aerodynamics

[hawkwind]

some resources to check out  
http://www.ae.su.oz.au/aero/contents.html
http://homepage.usak.ca/~drs694/fluidmechanics.htm#bluff-body%20aerodynamics
http://princeton.edu/~asmits/Bicycle_web/bicycle_aero.html
http://www.rmcs.cranfield.ac.uk/aeroxtra/exb2pdbase.htm
enjoy  

[Sumner]

Thanks for the links.  Learned some new stuff.

I think Jon should deduct 1/2 of your points on this post as I could only get 2 out of the 4 links to work   .

c ya, Sum

[panic]

[ack]

One of those surprised me quite a bit. According to the graph, with speeds under Mach .6 a sharp point (conical section) has lower drag than a rounded leading edge or "bullet" (ogive) shape.



Ddn't Carroll Smith (among others) rant for years that any any speeds a race car will see on a closed course an elliptical section is preferred?

Well if you found it on the internet it must be true.

[hawkwind]

one of our racers (dlra) adapted a ultra light fairing to his bike using shape B and with the same HP added 12% to his top speed, so in practice it obvisously works quite well
http://Http://photobucket.com/albums/y108/dlra2/speedtrials2005-010.jpg
I was very suprised as I expected very little if any inprovement , as this is the prefered shape for supersonic speeds   it appears to increase the bikes Cd without to much frontal area penality
Gary

[bbb]

think about the numbers the dustbins were running in the 40's and 50's. it wasnt clean hp making those numbers.

[ack]

When they learn of this heads are going to roll at Cessna, Beech, Boeing and at other manufacturers after they learn that their engineers have spent years and hundreds of millions of dollars developing those blunt nose shape of their subsonic aircraft.  Try this link for information on parabolic nose shapes.
http://www.aerospaceweb.org/question/aerodynamics/q0151.shtml

[hawkwind]

One of those surprised me quite a bit. According to the graph, with speeds under Mach .6 a sharp point (conical section) has lower drag than a rounded leading edge or "bullet" (ogive) shape.



Ddn't Carroll Smith (among others) rant for years that any any speeds a race car will see on a closed course an elliptical section is preferred?

Well if you found it on the internet it must be true.

I think we need to put the above data into perspective , looking at the data AND reading the points its making , the author is stating that at low subsonic speeds ,in this graph  below mach # 0.6 which equals 445 mph , there is not much difference between the shapes  maybe 0.03 Cd( and I assume they are balistic projectiles )   , the main point is at what mach # drag starts to increase ( due to the nose shape ) ,in the information from the site you supplied ( which was also on the internet   ) the data is missing an important bit of information  what mach # the model rockets achieved as this has a large influence on the drag created , also the picture examples  where of subsonic jets and a rocket  , the crusing speed of a 747 is 0.86 mach # or 670 mph  at this speed the shape of the nose is important .,all interesting stuff
I look forward to meeting you in person and having a chat  , your liner is a blast  
Gary

[ddahlgren]

it seems hard to believe that you can isolate one part of an aerodynamic body and consider it's performance without any association to the entire body. looking at the example motorcycle it would seem the base drag is the predominent factor in the aero losses. A 0.03 change in Cd of the nose seems pretty hard to prove. it certainly seems like it would not account for a 12% improvment in performace unless the original nose was a large flat plate. I had always thought the first place to clean things up was the base drag as it is generally the largest component of drag in land based vehicles. That is other than poking the smallest hole in the air to start with. The old CdA thing..
Dave

[hawkwind]

Dave that is correct , most improvements will be based around a reduction in base drag , and yes the overall picture tells the story ,where the CdA  figure is paramount ,in my opinion as a keen amateur  , the front shape of those 3 examples is not important  provided that there is no detachment of the boundry layer before the widest part of the vehicle especially at the low mach # we run at , a good experiment would be for my friend to replace the cone shaped nose for eitheran ogive or spherical  nose and see if there is any improvements .
Gary

