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02-17-2010, 05:29 PM
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been here awhile
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Join Date: Dec 2004
Location: North Alabama
Posts: 4,301
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Quote:
Originally Posted by VHS
I need the opinion of an aeronautical engineer on a physics of flight question that is now a blood feud. Can anyone with lots of impressive initials after their name contact me offline at vhsproducts@aol.com to see if you can stop the potential bloodshed?
Allen
VHS
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Even though few of us are genuwine aeronautical englenears, you really have our curiosity aroused...... 
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02-17-2010, 06:11 PM
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Join Date: Feb 2008
Location: Napa, CA
Posts: 293
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Aeronautical engineers?
Okay, the post may get nuked for not being appropriate to the APRS venue, but here it is: A question on a CAP (USAF-AUX) blog has been posed and argued from all sides. Imagine that you have an airplane on a runway that is a giant treadmill. The runway automatically changes speed to mirror the speed of the aircraft wheels. Can the Airplane take off?
I argue that of course it can, but my fellow Capper's ( many communists and admitted homosexuals) argue that it could not take off. I was hoping to find an AE who could settle the argument.......Pistols are just so passe'. I think you will be able to view the arguments here: http://captalk.net/index.php?topic=9902.0
Allen
VHS
Last edited by VHS : 02-17-2010 at 06:14 PM.
Reason: left out critical info
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02-17-2010, 06:22 PM
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Join Date: Jan 2009
Location: Gilbert, AZ (and missing TX)
Posts: 54
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Here you go
The merits of airspeed vs groundspeed. If you can find the whole episode they explain it in a little more detail.
http://www.youtube.com/watch?v=YORCk1BN7QY
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02-17-2010, 08:13 PM
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Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
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Pat and Allen,
I measured the antenna and the tip I cut off, and the remnant is 4.5". So Pat, that means we have the same items, which makes sense. I saw the 20" note in the COMANT description of the 122SP, but sounds like that is either a misprint or that describes another dimension (like a virtual or effective length) who knows, and not a biggie.
So at 20.5" mine is showing an SWR of 1.6:1 at 144.39, and Pat, it sounds like yours is better. Did you note the impedence at that freq? I think I'd go with the full length as well, if I had not cut it already.
As for grounding, the avionics shop confirmed what Allen posted. Since I have a doubler riveted to the frame, it appears the four screws and the BNC connection is providing a good ground.
Now if I can just get rid of that bleed over when the packets fire! I looked into a band-pass or band-reject filter for the range of my comm 2 (136-174 MHz, which would cover the APRS freq), but was quoted $395!  However, on the recent SAR practice mission, using comm 2 on the whip did not seem to cause a problem as it did on the ground. However, the APRS on the whip consistently causes a little buzz when it transmits. I may see what a band-reject filter for just 144.39 would cost. Any thoughts from the pros?
Cheers,
Bob
Last edited by rvmills : 02-17-2010 at 08:15 PM.
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02-17-2010, 08:43 PM
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Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
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Quote:
Originally Posted by VHS
Okay, the post may get nuked for not being appropriate to the APRS venue, but here it is: A question on a CAP (USAF-AUX) blog has been posed and argued from all sides. Imagine that you have an airplane on a runway that is a giant treadmill. The runway automatically changes speed to mirror the speed of the aircraft wheels. Can the Airplane take off?
I argue that of course it can, but my fellow Capper's ( many ********** and admitted *********) argue that it could not take off. I was hoping to find an AE who could settle the argument.......Pistols are just so passe'. I think you will be able to view the arguments here: http://captalk.net/index.php?topic=9902.0
Allen
VHS
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Allen,
I looked through that CAPTALK thread, and though I don't belong to either of the groups you mentioned (I redacted the titles for PC-ness  ), at first blush, I have to say I'm in the "Won't Fly" camp. I'm an Aero Major, but not an AE, so this is all from the (well "a") pilot's perspective.
