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Old Oct 04, 2011, 10:09 PM
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I have the David Wing that you were talking about. Those are two different wings. The pink one has much sharper leading edge.
The other day I had an RCSpeedo reading of 164.77 MPH. Still not sure how this can happen with a 2200KV motor on a 3s battery with a 5x5. However I get repeated runs in this range so I am pretty sure its not an error in RC Speedo.
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Old Oct 05, 2011, 12:56 PM
Grumpy old git.. Who me?
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Originally Posted by Bike Medic View Post
Still not sure how this can happen with a 2200KV motor on a 3s battery with a 5x5.
Simple answer is, if those facts are correct, that it cant. As I'm sure you will have calculated for yourself the no-load pitch speed of that set up is about 115mph.. Add some load on the motor and you would have a real world pitch speed of less than 100mph.

Only options that could explain it are:
  1. The kv of the motor is much higher than you think (like over 3500kv).
  2. The plane is basically 'gliding' along at 165mph with the prop contributing no thrust at all (in fact making lots of drag).
  3. RC speedo is coming up with incorrect results.
Option 1 should be pretty obvious because Amp draw would be through the roof but you could check if you have a spare motor. Option 2 can basically be discounted.
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Old Oct 05, 2011, 04:11 PM
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Originally Posted by JetPlaneFlyer View Post
  1. The kv of the motor is much higher than you think (like over 3500kv).
  2. The plane is basically 'gliding' along at 165mph with the prop contributing no thrust at all (in fact making lots of drag).
  3. RC speedo is coming up with incorrect results.
wrt #2 ... accorrding to a variety of books, the SR-71 "sucked it's way through the air with the intake provinding 75% of the thrust" at speed and altitude.

Clearly #2 is possible ... (lol).
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Old Oct 06, 2011, 01:58 AM
just Some Useless Geek
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Originally Posted by Cap_n_Dave View Post
...according to a variety of books...
Can you point to any online references? This statement doesn't pass the smell test. I'd like to look this one up for myself.
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Old Oct 06, 2011, 02:35 AM
Build straight - Fly twisty
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Quote:
I'd like to look this one up for myself.
Yup, it's not really as stated:
http://en.wikipedia.org/wiki/Lockhee...ird#Air_inlets
How did the SR-71 get on to a KF thread?
We've had owls, albatross and now blackbird...



Oh! And a vulture and the silver gull as well
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Old Oct 06, 2011, 06:29 AM
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Can you point to any online references? This statement doesn't pass the smell test. I'd like to look this one up for myself.
http://www.grc.nasa.gov/WWW/k-12/airplane/inlet.html

Read the "supersonic inlets" part.

Please realize I was being sarcastic in my comment, of course inlets don't produce "thrust" they produce pressure recovery and drag. The problem is that explaining how pressure recovery contributes to engine efficiency, and therefore thrust, is not a straighforward thing.

Ben Rich's book "Skunk Works" has a similar statement however he attempted to qualify his statement by invoking 'pressure recovery' but many laypeople don't understand and/or take it out of context.

Lastly, if you are an SR-71 fan, take a look at this book. The discussions in the book about the engine inlet are fascinating ... one thing I learned was the intake was FAR more complex than I had initially thought.
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Old Oct 06, 2011, 06:46 AM
Grumpy old git.. Who me?
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Quote:
Originally Posted by Cap_n_Dave View Post
wrt #2 ... accorrding to a variety of books, the SR-71 "sucked it's way through the air with the intake provinding 75% of the thrust" at speed and altitude.

Clearly #2 is possible ... (lol).
The SR-71 'suction' claim is plain wrong. In fact the SR-71 got most of it's thrust from ram-jet effect at high speed, the main jet engine compressor being largely bypassed in high speed flight. As the inlet air is decelerated from mach 3.5 to subsonic as it goes through the engine then there could be no great suction effect, the NASA link says nothing about suction. At very high altitude significant 'suction' is impossible anyway because the atmospheric pressure is so low that there is nothing to suck against.. you can only 'suck' down to a perfect vacuum and at altitude you arent far from a vacuum to start with.

Pressure recovery is a totally different thing, it's the conversion of dynamic pressure to static pressure which is how ram-jets work.

But it's also irrelevant to a plane propelled by a prop anyway.. A prop will only provide forward thrust while it's moving slower than it's pitch speed (in truth that's a bit of a simplification but near enough).

Steve
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Old Oct 06, 2011, 06:58 AM
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The SR-71 'suction' claim is plain wrong.
Yes, I know.

I was being sarcastic. The internet NEEDS a sarcasm emoticon.

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Old Oct 06, 2011, 07:18 AM
Jack
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Old Oct 06, 2011, 08:01 AM
Build straight - Fly twisty
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Please can we get back to the real topic of this "Discussion?"
Someone was building a wind tunnel. Any progress there?
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Old Oct 06, 2011, 08:25 AM
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It is unfortunate but sarcasm is often perceived as sarchasm by those self-appointed armchair experts who hang around here and specialize on making wordy pronouncements on things that they consider themselves experts on.
Erm, not what I had in mind.

Steve is very knowledgeable ... I don't put the blame on him, rather, the internet strips away some of what happens in "normal" (i.e. face-to-face) interaction between humans.
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Old Oct 06, 2011, 08:29 AM
Jack
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Eeek!

That was not an accusation aimed at Steve or anyone! It was a general observation about the foibles of posting anything anywhere, missed sarcasm, and the like.

I needed more emoticons I guess...I'll delete my post and hope that is takes the ripples out of an otherwise tranquil pond.

Jack
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Old Oct 06, 2011, 08:30 AM
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Speaking of that (oh my, an on-topic post!) I was dredging through the "plans page" http://www.theplanpage.com/index.htm and found plans for a wind tunnel scanned from the 1942 Air Trails. Kinda an interesting project, may have to make it a winter one..... Then I can start testing KFM airfoils - assuming that the differences are enough to measure with a simple "tunnel"

Sarchasm, I love it! he said off topically........
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Old Oct 06, 2011, 08:47 AM
Jack
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I had some trouble finding those wind tunnel plans, here are some more specific links to them:

Wind Tunnel Pt.1 - http://www.theplanpage.com/Months/2306/2306.htm
Wind Tunnel Pt.2 - http://www.theplanpage.com/Months/2307/2307.htm

For those that are interested in wind tunnels, here is another link to one:

http://www.techdirections.com/BuildAWindTunnel.pdf

And the last page of that document has more links to more wind tunnel stuff.

It must be something to this. Could it be that there is more to this wind tunnel thing than it being just a bunch of blow hards?

springer,

Thanks for mentioning those, I don't mean this post to criticize you!

Jack
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Old Oct 06, 2011, 01:24 PM
treefinder
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Ha! I get my own perverse kick out of finding out who will actually dig down and find the actual web page! Should have known it wouldl be you, Jack!
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