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Possible antennas for APRS: Used LORAN antennas.

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Pat Hatch

Well Known Member
Advertiser
Anyone looking at alternatives for an antenna for your APRS rig, take a look at these bent-whip, used Loran antennas: Comant CI-122SP.

I just bought one for $9.95 on eBay. They are apparently removed Loran antennas and there should be plenty of them around now. The nice thing is that they exactly match the two Comant CI-122's that I have for COM1 and 2 now. As near as I can tell, they are basically a CI-122 shortened to 20" for Loran. Obviously Loran, being in the 100 kHz range, is using some other multiple of wavelength other than 1/4 wave. So near as I can tell, 20" would be about a perfect 1/4-wave for 144.390 MHz; and they have a BNC connector, what's not to like?

I spoke to my avionics shop here on the field and they know of no reason why these shouldn't work. If anyone does, please speak up!

Here's the link:

http://cgi.ebay.com/ebaymotors/ws/e..._trkparms=algo=SI&itu=UCI&otn=18&po=LVI&ps=54
 
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Pat,

I'm using a CI-122SP as well, and switching it between the APRS and my Sheriff SAR Radio (150.025), and it works well.

The avionics shop here had an old spare on the shelf and gave it to me. However it was longer than 20", so I cut it to just over 20", and it works well for both APRS and SAR comm. I was told it was OK to cut the SP, whereas its not good to cut the normal CI-122, due to what is in the base (can't remember what it was...filter, or amp or tuning "stuff"). Did the one you buy come 20" long?

Guccidude Dan and others are using an inexpensive whip with the small round insulator on the base, and those are OK to cut too. But you're right...the 122SP looks like the 122, and has the same faired base, which is nice!

Cheers,
Bob
 
Comant CI-122SP

Hey Bob, yeah, I have an old Comant catalog and it describes the CI-122SP as follows, "Provides excellent reception by utilizing a 20-inch stainless steel element bent backward to maintain a maximum height, etc....Complete antenna features P-Stat anti-precipitation coating and is finished in a white Skydrol resistant polyurethane paint." I wonder if this coating is helpful in reducing that static buildup that Allen was describing. Did you measure from the BNC connector to the tip? I don't have the antenna yet to measure but I will when I get it and report back. For now, I'm using my COM2.
 
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Pat,

I'll measure the antenna and the remnant when I get home tomorrow. IIRC I cut about 3" off. I called COMANT before I cut it to discuss the use, and the gent said to measure from about an inch above the the flat base of the antenna (where the BNC attaches to the base).

I used 20.5", from the calc:

300/144.39 = 2.0777 meters = 81.77"/4 = 20.44" for 1/4 wave

Figured I'd leave it a little long then put an SWR meter on it when installed and tune (trim) it more closely for midway between my APRS and SAR radio freqs.

Might not even have to do that last part, as it works pretty well now, and it seems the APRS works well with a fairly high SWR (I believe Allen has said even up to 8:1). I'm going to fly a SAR training mission on saturday and test the Kenwood radio on it, and see if I need to go a little shorter for voice. Will be transmitting APRS on the gear-leg j-pole on that mission. Should be an interesting test.

Will let you know what the SP measured before cutting when I get home.

Cheers,
Bob
 
LORAN Antennae

Looking at E-Bay, I found these types of antennae all over the place. I could not resist when I saw one that was brand new in the box for $15.00!
http://cgi.ebay.com/ebaymotors/ws/e...0394136445&viewitem=&sspagename=STRK:MEWAX:IT

I have never dissected a LORAN antenna before, so I don't know if there is a loading coil or other special matching balun, but I am anxious to try these out. Will advise. ( I think it will look cool on my Honda Mini-Van too!)

Allen
VHS
 
I have never dissected a LORAN antenna before, so I don't know if there is a loading coil or other special matching balun, but I am anxious to try these out. Will advise. ( I think it will look cool on my Honda Mini-Van too!)

Allen
VHS

Allen, the avionics guy here on the field said that the impedance matching was done external to the antenna. I hope this is accurate, but he claimed there was essentially no difference between the CI-122 used for VHF com and the CI-122SP used for LORAN except for a small difference in the element's length and the fact that they are coated. Let us know what you find out!

These antennae will be a great deal if they actually work out.
 
I think you should put a SWR meter on your antenna. I am guessing you will not be happy about the readings. The antenna might work, but I am curious how well. Interesting.
 