If the treadmill matches the wheel speed exactly, then the prop will make the treadmill turn instead of making the airplane move forward. If the airplane doesn't move forward, there is no relative wind other than a small amount of propwash flowing over the wings. I would say that unless the airplane is very specially designed, and very, very light, it will not fly with the small amount of lift generated by propwash flowing over a small part of the inboard portion of the wings. Maybe one of those balsa wood/rubber band toys we used to play with (OK I still play with 'em).  Sounds like the referenced mythbusters episode allowed the airplane to move forward, which can't happen if the treadmill exactly matches the wheel speed of the plane.
If you can design an airplane that will take off with just prop wash flowing, definitely patent it...it'll be one heck of a VTOL aircraft, and you'll be a very wealthy man (can I be your VP marketing and test pilot?!) Helicopters do it by moving the wing through the air and creating relative wind. I don't know of a fixed wing airplane that can do it with the prop.
On the other hand, if the treadmill runway could be turned into the wind, and the wind was over...hmmm...say 60 knots, then maybe and RV could take off...maybe! In the Navy, we call that kind of runway an aircraft carrier...but we replace the treadmill with a catapult...but it still moves the airplane forward (which the theoretical treadmill will never do!)
I'd love to hear Kevin Horton's take on this!
All tongue-in-cheek, of course. You know, it'd be interesting to spark that debate on VAF...could be colorful!
Cheers,
Bob
Last edited by rvmills : 02-17-2010 at 08:50 PM.
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02-17-2010, 08:54 PM
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Join Date: Feb 2008
Location: Napa, CA
Posts: 293
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Bob,
Did you watch the Mythbusters video? They actually did the experiment as described and it flew, surprising even the pilot!
Allen
p.s. I am not known for being politically correct....
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02-17-2010, 10:28 PM
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Join Date: Oct 2009
Location: Sydney, Aust.
Posts: 820
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Quote:
Originally Posted by VHS
Okay, the post may get nuked for not being appropriate to the APRS venue, but here it is: A question on a CAP (USAF-AUX) blog has been posed and argued from all sides. Imagine that you have an airplane on a runway that is a giant treadmill. The runway automatically changes speed to mirror the speed of the aircraft wheels. Can the Airplane take off?
I argue that of course it can, but my fellow Capper's ( many communists and admitted homosexuals) argue that it could not take off. I was hoping to find an AE who could settle the argument.......Pistols are just so passe'. I think you will be able to view the arguments here: http://captalk.net/index.php?topic=9902.0
Allen
VHS
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YES, the airplane WILL takeoff. Not only because MythBusters did it, and they're never wrong, but here's why.
As the aircraft begins its' takeoff roll, the wheel RPM will increase, thus causing a corrosponding increase in treadmill speed. This will continue through until the aircraft is at liftoff speed at which point, the surprised pilot shall commit aviation forthwith.
As the aircraft wheels have no bearing on the speed of the aircraft, they, and the treadmill speed are immaterial in the takeoff performance of the airplane. IF you have a car, then it'll be a different story...
__________________
Once you have tasted flight you will forever walk the earth with your eyes turned skywards, for there you have been and there you will long to return - Leonardo DaVinci
My Flickr gallery: http://www.flickr.com/photos/35521362@N06/
RV-9A - Finished on 10th February 2016 after 4 years, 9 months and 19 days! The 1020th RV-9 flying.
First flight 26th March 2016. Essential specs 145KTAS @ 2400RPM, 8000', 24.2LPH, Initial RoC 1800FPM.
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02-18-2010, 10:52 PM
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Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
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Quote:
Originally Posted by VHS
Bob,
Did you watch the Mythbusters video? They actually did the experiment as described and it flew, surprising even the pilot!
Allen
p.s. I am not known for being politically correct....
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Allen,
I didn't see the episode, but googled it and watched several clips and read many reviews. The argument rages, sometimes in not so civil tones. Of course, VAF is a very civil place, so I'll respectfully stay in my "it's busted" camp, despite what the TV show (which I like) showed. So perhaps I'm getting sucked into an impossible debate  , but what the hey, I'll take a bite of the apple.
