asmund

what? did you measure the speed of the plane with an old guitar??? I`m waiting to buy me a Bushnell radar gun but maybe I should see if my mother inlaw have an old guitar in the shed instead[sm=50_50.gif], he he

Flyboy Dave

My Feedback: (21)

I would say your sound/speed estimate is very good....I would estimate the top speed
at about a buck twenty myself. The extraordinary thing about the flight was the
take-off....or the "launch"....it was very good.

I have special insight into just how spectacular it really was, because I use those planes.
those engines, and those props. The second biggest "prop" disappointment I have ever
suffered in my career was with my 60 size Cermark F-20, with a Tower .75 in it. I might
have bought two of the very first 75's Tower sold. I put a 12-6 APC prop on it because I
wanted to give the APC's a try.....I thought there might be something special to this new
prop design.

I could not have been any more disappointed. I had high hopes for some good performance
out of my new "speed plane". I gunned the engine for take-off, and the plane left the line
and I quote "like an old lady in a wheel chair"....no kidding, and no offense to anyone.

After a few seconds I wasn't really sure if the plane was going to take off or not, but I
let it run about 75 yards and lifted it up. I had it full throttle....my friend said "open it up",
I told him it was wide open. It went a couple laps at a miserable top speed of about 75
MPH, or so, with the gear up....and then disaster struck.

The performance of the plane with the 12-6 APC prop was, in one word, pathetic.

Fast forward to the performance of the Bobcat in the video with the 12-6 Zinger. The
difference in performance between the two props could not be more dramatic, trust me....

.....anyone who says a 12-6 APC is even close to the performance of the Zinger certainly has not
used the props on these engines in a performance application....perhaps is guessing....or
just doesn't have a clue what they are talking about....that is my opinion.

And yes, the 11-8 Zinger will give quite a bit more top speed than the 12-6, which is just a
mediocre prop anyway, the thing is....pushers don't come in all sizes.

And BTW, I tried other APC's on the F-20....the 12-7 and 8. They stunk too. []

FBD.

djlyon

Well doing the math that works out to 22880rpm with a tip speed of 816 mph or mach 1.1. And somebody said Jaka wasn't credible because of the props being a little modified or some such thing. This prop and engine are magical. Actually I think BLEs doppler shift estimate methodology needs some work.

Denis

B.L.E.

Many years ago, Model Airplane News published some results from flight data recorders installed in model airplanes. A finding that surprised them was that most of the planes they tested flew faster than the prop's theoretical pitch speed. The reason is that the prop's pitch is usually defined by the bottom of the airfoil, not by the zero-lift angle of attack. I remember something about a Clark Y airfoil having a zero-lift angle of attack that is about 6 degrees negative from the flat part of the airfoil bottom.
The blade angle of a 12x6 is about 9 degrees at the prop tip. Assume that the airfoil makes zero lift at a minus 6 degree angle of attack and now the angle is 15 degrees at the prop tip which means the prop has to advance about 10 inches per revolution in order for the thrust to drop to zero. Take that into consideration and 120 mph (176 ft per second) can occur with the engine turning at 13,000 to 14,000 rpm.

B.L.E.

That Bushnell radar gun is probably less expensive than my wife's Martin 000M

Harry Lagman

OK, I was seduced by Flyboy Dave's Zinger promo and finally got the video to download (I'm on a bad 56k dialup - feel my pain), expecting to see a technological revelation and what did I see?

An aerodynamically clean 40/50 size model with a .75 two-stroke, rolling along a paved runway, taking off and climbing out at 30 or 40 degrees and doing a few low passes at about 100 mph (Using the second harmonic, I got 540 Hz approach (after the dive) and 420 Hz departure (just prior to climbout), which is 96 mph, inflight rpm of about 14k). Nice in-flight performance but nothing outside of the ordinary.

Is this the incontrovertable evidence of the Zinger's total superiority over an APC? Is this the video that proves it beyond all doubt?

Perhaps I downloaded the wrong video...

Flyboy Dave

My Feedback: (21)

....

speedster 1919

My Feedback: (3)

Yeah Living out here in HOOTERVILLE with dial up watching Dave's video was a joke. I guess I have to move to PIXLEY to watch it............

B.L.E.

After reading this, I redid my experiment using a guitar as a pitch reference. My wife was out of town so I could really turn up the volume on the video and I found that the notes C and G-sharp were the best match for the approach and departure frequencys. Converted to Hz that's 523 and 415 which translates to 90 mph assuming that the speed of sound is 350 meters/sec. The speed of sound was based on it probably being 90 degrees F, this was shot in AZ after all. This is a much more believable result than my first hasty experiment yielded. Garbage in=garbage out.
Interesting note, my doppler effect derived speed was about 15% faster than the theoretical "pitch speed" of a 12x6 prop turning 13880 rpm, the rpm that corresponds with the center frequency of 462.78 Hz on the second harmonic. MAN magazine found similar results when they flew several models with data recorders.

Jim Thomerson

They are in fact little rotating wings, not airscrews.

djlyon

Those results are quite believable. I've been looking at some airfoil wind tunnel data and see if some math will confirm it. Didn't need the MAN stuff to know that all non symmetric airfoils will have some to a lot of cl at negative angles of attack. So I agree my earlier post was a bit unfair for not taking that into consideration. Christmas is now interfering so it'll be a couple of days before I do it.

Merry Christmas everybody.