Antenna Test

Drew,

That's exactly my plan. I plan to test the unit with an antenna analyzer, and then static test the SWR and ERP on my minivan. It would be nice not to have to shorten it and damaged the units water-resistant integrity, but I think I can lop of short lengths and verify a reasonable SWR, and then tool-dip the end. Probably effective for a car, but possibly not quite robust enough for an aircraft....

I know that the wavelength of the LORAN signal is a tad bit longer ( the wave is about 2 miles long, and a quarter wave receiving antenna would be a little cumbersome) At 2 meters, if its just a simple whip, its pretty darn close Even a little over-length will probably ( I surmise) outperform a 3 dB rubber duck.

Allen
VHS
 
Allen,

I'll be very interested to see what you find out with the meter, and if you dissect it, what is inside.

I talked to both the local avionics guy and a rep at COMANT about the 122SP, and from what I gather, it is a simple whip. The local guy said the SP was OK to trim, and while the COMANT guy wouldn't/couldn't say that, he gave me the "verbal wink". I went by the avionics shop today to ask again how the 122 and 122SP are different (its what's inside the base of the 122, I just can't rememeber the tech terms), but they were out of the office workin' on planes. In the COMANT antenna guide, the dimensions shown on both antennae are the same.

I put an SWR meter on both the CI-122SP and my Howell j-pole antennas today. One caveat here, the meter was connected at the feed lines, right near the transmitters in the cockpit. I know that it would be better to check the SWR right at the antenna, but the j-pole is permanently connected to the feed line, and I didn't have time to pull the floor to test the whip at the BNC. But here are the results.

1. CI-122SP (shortened to 20.5" measured along the whip from tip to 1" above the BNC):

144.39; SWR 1.6; IMP 118 ohms

OK SWR, but an impedence mismatch. I tuned the meter up and down to the first freq that showed 50 ohms, and found this:

138.15; SWR 1.8; IMP 50 ohms
151.87; SWR 1.8; IMP 50 ohms

It had a fairly flat SWR curve (if that is a correct term) and it's best SWR was the 1.6 at 144.39. Does that mean I cut it to the correct length? I dunno!

2. Howell j-pole (twinlead TV antenna down the right gear leg):

144.39; SWR 3.1; IMP 4 ohms

SWR is higher, and it has a big impedence mismatch at that freq as well, which probably corresponds to it's lower performance compared to the 122SP. I also tuned the meter up and down to see where it hit 50 ohms, and found the following:

121.72; SWR 3.6; IMP 50 ohms
160.76; SWR 2.9; IMP 50 ohms

The j-pole's lowest SWR was 1.3 at 129.81 MHz, with an impedence of 30 ohms. Not sure if that means the j-pole is currently tuned for 129.81, and thus needs to be shortened...in fact I don't know if any of those numbers mean anything of significance. Any thoughts Allen (or anyone)?

After testing, I did a test flight, and switched between those antennas in flight. I flew out to a practice SAR target on guccidude Dan's wing, so it's an interesting comparison of the two tracks. He's in green, I'm in blue:

aprs12feb10.png


He transmits via a bent whip with a small porcelin base, and had a good track throughout most of the flight, both hi and low. I transmitted via the jpole on the way out and in the target area, and on the CI-122SP during one turn up north which you can see, and as we got close to the airport homebound. The target area is to the northeast, and down low the j-pole did poorly (it's pretty sparse up there too). The 122SP did well on the way back, and during the landing.

So the 122SP really isn't doin' too bad so far. We fly the SAR practice again tomorrow, and I think I'll try doing the whole mission on the 122SP. Of note, when I run the APRS on the whip, my SAR radio is switched to the j-pole, and it worked well today for voice comm with guccidude and the ground crew...pretty interesting for a wire down the gear leg (granted, it was all at close range!)

Kinda fun to play with this stuff though...gadget-time! :)

Cheers,
Bob
 
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Bob, its probably been 30 years since I cracked one of my EE textbooks, but I did today! Just wanted to refresh on VSWR and impedance mismatch, etc.

Anyway, your VSWR's are very good. Reflected power (bad) at 1.6:1 VSWR is about 6%, thus 94% is getting through your antenna. The worst VSWR I saw was 3.6:1, this is about 31% reflected power, not real good but not awful either. Interestingly, in the COMANT manual that I have, it shows the CI-122 rated at 50 ohms and VSWR of 3.0:1, which is a 25% loss (reflected power).

