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  #21  
Old 07-20-2011, 09:36 PM
elippse elippse is offline
 
Join Date: Dec 2006
Location: Arroyo Grande, CA
Posts: 938
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Quote:
Originally Posted by digidocs View Post
Paul,

I'm curious --- how do the total blade area and the aspect ratio change as you add more blades to your designs?
Prop pitch at a given speed is based upon the design CL. Increase CL for a given speed and HP, the chord and area decrease. As you add more HP for a given drag CD your speed will increase and so will your pitch at a given CL, but the prop will require more area.

On a wing and a propeller aspect ratio is not necessary in computing induced angle of attrack; the only thing that counts is the span. AR is just a short-hand method of trying to arrive at a 3D set lof lift-drag polars, but if you solve directly for the IAOA, you can use the airfoil 2D L-D polars.

I'm not trying to make a case for the relative improvements of each mod; I was just trying to celebrate, along with Jim, his new barrier-busting speed with the Vetterman-designed outlet. If you go back into some of my previous postings you will see the relative speed and rpm changes on Jim's plane from the two-blade Aymar-DeMuth to the three-blade Ellipse to the new wingtips. Then you can derive the total increase form the Vetterman-designed fairing and LSE Plasma ignition.

I sent Jim's pictures of his fairing to Bob Axsom since my efforts at posting pix have come to nought. Perhaps if you were to impose on Bob you could get him to post some of the pertinent pix.

The point about the clunky, un-aerodynamic hub shape is that it is not covered by a spinner but is usually out in the breeze. Even if you have an aerodynamic shape right up to the spinner and you don't seal the blade to the spinner you will get a circulation from the higher relative pressure on the blade's bottom surface into the spinner and out on to the lower pressure on the top surface which will add turbulence to the airflow on the spinner and add drag. And yes, new prop designs for the military and commercial planes with their six and eight blade props are adding rotary seals to the hub-spinner interface. Do a search on the Hamilton-Sunstrand eight-blade AR 2000propeller that is being tried on the C-130J and look at the airfoil going all the way to the spinner and the rotary seal. Finally prop designers are getting it together!

I won't divulge Tom's actual rpm because that gives away proprietary info.
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  #22  
Old 07-21-2011, 12:49 AM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Three of the photos

Here are three of the photos I received. There are others in the sequence of Jim Smith's buildup but I think these show the product. He has smooth flow contours and that seem natural. Intuitively, I like his approach and it gives me some ideas for another similar approach of my own. It is compatible with an A model for a cowl flap implementation. where the lower surface fwd of the exhaust pipes could be hinged up by the cowl and drop down like the cooling outlet on a Mustang. Still thinking but I like the ideas he has built into the surfaces that at not necessarily apparent in the two installed fairing photos. The other one hints at inner works. If I get to try something I will probably incorporate aluminum or stainless steel into the design. I have already cut off the down turned ends of the pipes to avoid the exhaust blasting into the slipstream which will give me more area to include in the flap but I will have to think about the pipes - I don't want to leave them terminated way back in the faring under the flap - hmmmm. More thought required but I like Jim's approach to this drag problem.

Bob Axsom




Last edited by Bob Axsom : 07-21-2011 at 01:02 AM.
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  #23  
Old 07-21-2011, 09:30 AM
gereed75 gereed75 is offline
 
Join Date: Sep 2007
Location: pittsburgh pa
Posts: 533
Default I have been working on another approach

to this same area. My approach will be much simpler with less structure. It is based on the concept of outlet "chimneys" that are currently in use in F1 auto racing to optimize engine compartment outlet flow.

My approach should get the outlet airstream accelerated to free stream speed efficiently with minimum turbulence. If this exit air is smoothed, there should be a drag reduction and also more efficient cooling.

From observation, I know that the corner of the outlets (where the cowl exit meets the fire wall) is extremely turbulent. Air in this corner actually flows back into the cowl. That area is my current focus.

I hope to have some of this work done (after visiting Osh Gosh) by the Indy SARL race. Pictures and results by then I hope.

This chasing speed thing is addicting!!
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  #24  
Old 07-21-2011, 03:47 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default A true believer in the need for speed

Quote:
Originally Posted by gereed75 View Post
to this same area. My approach will be much simpler with less structure. It is based on the concept of outlet "chimneys" that are currently in use in F1 auto racing to optimize engine compartment outlet flow.

My approach should get the outlet airstream accelerated to free stream speed efficiently with minimum turbulence. If this exit air is smoothed, there should be a drag reduction and also more efficient cooling.

From observation, I know that the corner of the outlets (where the cowl exit meets the fire wall) is extremely turbulent. Air in this corner actually flows back into the cowl. That area is my current focus.

I hope to have some of this work done (after visiting Osh Gosh) by the Indy SARL race. Pictures and results by then I hope.

