Canard Community Forum  

Go Back   Canard Community Forum > FLYING INFO AND STORIES > FIRST FLIGHTS & COMPLETIONS
FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Display Modes
  #271  
Old 07-06-2005, 12:41 PM
bferrell's Avatar
bferrell bferrell is offline
Member
 
Join Date: May 2005
Location: Cincinnati, OH
Posts: 174
Default

Quote:
Originally Posted by Don P-Factor
Golf balls are dimpled? Is that to increase lift and decrease drag? Does this only work at lower speeds. What does the blunting of the trailing edge if an airfoil accomplish? Wind tunnels are a good tool but when scaling, there is no direct correlation to real world flight testing. Real flight testing in full scale is the only way to quantify flight characteristics. Don.
I believe the dimpling is for drag reduction, but it only works for certain shapes. I saw a show on the "Science of Lance Armstrong" where wind-tunnel testing showed that the diameter of a bicyclist arm and upper leg would benefit from 'dimpling drag reduction', so their racing jersey and shorts incorporate dimples in those areas, but the rest of the uniforms are very smooth, because the larger the object the less the benefit from the uneven texture.

I disagree that full scale testing is the only way to quantify flight characteristics. Many designs are modelled in wind tunnels and/or computers and test flown long before they really fly, and can be scaled reliably as long as the rules of similitude are followed. Flight testing is the only way to prove that your model really has similitude with the full scale, but it can be very reliable when done properly.
http://en.wikipedia.org/wiki/Similitude_(model)

Brett
__________________
Brett Ferrell
Cincinnati, OH
www.velocityxl.com www.eaa974.com
Reply With Quote
  #272  
Old 07-06-2005, 02:06 PM
Marc Zeitlin Marc Zeitlin is offline
Member
 
Join Date: May 2004
Location: Tehachapi, CA 93561
Posts: 1,083
Default

Quote:
Originally Posted by rv6ejguy
Airfoil shape has little or no effect on lift coefficient even at low alphas? News to me. A symetrical airfoil or piece of plywood has a lift coefficient of 0 at 0 alpha. Many airfoils have a CL of .25-.35 at 0 alpha.
OK, let me get a little further into the theory. If you take a look at the lift curve slope of a given airfoil (or plywood sheet, as the case may be), there will be some AOA at which the airfoil develops zero lift. Call this the zero lift AOA. Using that AOA as a reference, if you measure the Lift Coefficient at DELTA AOA's from that point, you'll find that the CL not dependent upon the airfoil shape. This means that the lift curve SLOPE is not a function of the airfoil shape.

So, if you're 1 degree AOA above the zero lift AOA, a sheet of plywood and a Eppler 1230 mod. (with the same size and planform) will develop the same CL. Same goes for 2 degrees, 3 deg. etc., until you reach the AOA at which the sheet of plywood will stall due to the crappy shape, whereas the real airfoil can keep increasing AOA. Since at high speed in cruise, the CL and DELTA AOA is relatively low, this is not an issue in cruise - only at lower speeds.

Does that help explain what I meant and how airfoil shape does not affect lift generation? If you look at the equations that determine lift, they're dependent upon wing size, wing aspect ratio, sweep angle and a few other things, but airfoil shape is NOT one of them, at least to a first and second order approximation.

See:

http://www.rmcs.cranfield.ac.uk/daps...nitewings/view

For some more lift curve slope info, and some graphs.

Quote:
Originally Posted by rv6ejguy
Structural considerations aside, must be a good reason why most non-aerobatic aircraft use unsymetrical airfoils instead of flat plates.
Yes, there is. They're more efficient (meaning less drag for a given amount of lift) in the aircraft SYSTEM. If you don't need to fly upside down, there's no reason to use a symmetric airfoil.

Quote:
Originally Posted by rv6ejguy
... Maybe there is something that I'm just not understanding here and I bow to your superior knowledge on this subject.
No bowing required. I'm happy to explain what I know for those that wish to learn, and I'm happy to learn from those that can explain stuff to me, especially when I'm mistaken about something.
__________________
Reply With Quote
  #273  
Old 07-06-2005, 02:27 PM
Glos Glos is offline
Member
 
Join Date: Mar 2005
Location: Roselle, IL
Posts: 252
Default

Mark

I'm not sure I completely understand either. Almost there though. Maybe you can help explain this.

I have the old (original ) shape on my Long EZ. With no apparant change to the AOA ie flying happily along, I entered rain for the first time. Bang, an immediate loss of lift. Nothing uncontrolable but a loss none the less.

After that I became more alert in the rain and always kept bugs off the canard.