Seems the basis for the yes-no argument is the interpretation of what this part of the myth states: "The runway automatically changes speed to mirror the speed of the aircraft wheels" or similar wording.
My interpretation is that that means the plane doesn't move forward, relative to the surrounding ground (and air), because the treadmill, conveyor, etc. is matching the rolling speed of the wheels exactly. No aircraft movement, no relative wind, no takeoff. I picture this mythical treadmill to be equivalent to placing the three wheels of the airplane in three frictionless dynomometer wheel-like assemblies (think of sitting on top of three pairs of those silver roller thingy's...good tech term!) Perhaps that is the flaw in this interpretation. If so, I'm open minded, so what does the problem's premise really state?
On the show, there was no treadmill, they put the airplane on a tarp, and pulled the tarp in the opposite direction. Once they got the tarp to stop tearing, they had the aircraft start its takeoff roll, and gunned the truck to pull the tarp in the other direction. The airplane moved forward and took off. However, I question whether the part about the surface moving at exactly the speed of the wheels was adhered to.
So...if the question is whether an aircraft can generate enough thrust to move itself forward along a treadmill moving in the opposite direction at the plane's normal takeoff speed, then I'd agree...of course it can, and it will takeoff. The wheels will be turning at double the airspeed required to takeoff in that case, and the takeoff roll will be longer than normal, but the plane has to move to takeoff, just as it did in the MB episode.
But in that scenario, the treadmill didn't increase to match the speed of the wheels, as stated in Allen's post. I don't think the MB guys tested the original premise well, but maybe I'm missing something, or am interpreting it incorrectly. All in fun, of course!
Pretty good thread drift here...perhaps record setting! So Pat, how's that Loran antenna working on your APRS, and Allen, any pics of your ground assault vehicle with the CI-122SP sticking out of its forehead?
Cheers,
Bob
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02-19-2010, 01:10 AM
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Join Date: Oct 2009
Location: Sydney, Aust.
Posts: 820
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Quote:
Originally Posted by rvmills
<Big Snip>
But in that scenario, the treadmill didn't increase to match the speed of the wheels, as stated in Allen's post. I don't think the MB guys tested the original premise well, but maybe I'm missing something, or am interpreting it incorrectly. All in fun, of course!
<Small Snip>
Cheers,
Bob
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I'd have to disagree with you there.
Unless you have wheelspin, or wheelslide, the wheel RPM is matching the treadmill speed. As the aircraft develops its' thrust independent of the wheels, they can spin at whatever speed they like while the airplane gets in the air.
Think of a normal takeoff, into a 15 knot headwind. Now take that airplane and make a downwind takeoff. You will still liftoff at the same airspeed, but your ground (treadmill) speed will be different by 30 knots. Therefore, the wheel RPM will be off by whatever RPM it is that matches the 30 knot difference in airspeed.
__________________
Once you have tasted flight you will forever walk the earth with your eyes turned skywards, for there you have been and there you will long to return - Leonardo DaVinci
My Flickr gallery: http://www.flickr.com/photos/35521362@N06/
RV-9A - Finished on 10th February 2016 after 4 years, 9 months and 19 days! The 1020th RV-9 flying.
First flight 26th March 2016. Essential specs 145KTAS @ 2400RPM, 8000', 24.2LPH, Initial RoC 1800FPM.
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02-19-2010, 07:30 AM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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This treadmill thing is much too old (by several years) to argue!  It does easily takeoff.
A few years back, when this was being discussed, I had models with free wheeling wheels attached to my treadmill with rubber bands. The rubber bands hardly even stretched, even as I pulled the model forward by hand...no matter what the speed. The treadmill just doesn't hold it back. Back then, I was in the NO takeoff camp. I've changed my mind.
L.Adamson -- RV6A
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