This chasing speed thing is addicting!!
Thanks for the information. I guess I'm going to have to exercise Google to find out what an F1 exhaust chimney looks like. I looked at the www.sportairrace.org "who's In" list for the Indy Air Race on August 13 and I see only 2 RVs (both RV-6s that I'm familiar with). Have you let them know you are racing yet? I hope to be back in the saddle by the end of the season or next year and it is always good to meet a new speeder.

Bob Axsom
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  #25  
Old 07-21-2011, 04:12 PM
Mike D's Avatar
Mike D Mike D is offline
 
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Location: Houston, TX
Posts: 456
Default

Some interesting reading on F1 exhaust
http://f1-dictionary.110mb.com/diffuser_blown.html
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  #26  
Old 07-22-2011, 10:14 AM
elippse elippse is offline
 
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Location: Arroyo Grande, CA
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Howard, you like to calculate the efficiency of my propeller, so I'll give you a little comparison. Van's quotes the TAS of an RV-6 solo with 150 HP at 187 mph at 8000', which I would assume was 2700 rpm since it was supposed to be at 75% power. Jim got 186 mph with his Aymar DeMuth at 2798 rpm, and with my three-blade Ellipse it gave 190.5 mph at 2700 rpm back before he did any aero clean-up. I don't know what prop Van's used to obtain his data but I'm sure you know or could find out. As you know, the relative efficiency is based on (TAS1/TAS2)^3 X RPM2/RPM1. Just using these, its shown that the efficiency difference of the Ellipse ove the A-D is +11.3%. The Van's relative to A-D was +5.3%, and the Ellipse relative to Van's was +5.7%. Now with the wingtips and the cooling outlet fairing, Jim's plane at 202 mph is 26% more efficient than a standard Van's RV-6; that's like having 189HP but without the extra fuel flow.
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  #27  
Old 07-22-2011, 10:23 AM
gereed75 gereed75 is offline
 
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Location: pittsburgh pa
Posts: 533
Default Thanks

thanks Mike - That is a great source I had'nt seen.

The F1 world is a great source for this type of thing. They have huge budgets and do a lot of wind tunnel work and much of it is in the same speed range where we are - 180 mph or so with similar goals - air flow management for drag reduction or lift.

I am contemplating some diffuser work inside the cowl at the exit area -more to shape and direct airflow to get it out of the cowl efficiently, similar to what Bob has done but simpler, and similar to what the F1 guys do around the front wing and side pods and chimmneys.

I'll share as I go.

Fly safe!!
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Last edited by gereed75 : 07-22-2011 at 10:35 AM.
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  #28  
Old 08-01-2011, 03:33 PM
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brian brian is offline
 
Join Date: Jan 2006
Location: Cornish, NH
Posts: 391
Default i wonder what the fairing cross-section is

Quote:
Originally Posted by Bob Axsom View Post
Here are three of the photos I received. There are others in the sequence of Jim Smith's buildup but I think these show the product. He has smooth flow contours and that seem natural......
I have a question about what the inside of that fairing is like. We have a pic of it looking forward, but not one of it looking aft. Is the fairing interior open to the aft of the cutout for the exhaust, or is it blocked off? It seems to me that, if it's open, the air coming out of the cowl will go in to the opening aft of the cutout, catch in that dead open space, swirl all around, and create drag as it comes dumping out. It seems to my seat-of-pants engineering that the fairing should have a vertical wall from the aft part of the cutout up to the belly of the aircraft, preferable one that slopes fwd at about 45 degrees, so the air coming from the cowl gets dumped smoothly into the airstream, rather than going into a dead space pocket. I hope I'm describing this clearly so you all can understand what I'm saying.

On similar note, I attended a "Reducing Drag" seminar by Jim Bede at OSH. One thing he mentioned was that slots in the back of wheelpants MAY help reduce drag by dumping the air caught into the wheelpant out to the aft, rather than having it swirling and pouring out as I described above for this exhaust fairing. The other big thing he mentioned was that any and all GAPS create drag, and that we should try to eliminate all the gaps we can.
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  #29  
Old 08-01-2011, 06:41 PM
hevansrv7a's Avatar
hevansrv7a hevansrv7a is offline
 
Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,587
Default Prop Efficiency