I was understanding that the contamination was some what altering the airfoil shape, something similar to ice on the leading edge.

What are the physics here?

Rick
Reply With Quote
  #274  
Old 07-06-2005, 02:52 PM
rv6ejguy's Avatar
rv6ejguy rv6ejguy is offline
Member
 
Join Date: Dec 2004
Location: Canada
Posts: 578
Default

I understand the concept of a zero lift line vs a chord line. If our typical wing runs at 1 degree alpha in cruise, I'd still have to think that a non-symetrical airfoil has an advantage even here in lift over a symetrical section or flat plate.

I've flown a Zlin with symetrical section and it is the slowest, worst climbing aircraft I've ever had the non-pleasure to fly, despite a normal aspect ratio and plenty of wing area. Granted the climb is at higher alpha than cruise, but this thing was inferior in every respect to something like a Warrior having far less power. The Zlin has a supercharged six.

In my model aircraft days I used to compete in free flight Wakefield and glider events. I built dozens of these of my own design, tweaking airfoils, aspect ratios and experimented with turbulators and vortex generators. Even within the same design and planform, I found airfoils made a marked difference in L/D and glide ratio. I'd have to assume that drag and lift therefore were affected strictly by airfoil shape. Were were getting glide ratios of almost 40 to 1 eventually and my windy weather ship out maxed the previous years championship winning calm ship in dead calm test conditions one night at the nationals. In glide, these operated at very low alphas although I had no way to measure it.

Certainly these observations don't have the carefully controlled condition set as a wind tunnel might have but it is hard for me the fathom this. Of course I could be wrong!

Reply With Quote
  #275  
Old 07-06-2005, 03:24 PM
Marc Zeitlin Marc Zeitlin is offline
Member
 
Join Date: May 2004
Location: Tehachapi, CA 93561
Posts: 1,083
Default

Quote:
Originally Posted by Glos
I have the old (original ) shape on my Long EZ.
That's the GU canard.

Quote:
Originally Posted by Glos
With no apparant change to the AOA ie flying happily along, I entered rain for the first time. Bang, an immediate loss of lift. Nothing uncontrolable but a loss none the less.
That's the standard response for most GU canards when contaminated and losing laminar flow.

Quote:
Originally Posted by Glos
I was understanding that the contamination was some what altering the airfoil shape, something similar to ice on the leading edge.
Not really - what it does is alter the effective shape due to airflow detachment, which moves the zero lift AOA (and a minor change in Lift Curve Slope), which then requires elevator trim change to correct. Since the canard needs a higher AOA to produce the same amount of lift, you need more elevator to produce the higer effective AOA.

The Ice issue IS in fact an airfoil shape change, but again, it will move the zero lift point, and not substantially change the lift curve slope. Both Ice and Contamination WILL substantially effect the MAXIMUM Lift Coefficient that can be produces, which is why the stall speed of the aircraft increases dramatically in these cases.

See the "GU / Roncz Canard #2" slide from:

http://www.cozybuilders.org/Oshkosh_...esentation.htm

Todd Parker supplied the Xfoil analyses of the airfoils. You can see the GU airfoil zero lift point change SUBSTANTIALLY from about -7 degrees to -4 degrees. In this case, you can also see that the GU canard's Lift Curve Slope does change a bit as well - much more so than do the other airfoils when contaminated. But you can also see that the LCS's of all the airfoils (even the others when contaminated) are almost identical, even with very different shapes.

Quote:
Originally Posted by Glos
What are the physics here?
I can't say that I can fully explain the modest change in LCS of the GU airfoil when contaminated, other than to say that the detached airflow decreases the pressure differential between the top and bottom surfaces. But as the web page quoted in my above message indicates, for relatively thin airfoils (which almost all that we use are) at low AOA's (well below stall), the LCS is not dependent upon the shape of the airfoil.

Hope this helps a bit more.
__________________
Reply With Quote
  #276  
Old 07-06-2005, 03:42 PM
Marc Zeitlin Marc Zeitlin is offline
Member
 
Join Date: May 2004
Location: Tehachapi, CA 93561
Posts: 1,083
Default

Quote:
Originally Posted by rv6ejguy
... Even within the same design and planform, I found airfoils made a marked difference in L/D and glide ratio.
Absolutely so, and that's because airfoil shape has a HUGE effect on the DRAG and on Cl/Cd ratios at given AOA's. The EFFICIENCY of the airfoil is hugely dependent upon the airfoil shape, but the LIFT generated at a given angle above the zero lift line is not.

