined Feb 2011 639 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 .
 Nov 26, 2012, 02:17 PM B for Bruce The 'Wack, BC, Canada Joined Oct 2002 14,678 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.
 Nov 26, 2012, 03:23 PM First pull up, then pull down Thousand Oaks, CA Joined Mar 2004 2,831 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.
 Nov 27, 2012, 12:22 PM Suspended Account Joined Feb 2011 639 Posts Well I guess you may be right... it's true that this paper don't give the smallest explanation about the results.
Nov 29, 2012, 09:18 AM
Registered User
United States, UT, Salt Lake City
Joined Oct 2007
10,001 Posts
Quote:
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.
 Nov 29, 2012, 01:43 PM First pull up, then pull down Thousand Oaks, CA Joined Mar 2004 2,831 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.
 Nov 29, 2012, 03:44 PM Registered User United States, UT, Salt Lake City Joined Oct 2007 10,001 Posts You are serious? if this idea really worked , there would be saw tooth props everywhere .
 Nov 29, 2012, 06:24 PM First pull up, then pull down Thousand Oaks, CA Joined Mar 2004 2,831 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.
 Nov 30, 2012, 10:54 AM Suspended Account Joined Feb 2011 639 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
 Nov 30, 2012, 11:47 AM Registered User United States, UT, Salt Lake City Joined Oct 2007 10,001 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 Last edited by richard hanson; Nov 30, 2012 at 11:56 AM.