Quote:
Originally Posted by elippse View Post
Howard, you like to calculate the efficiency of my propeller, so I'll give you a little comparison. Van's quotes the TAS of an RV-6 solo with 150 HP at 187 mph at 8000', which I would assume was 2700 rpm since it was supposed to be at 75% power. Jim got 186 mph with his Aymar DeMuth at 2798 rpm, and with my three-blade Ellipse it gave 190.5 mph at 2700 rpm back before he did any aero clean-up. I don't know what prop Van's used to obtain his data but I'm sure you know or could find out. As you know, the relative efficiency is based on (TAS1/TAS2)^3 X RPM2/RPM1. Just using these, its shown that the efficiency difference of the Ellipse ove the A-D is +11.3%. The Van's relative to A-D was +5.3%, and the Ellipse relative to Van's was +5.7%. Now with the wingtips and the cooling outlet fairing, Jim's plane at 202 mph is 26% more efficient than a standard Van's RV-6; that's like having 189HP but without the extra fuel flow.
Paul, your (Jim's) numbers here are different than quoted at other times and other cases, but I did a quick look at these numbers. I used my triangle tool which has already been done for the CAFE 6A and which balances well to Van's data from his well known long-distance test as often quoted by Jack Norris. I did once inquire of Van's staff some details about the numbers he uses for the 6/6A and the answer I got suggested that nobody really knows anymore. I did not find that very helpful.

The CAFE flight's prop efficiency was most likely 78.8%. If it had flown at 8000' it would have needed 121.32 BHP to go 187 mph. To go 190.5 on the same BHP the prop would be at 82.5%. I did not use your formula because it is not completely accurate. The tool includes the drag polar and the density altitude. It's on my website and anyone can try this by downloading it and using it. The gain in efficiency from 78.8% to 82.5% is 3.7% which is correct in terms of BHP. But comparing them to each other, the gain is 82.5/78.8 or relative 4.7%.

CAFE used a Warnke 70 x 74 wood, fixed pitch prop. FWIW.

By way of comparison in a more general way, the Elippse prop on Jim's 6A is very close in efficiency, as best I can tell, to my Catto 3-blade on my 7A.

Since the A-D prop was at a different RPM and I don't have a really good way to evaluate the HP delta, I did not include it. That said, a minus 1 mph at a plus 3.6% RPM (full throttle or equal MP) is not favorable.

75% of 150 is 112.5. But, 121.32 is 80.8% of 150. This is consistent with what Kevin Horton found when he looked at the question of power versus altitude and all my work comes to the same conclusion. In other words, these numbers are probably right for a nominal 150 BHP engine at 8000'. Thus, they are consistent with the CAFE findings.

Many have pointed out that these are hand-build airplanes and must therefore vary. Yes, that is true. However, we need to assume a high degree of similarity, even in the 6 series which was not pre-punched (?). Comparing 7's, 8's or 9's would be even better because their alignments would be more nearly equal.

Anyhow, thanks for the additional data.
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  #30  
Old 08-01-2011, 07:20 PM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default His implementation is similar to your description

Quote:
Originally Posted by brian View Post
I have a question about what the inside of that fairing is like. We have a pic of it looking forward, but not one of it looking aft. Is the fairing interior open to the aft of the cutout for the exhaust, or is it blocked off? It seems to me that, if it's open, the air coming out of the cowl will go in to the opening aft of the cutout, catch in that dead open space, swirl all around, and create drag as it comes dumping out. It seems to my seat-of-pants engineering that the fairing should have a vertical wall from the aft part of the cutout up to the belly of the aircraft, preferable one that slopes fwd at about 45 degrees, so the air coming from the cowl gets dumped smoothly into the airstream, rather than going into a dead space pocket. I hope I'm describing this clearly so you all can understand what I'm saying.

On similar note, I attended a "Reducing Drag" seminar by Jim Bede at OSH. One thing he mentioned was that slots in the back of wheelpants MAY help reduce drag by dumping the air caught into the wheelpant out to the aft, rather than having it swirling and pouring out as I described above for this exhaust fairing. The other big thing he mentioned was that any and all GAPS create drag, and that we should try to eliminate all the gaps we can.
Jim's fairing is curved and shaped to get the cooling air our in an orderly way - no traps or unnecessary changes in direction an no excess cross section. He told me it is actually the third generation of the cooling outlet fairing by Larry Vetterman. I can see the evolution and the improvement over the previous two designs. I intend to adapt the general idea on my RV-6A with the addition of a flap extending to the forward edge of the outlet. In otherwords I think it is a good design.

As far as cutting outlets at the read of the Landing Gear fairings, I think that is a bad idea that promotes airflow through the fairing from the large opening area around the tire through a restricted outlet. If you think about it I believe you will realize this is not a drag reduction thing. However if you think it has merit you should try it (like I did with the internal volume reducing bulkheads inside the front and back of the fairings - another bad idea).

I am currently involved in a 24 hr a day 7 day a week situation that is not an arbitrary choice but when this resolves I plan to cut the wingspan down by 18" giving a wingspan of 20' with 3" racing tips and make custom deep well wheel fairings based on my stock fairings and custom subfairings and make a cooling air outlet fairing similar to Jim's with a flap. We just keep struggling for knots.

Bob Axsom
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