The only caveat is that VSWR should be measured at the antenna, and if not, could be giving a deceptively better than actual VSWR reading. This is because of the losses in the coax and connectors that could be attenuating the reflected wave.

Hope this gives you some perspective.

P.S. To tune your J-pole, you pretty much have to connect the meter at the antenna and then just trim to minimize VSWR (at 144.39 MHz), just as you suggested. I wouldn't worry too much about impedance at that point, just VSWR. Good data!
 
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J-Pole

My comments are in defense of the J-Pole which has been successfully used in the Ham Radio world for many years. Surprisingly it is working as well as it is in this application mounted tightly to a metallic member or support, I'm sure the designer never intended it be mounted in this manner.

Any time a object is placed near an antenna it has an effect, this effect can be positive or negative depending on the design criteria and intended use. This method is used to detect stray metal when packaging food products.

Using a 1090 MHZ transponder antenna as an example: when a metallic object is placed within 1 wavelength (about 10.8") of the antenna the effect can be measured on a network analyzer.
Move the metallic object within 1/2 wavelenght and it will effect the antenna properties, move it to within 1/4 wavelenght (2.7") and the intended design properties will no longer be available.
Scale these dimensions out to the APRS frequency and 20.5" becomes the point where the intended properties are no longer available.

Now with that said, the antenna may still work under those conditions but the performance will be severely degraded.
 
Pat and Don,

Copied both replies, and thanks!

On the SWR...Pat, thanks for dusting off the ol' textbooks! I've been told that < 2:1 is desirable, but concur with you that what I'm showing is not too bad, given the situation. Allen said the MT-8000FA would be OK up to even 8:1! The readings on the j-pole were very sensitive to movememnt and touching, and I had the end of the feed line taped up near where it attaches under the panel, to try to simulate the installed position.

Next time I have the floor up, I'll test the SWR of the 122SP at the BNC, and swap in a backup 122 that I have, and see what it shows in comparison to the 122SP. To really give the SP a good test, I really should fly a X-C to CA and back, do a little hi and low flying in the Central Valley or on the coast, and then swap the 122 into place for the way back. The areas I've been testing are pretty remote, and line of sight to digis or repeaters is often questionable. I'll touch on today's flight below.

Don, In this quest for a no-drag antenna, using Pete's j-pole like this is really putting it through about the most odds-against-success scenario...and it's not doing half bad. It's threaded down the aft part of the gear leg fairing, and runs down the fairing about an inch from the titanium strut. Then it wraps around the top of the gear leg and up the motor mount with standoffs. The feed line is stood off from another part of the motor mount, and then penetrates the firewall with other wires. It probably shouldn't work at all! But it gets pretty good high reception (like Sam said, a coat hangar probably would too! :))

Pete and I may tune it, if, no when (right Pete), he comes out for the races sometime. I owe him a very nice lunch or dinner for all the help and the DDRR antenna, and its become a bit of a quest to see what we can make it do.

In the meantime, our expectations are in line with reality, and its fun to see how these antenna perform.

Today I flew a practice SAR mission north of Reno and Pyramid Lake, up and down what's called the Smoke Creek Desert. It's a pretty inhospitable area, to hunters and ATVers, as well as APRS tranmitters :rolleyes:. It was a bit of a marathon...3hr session. We spent most of it below 1000' agl, and much of it at 500 agl. I alternated between the j-pole and the 122SP for APRS and the SAR radio. The j-pole did pretty well as a comm antenna (150.025), but we had some comm issues at times, so I used the whip as well for the mission comms. Hard to tell if there was a difference, as we were swapping between simplex and duplex channels, and talking to ATV SAR crews that had hand-helds and panel-mounts. Pretty fun mission, and we made verbal and visual contacts with 3 ground SAR teams and the practice "victims".

In the screen shot below, the majority of the hits were either on the whip, or on the j-pole when over 1000' agl. There is a big gap in coverage when I went up north near Gerlach, as we were 500' above the RR tracks in the desert during that part of the search, and using the j-pole (tough duty for any antenna).

aprs13feb.png


To give you a feel for the arduous terrain, here's a closer shot of the area in the satellite mode:

aprs13feb2.png


Lots of ridges to block packets, and its really in the boonies! If we can find an internal antenna to work in this SAR environment, and not cost speed for SARL racing, it'll be a real homerun. In the meantime, I'll swap between them with the MFJ-1703 switch I installed, and try to get the best of both worlds!