[ack]

Hawk:

When I started the project a couple of years ago I knew almost nothing about aerodynamic theory (and still don?t).  My approach was fairly simple, don?t be influenced by any prior designs and look to other vehicles that travel at mid range subsonic speeds and have had many years and millions of dollars spent on R & D i.e. aircraft, missiles and bullet trains.  One thing that always struck me as wrong during my previous two trips to Bonneville was the sculptured shapes around wheels and other chassis parts on most of the streamliners.  My thoughts were that you don?t want to disturb the direction of the moving air any more that is necessary.  You won?t see bulges bumps and protrusions on aircraft fuselages missiles or even boat hulls for that matter.  I thought the ?cool look? might have a lot to do with it or ?that?s the way other streamliners look? train of thought.  When the project was getting to the stage where we were ready to start finalizing the body shape a friend of mine brought Ken Mort by.  Ken had worked on the  wind tunnels at Ames NASA in Mountain View for a number of years and had much practical knowledge of various designs.  Ken liked the project and volunteered to help with the design.  He liked my initial design but had me make a number of changes.  First  he liked the parabolic nose but did not like the symmetrical design.  Ken wanted to change the cord to about a 10 degree downward angle for some down force.  I just kept moving the curve down on the computer until Ken said I like that then built the nose tank.  Next we discussed the shape of the windscreen I had two designs one with a long tapered windshield similar to Sam Wheelers and the other a more conventional upright design.  Ken said that up until almost supersonic speeds either design would perform about the same however on the upright design it was very important to have the proper shape at the body transition areas.  When we built the plug for the mold Ken came by and  we just kept changing the transition area until he liked it.  The major change he suggested was to add a couple of feet to the rear of the body to move the center of pressure back and he concurred with my conclusion that a tail would just add drag and increase the effects of a cross wind on stability.  Ken also wanted the bottom profile to be narrow and rounded as opposed to any type of flat surface.  Also with the body as close to the ground as possible.  By accident ,rather than design, the body ended up about 1 ? ? higher in the rear than the nose.  This seems to work well as it seems to produce down force at the rear.  Ken estimated that an open tail would add 10 -20% to the total drag however if the overall drag was low this would not be a great penalty especially with the power we had available and it simplified the design.  We never did any wind tunnel testing.  Ken wanted to tape a few hundred pieces of string to the body and follow it at about 100 mph to observe the air flow however we never had time to do this.  As luck would have it the design has worked very well and we have seen almost no wheel slip at up to 336 mph.  I don?t know if any of this will be helpful in your endeavor but I wish you the best.  I guess my point is that common sense, practical knowledge and luck can often trump theory or commenly accepted principals.  As Ken says ?I would rather be lucky than good any day?.

[panic]

[ack]

Panic:

Rude? Im not as rude as Jack D but I will try harder.  What I thought you meant by your post was Carroll Smith and others were obviously wrong based upon this graph.  What did you really mean? Dave Dahlgren has it right I believe you can not take a finite element such as ballistics data and come to conclusions about a streamlined body.  Thats why I posted the nose cone link for Hawk.

[JackD]

Like speed, rude has all manor of expression, reaction and results. One person's rude/speed is another person's cause to stop and think. The end result should be that a lot of the stopping and thinking has been done before you and the value will be in the results. That is all that will last past today anyway is results.
To fold easily and edit out your remarks often means you are not comfortable with the information in the face of any other ideas.
What works for one might not work for another, but still has to work.
If everybody failed to stop and think and were afraid to try something the result would be a lot closer to the tried and true Highboy Roadster that has not been perfected yet.
You can be nice and rude or mean and rude. In the end, the results will establish the value.
You wanna see rude? Spend a bazillion dollars and get a 2 bit result. That is rude.

[hawkwind]

some interesting points there Ack    cheers , nice to have a friendly neigherhood aerodynamist willing to assist