Denis

djlyon

I couldn’t find coefficient of lift data for a Clark Y with a quick look on the enternet. So I dug through my NACA stuff and came up with a NACA 4412 looking much like a Clask Y and also looking like a good airfoil for a prop. The 4412 goes to 0 cl at neg 4 deg which is quite good. Most of the airfoil data that I have goes to 0 cl at between 1 and 3 deg. The 4412 has a .3 cl at neg 2 degs. For a 12/6 prop this is an effective pitch of 7.32 inches.

To get to 120mph the engine would need to be turning 17300

To get to 90mph the engine would need to be turning 13000. This is very reasonable and also represents a 21% increase in speed over the geometric pitch.
14000 rpm gets 97mph. Also reasonable numbers.
BLE this stuff seems to bracket what you see nicely The forgoing is based on airfoil data taken back when the Wright brothers were in diapers.

Denis

JettPilot

My Feedback: (6)

I dont buy for a minute that the Zinger beats the APC. I think Flyboy Dave has such a pro Zinger agenda that he will say anything, or even twist the results to make APC look bad. I will be testing for myself, and publishing the vidoes with each prop. I have live telemetry that shows speed readouts of the plane, and rate of climb info, I will be testing an APC 12x6 against the Zinger 12x6 on a typical .46 sized aerobatic semi - symetrical wing trainer. No twisting of results, no spin, you all will be able to see the video with data for yourselves

Its probably not a great idea to piss off the moderator on his own forum, but this is important, and it will be a great service for everyone here to find the real story which prop is better. My results will be posted accurately weather I am right or wrong. My only agenda is to fly the best performing prop I can find


JettPilot

Flyboy Dave

My Feedback: (21)

WOW !! ....a trainer....with a "typical .46"....

....sounds like Jettpilot is going all out on this one....

I didn't know "Jettpilot" was into this high performance stuff....I can't wait for the results
of this big challange.

speedster 1919

My Feedback: (3)

Jett I look forward to your test. Now -Keep in mind props don't always match up size for size. One brand would be a 12x6 and a 12x5-6-7 from another brand. The only APC I would consider buying would be the 12.25x3.75 size. If you lay down say MAS props like size 7-11 inch they all are proportioned the same ( like they are scientificly designed ) and then you take those APC computer scientificly designed props from 7-11 inch and they are all different shapes.

jaka

Hi!
Well I assume that APC 's props have different shapes because of functionality.
This discussion is really strange...I thought it was a well known fact world wide that APC is or was the preferred prop before Ma, Zinger and Top Flite!?

I must repeat that static tests is worthless ...it's like asking how much current a glowplug draws. Not that important.
The moment an airplane starts to roll the static trust measurements don't apply anymore...they only say something when the plane is in a hover or when the airplane sits still on the ground before take off.

I wasn't that impressed with the performance of Fly Boy's pusher and think he would have better performance if he had chosen a correct size APC prop...and eventually modified it...or modified the Zinger he used.
It's acommon fact among race pilots that a wooden prop with as blunt trailing eddges as the Zinger and bluntly shaped leading edge ..and top that rather thick... isn't going to perform as good as a thinner better shaped prop.

speedster 1919

My Feedback: (3)

Jaka I think it was said here before , this isn't about race props this is about 98% of flyers that fly sport and not competatively. This is about size for size they don't match up exactly. This is about if a certain prop has better static rpm and thrust it should be better in the air and over all sport flying. I know you love APC (maybe you sell them) and hate MAS and Zinger. Some people love wood props . I 've got Top Flites coming to see how they stack up with Zinger. Even though the whole world seems to say Top Flite stinks.
Glow plug voltage not that important . Bull -Take Fox plugs - they make 1.5 and 2 volt plugs. Take a 1.5 volt plug and put 2 volt to it and it's pretty much a gonner. But don't worry , there's a guy in Sweden that will replace all your burnt up plugs because voltage does not make that much difference...........

Flyboy Dave

My Feedback: (21)

.....speedster....Top Flite makes good props, always have.

http://www2.towerhobbies.com/cgi-bin...P?&C=QBA&V=TOP

jaka

Hi!
I did not mention voltage...I said current...matters you know![>:]
I don't care what the props are named or are called...as long as they are good...and APC is not always the best prop, there are other props that are superior like wooden Menz...or the carbon fiber Mejzlik for larger models.
But for most sport flying applictions, not racing, when you use props in the size from 5"-18" diameter, APC are good or at least better than most other props like for instance Zinger, MA or Kyosho (To name the worst).

speedster 1919

My Feedback: (3)

Jaka What is your all time favorite APC sport size ?

blw

My Feedback: (3)

Jaka,

Just how do you arrive at such a broad assumption?

speedster 1919

My Feedback: (3)

Jaka What the heck is wrong with you ? We are talking $2-$4 props and your throwing in carbon fiber props and special wood props $17-$43 starting prices and the smallest menz I could find was 15" and 16" in mejzlik........And yes you did say current but with current comes voltage hand in hand. But we keep up the spirited debate. My props have come in so maybe I can do phase 2...........

Flyboy Dave

My Feedback: (21)

....it's a Jaka prop myth....

JettPilot

My Feedback: (6)

That has been exactly my experience when flying my models. They always performed much better with APC than with any wood prop I ever saw. The old guys that taught me to fly were die hard wood fans, and thats all I ever used for years. Then one day I tried an APC, and I was like WOW The performance increase was great

The difference between me and most people, is that I dont care about what I want to be ture, nor am I so closed minded that I am unable to change... I just care about performance, and I will use which ever prop performs the best.

Now I have the ability to measure the numbers and performance, its time to prove waht I have seen for years.

JettPilot

MyWay

OS 55ax + Bolly 11.5x6= best combo for thrust and speed.