I know we're driving Allen crazy with all the wacky antennas, but the experimenting is part of the fun! Thanks for bearing with us Allen, and look forward to finding out what you see with the CI-122SP.

Cheers,
Bob
 
Pretty Geeky, Bob!

And I k mean that is a good way....... Good test for the antennas today! It does look like some rough country up there.

The great thing about the ham radio side of things is that we can test new ideas for $10 in parts - hard to do that with airplane stuff! Sometimes ideas work out, sometimes they don't, but we get to fly to try them out and that is not bad!

We will find something that works!
 
Comant CI-122SP

I know we're driving Allen crazy with all the wacky antennas, but the experimenting is part of the fun! Thanks for bearing with us Allen, and look forward to finding out what you see with the CI-122SP.

Cheers,
Bob

Bob,

I received my CI-122SP today and it measures approximately 25" from tip to about an inch above the BNC connector. I was surprised since the Comant literature describes it as a 20-inch element. How much did you cut off or how long did yours end up? Also, on mine it appears the base is painted, so now I'm wondering if it was intended as a ground plane antenna. Did you remove the paint on yours and did you mount it to get good ground plane contact?

Also, I measured and my two CI-122 COM antennae are 27.5" apart. I am now consistently getting bleed over from the #2 antenna at each packet transmission in flight (on COM1). I must not have noticed it on my first flight. My plan is to mount the new antenna aft of the baggage compartment which will give about a 58" clearance from COM1 antenna. I'm hoping this will be enough to eliminate the RF interference. Will let you know.

I hope we'll be hearing from Allen when he gets his LORAN antenna, and it will be interesting to see what kind of VSWRs he comes up with at 144.39 MHz.
 
"new" antenna

My so-called "New, in-box" antenna made it in today. ( paint chips and bug splatter lead me to believe that it is not quite as advertised) I did a quick test on the bench at 144.00 MHZ with the antenna free-standing ( no solid ground mount) and had an SWR of 1.5:1, which is not bad. I will try it again when I have had more than two hours sleep, and can jury-rig some kind of ground plane and operate a little closer to 144.390.

Allen
VHS
 
Bob,

I received my CI-122SP today and it measures approximately 25" from tip to about an inch above the BNC connector. I was surprised since the Comant literature describes it as a 20-inch element. How much did you cut off or how long did yours end up? Also, on mine it appears the base is painted, so now I'm wondering if it was intended as a ground plane antenna. Did you remove the paint on yours and did you mount it to get good ground plane contact?

Also, I measured and my two CI-122 COM antennae are 27.5" apart. I am now consistently getting bleed over from the #2 antenna at each packet transmission in flight (on COM1). I must not have noticed it on my first flight. My plan is to mount the new antenna aft of the baggage compartment which will give about a 58" clearance from COM1 antenna. I'm hoping this will be enough to eliminate the RF interference. Will let you know.

I hope we'll be hearing from Allen when he gets his LORAN antenna, and it will be interesting to see what kind of VSWRs he comes up with at 144.39 MHz.

Pat,

Not sure on the base of my 122SP...will pull it and take a look tomorrow, but with the bases of these 122 style antennas contacting a painted belly (and some have a cork gasket ta-boot), I thought the grounding would occur at the BNC and at the four screws that mount it to the nut plates in the doublers that I have riveted to the airframe. I could be totally wrong on that, and will bounce it off the avionics shop tomorrow too.

Hmmm, I wonder if we have a grounding issue along with the proximity issue. Our antennas are the same distance apart, and I hear it airborne as well. It'll be interesting to see if it goes away with the larger separation you use (I'm guessing it will).

I'll measure that antenna tomorrow as well (keep forgetting to do that :rolleyes:).

And Allen, thanks for the data...are you measuring it at full length (uncut to 20.5")? Please let us know what you recommend on cutting once you put it through its paces...thanks!!

Cheers,
Bob
 
Pat,

Not sure on the base of my 122SP...will pull it and take a look tomorrow, but with the bases of these 122 style antennas contacting a painted belly (and some have a cork gasket ta-boot), I thought the grounding would occur at the BNC and at the four screws that mount it to the nut plates in the doublers that I have riveted to the airframe. I could be totally wrong on that, and will bounce it off the avionics shop tomorrow too.

Hmmm, I wonder if we have a grounding issue along with the proximity issue. Our antennas are the same distance apart, and I hear it airborne as well. It'll be interesting to see if it goes away with the larger separation you use (I'm guessing it will).