Quote:
Originally Posted by rv6ejguy
I'd have to assume that drag and lift therefore were affected strictly by airfoil shape.
See above - drag, absolutely - lift, very little effect, if any measurable at all.

Take a look at the drag polars of various airfoils - you can see drag coefficients at various lift coefficients, and see the huge range of efficiencies that can occur based on operating AOA.

Then take a look at the lift curve slopes of any arbitrary airfoil, and normalize them for zero lift line AOA's - they're all essentially identical.

Am I getting any closer?
__________________
Reply With Quote
  #277  
Old 07-06-2005, 04:23 PM
rv6ejguy's Avatar
rv6ejguy rv6ejguy is offline
Member
 
Join Date: Dec 2004
Location: Canada
Posts: 578
Default

Thanks Marc.

It does make sense if you look at the lift and drag polars for some different airfoils. Hopefully I haven't wasted your time with my ramblings. Good to have you back here again.
Reply With Quote
  #278  
Old 07-06-2005, 04:23 PM
Glos Glos is offline
Member
 
Join Date: Mar 2005
Location: Roselle, IL
Posts: 252
Default

Thanks Mark

This was a really good explanation, the slides helped as well. This is not exactly my science but I understand it better now.

Rick
Reply With Quote
  #279  
Old 07-06-2005, 04:37 PM
Glos Glos is offline
Member
 
Join Date: Mar 2005
Location: Roselle, IL
Posts: 252
Default

Back on topic

Marks education on lift got so interesting I forgot to say congratulations to John.

From now on it's 40 point something hours and counting.

All I have is a O 235 L2C, wishing it were bigger.

But you have made a giant leap. Don't know that I would have the courage to do what your doing.

Keep everyone posted on the progress

Rick
Reply With Quote
  #280  
Old 07-07-2005, 08:51 AM
Don P-Factor Don P-Factor is offline
Don P-Factor
 
Join Date: Feb 2005
Location: Indian River City Fl.
Posts: 154
Default 40+

John, Congratulations on your milestone. I really enjoy reading your progress reports and wish you many carefree hours of flying enjoyment. Don.
Reply With Quote
  #281  
Old 07-07-2005, 09:49 AM
John Slade's Avatar
John Slade John Slade is offline
Flying TurboRotaryCozyIV
 
Join Date: Apr 2004
Location: KWST
Posts: 3,836
Default

Quote:
Keep everyone posted on the progress
Will do. I'll keep the Hobbs ticker onthe main page up to date. I'll probably use this forum for short updates. No progress since July 4th. I did something to my back over the weekend and can barely make it from the couch to the computer. IFR class tomorrow. Not sure if I'kk be able to make it.
Reply With Quote
  #282  
Old 02-08-2006, 04:55 PM
John Slade's Avatar
John Slade John Slade is offline
Flying TurboRotaryCozyIV
 
Join Date: Apr 2004
Location: KWST
Posts: 3,836
Default

As promised, here's a short update:

The kitten now has 52.3 hours and is running purrrrfectly. A quick Hmmmm around the pattern today showed all engine readings in the green and the IVO prop doing it's job admirably. I fitted a small ammeter in the prop circuit ($12 from mouser) so I now have a good reading of the current prop setting. Today I left it on full fine. RPM on runup was 5000 and 5600 as I got airborn very shortly afterwards. Climb was excellent with plenty of throttle to spare.

I'm running the stock turbo again after trial fitting the T04, measuring and ordering the exhaust pipes and fittings. She's performing so well with Bob's turbo that I'm beginning to think that the hybrid modification I had done in AU was leaking oil from the bearings and into the intake from day one. Not much. Just enough to screw everything up with mixture and ignition. These days the plugs and turbo wheel are a nice grey / brown color.

I can't put into words how good this engine feels now, after all the frustration I've been through over the past few years. Idle is a pure purr at around 800. I was considering trying to add a muffler during the T04 install but, on second thoughts, I dont think I'll bother. It's simply not needed.

One final (I hope) annoyance is that the kitty piddles after shut down
I have a minor coolant leak somewhere that's letting go during the heat soak. About a half cup of coolant comes out after every shut down, and I don't think it's from the overflow. Once I find this problem, its travel time.

I have new Matco disks to install to get rid of a bit of shimmy on the brakes, the T04 turbo will go in a few weeks and I'll be upgrading the redrive from RD1A to RD1B some time in April or May.

At some point I'll get around to bring the web site up to date, but for now I'm just keeping the hobbs ticker current.
Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -4. The time now is 12:37 PM.


Powered by vBulletin® Version 3.6.7
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.