I'll measure that antenna tomorrow as well (keep forgetting to do that :rolleyes:).

And Allen, thanks for the data...are you measuring it at full length (uncut to 20.5")? Please let us know what you recommend on cutting once you put it through its paces...thanks!!

Cheers,
Bob

Bob & Allen,

According to my calculations the correct length for a 1/4 wave whip at 144.39 would be 19.45" (2808/144.39). Probably not real critical, but just in the interest of accuracy, you may be off about an inch with your calculation, Bob. I think the formula you used is intended to be just an approximation (300/144.39 = meters). If correct, that would mean I would need to cut 5.5" off my 25" antenna. However, I'm curious about what Allen intends to do so I think I'll just mount it without trimming and wait to hear from Allen if he trims his based on his VSWR readings.

As to the ground plane, I'm just wondering if these LORAN antennae were never intended to be used as ground plane antennae and thus were isolated by painting the base and using a gasket. In our application, however, I believe they need to be firmly attached to a ground plane so I'm going to remove the paint from the base and on the airplane where it will be mounted. The hardware helps make the ground plane, but I think it would be best to have the antenna base make contact with the aircraft skin. Think I'll apply alodine to the surfaces for corrosion protection as a precaution.

What do you think, Allen?
 
Loran antennae

Pat et al,

I think that the Loran antenna is designed to be mounted to ground. Although in normal operation its a receive-only antenna, its so short for the band it covers, that it would be awful without some kind of a ground plane. Also, the base plate on mine is at ground (connected to the outer conductor of the BNC connector) and the screws that pass through for mounting are also electrically coupled to ground. So one way or another, the unit is going to be electrically connected to your ship's ground, the only question is whether that is desirable, and I would give that an unqualified "Yes".

I was impressed by the un-modified length SWR, and I kind of like the idea of a semi horizontal/semi vertical antenna on the roof of the Mini-Van. I can set my alternate configuration to hit one of the satellite uplinks, and possibly stand a chance of hitting one with 10 Watts ( Or I might use one of our 35 Watt units) There is a small chance that this much power could cook the antenna, but I guess I will find out.....

Allen
VHS
 
Pat et al,

I think that the Loran antenna is designed to be mounted to ground. Although in normal operation its a receive-only antenna, its so short for the band it covers, that it would be awful without some kind of a ground plane. Also, the base plate on mine is at ground (connected to the outer conductor of the BNC connector) and the screws that pass through for mounting are also electrically coupled to ground. So one way or another, the unit is going to be electrically connected to your ship's ground, the only question is whether that is desirable, and I would give that an unqualified "Yes".

I was impressed by the un-modified length SWR, and I kind of like the idea of a semi horizontal/semi vertical antenna on the roof of the Mini-Van. I can set my alternate configuration to hit one of the satellite uplinks, and possibly stand a chance of hitting one with 10 Watts ( Or I might use one of our 35 Watt units) There is a small chance that this much power could cook the antenna, but I guess I will find out.....

Allen
VHS

Thanks Allen,

Yeah, at 1.5:1 maybe we're asking too much to disrupt the integrity of the antenna just for a few more tenths of SWR. I mean that already exceeds the published SWR for the CI-122 COM antenna at 3.0:1. I'm interested in what you're going to see once you have the antenna mounted on a ground plane, and whether you decide to tune it or not. Inquiring minds want to know!:)
 
Aeronautical engineers?

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 [email protected] to see if you can stop the potential bloodshed?

Allen
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 [email protected] to see if you can stop the potential bloodshed?

Allen
VHS

Even though few of us are genuwine aeronautical englenears, you really have our curiosity aroused...... ;)
 
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
 
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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! :eek: 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
 
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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

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
 
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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....
 
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
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...
 
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....

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? :D

Cheers,
Bob
 
<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
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.
 
This treadmill thing is much too old (by several years) to argue! :D 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
 
APRS loran antennas on a conveyor belt

Oh I can't resist...

I think rvmills gave a good explanation of the problem.

The airplane is sitting on a conveyor belt. Suppose the airplane wheels have a radius r and are turning with angular velocity w; then the linear velocity of the airplane relative to the conveyor belt will be v = 2 pi r w.

Like rvmills, I understand the setup for the thought experiment to be that the conveyor belt is operated so it always moves with velocity -v = -2 pi r w with respect to the ground; so the velocity of the airplane relative to the ground is always v - v = 0. If you stick with that, and assuming still air, there will then be no relative wind, except for prop wash. So, of course :))) the airplane won't take off.

But can you build a conveyor belt to do that? (Clearly the Mythbusters belt isn't doing that.) Wheel angular velocity w is a free parameter. What determines w? Wheels on an airplane are not driven (I think I heard once that C-5A wheels are, but ignore that!).

Instead, the airplane engine and prop are producing thrust. Now, if the conveyor belt is going to keep the airplane from accelerating relative to the ground, it has to apply a force to oppose that thrust. How can it do that?

One way, which theoretically works even with frictionless bearings in the wheels, would be to constantly accelerate the conveyor. Since the wheels have mass, this will apply a linear force at the point of contact of the tire and conveyor, which can be adjusted to exactly oppose the thrust.

Another way is to take advantage of the friction of the wheel bearings. Suppose the torque resisting rotation of the wheels has a static term and a term depending on rotational velocity, e.g. t_s + w t_d. Then the conveyor speed -v just needs to be adjusted to turn the wheels fast enough so that the tangential force applied by the conveyor balances the thrust.

If the static rotational friction is small, w may need to be large enough that the wheel gets pretty hot. But what if the static term on its own is large enough to balance the thrust? Then the conveyor belt doesn't need to move at all!

And that's a realistic situation... put your brakes on. If they're good enough, the airplane won't take off then either :).

--Paul
 
Help, I've fallen into this and I can't get up :D Mike S, Pierre, guccidude, Greg, Sam, Pete, Pat, Allen...reach in and pull me out! ;)

OK, it really is all tongue in cheek...don't want to appear to be defending my position too hard...if there is something out there that will sway this the other way, I'm open-eyed. That being said...

KR, you are correct on the effect of wind on groundspeed and wheel speed. In fact that's one baseline assumption in my "busted" call (that wheelspeed and groundspeed are the same, as long as the plane is on the ground). Here's what I'm basing my call on (and this is not meant to sound like schoolin', just a review of my assumptions...if anyone sees a fly in the ointment, please shout!)

Let's assume zero wind, as wind is not mentioned in the original problem.

1. A plane uses thrust to move forward down the runway and accelerate to liftoff speed.
2. While on the ground, the airplane's wheels must rotate for the aircraft to move forward.
3. The original premise says that the treadmill runway will exactly match the speed of the wheels, so as soon as the pilot adds power, and the airplane attempts to roll forward, the magical treadmill moves in the opposite direction at the exact same speed.
4. If the pilot adds power/thrust and the plane attempts to accelerate, the magic treadmill also accelerates in the opposite direction.

I know the wheels of the plane are not driving, but as the thrust attempts to pull (in the case of a prop) the plane forward, and the wheels start to rotate, the treadmill moves in the opposite direction at the exact same speed. Thus, the airplane never moves...it has groundpeed, but no airspeed, and can never fly (IMVHO).

If the treadmill is set at a fixed speed, and the airplane applies thrust against that fixed speed, I believe that the airplane (with enough thrust applied) will overcome the reverse spinning and accelerate forward. However, the problem says the treadmill matches the wheel speed exactly, so in the problem the treadmill is not at a fixed speed.

I saw a youtube video of a gent that showed that an Airhogs model plane would accelerate forward at any speed they placed the treadmill, and LA, I think that's what you showed as well. However, these treadmill tests seem to be at various fixed speeds (or they certainly accelerate much more slowly than the model airplane can). Also, a rubber band airplane or airhogs airplane are likely capable of enough thrust to go well faster than a treadmill can turn, so the "matched wheel speed" part of the test was not met in the youtube video, or (I believe) in your test LA, with all due VAF brotherly respect...come back to the "won't fly" camp, brother! :)

If there's something I'm missing that would allow the plane to accelerate forward despite the treadmill's perfectly matched reverse force, I'm really keen to see it. If its there, I'll do the V-8 salute, and buy the cold ones! :)

I really think the plane would just look like George Jetson on the treadmill, and sit there with the wheels spinning until the bearings melted!

Cheers,
Bob
 
Help, I've fallen into this and I can't get up :D Mike S, Pierre, guccidude, Greg, Sam, Pete, Pat, Allen...reach in and pull me out! ;)
Cheers,
Bob

Ok, I can help.

Thread is now closed since this topic has been discussed ad nauseum for years.

Feel free to begin another thread that deals with..........antennas.. :)
 
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