Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

.46AX behavior without baffle

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Old 05-16-2008, 01:11 PM
  #1  
Jburry
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Default .46AX behavior without baffle

Hello all. I'm getting some strange (though very positive) behavior from my OS 46AX engine, the one installed on my Dynaflite Fun Scale 40 Mustang.

The plane's kitbashed a few ways, weighs 5#2oz, spans 50". Fixed gear with 2-3/4" sullivan tires. Stock OS46AX with the baffle gone (lost it by draw bolt failure last year). It's turning an 11x6 prop at 13K (300 below peak) on the ground, give or take a few hundred for the weather. Omega 15%, 1.5 gallons thru her now.

When I take off, the plane leaves the ground typically at half throttle. I immediately advance the throttle to WOT, and climb out agressively, 45 degrees or so, and begin my first circuit. The engine almost immediately achieves the ground RPM, plus a bit due to air speed, and then just keeps accellerating. It's turning about 15K by the top of the climb, and then RPM's take another large boost as she levels out. Dopplers from the back straight are giving me RPM's in the 16.5K range, straight and level off the top of a climb. Then I roll in for a speed run, entered with a shallow dive, leveling out about 150-200 yds from the pilot station. More RPM's. Dopplers from the speed run show 17-17.6K as it passes by. It's achieving 100mph on an 11x6. I've dopplered 4 separate passes on this plane, being careful to have a long level run before the pilot station, and I've pitted it against a small 30" 'leccy speed plane a buddy of mine flies that consistantly passes at 95mph (easier to doppler the smaller plane, as it's RPM's hold steadier. I know RPM's changing will ruin a doppler pass), and the 'stang is slightly quicker than it. 11x6@17.6KRPM = 100.0mph pitch speed. I know I shouldn't be achieving pitch speed, but it seems I am.

Does anyone have any observations or thoughts on this? Anyone else seeing such huge unloading on stock 46AX's less baffle?

I gotta say, she really sounds awesome on those high-low passes!

J
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Old 05-16-2008, 02:03 PM
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daven
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Default RE: .46AX behavior without baffle

I'd suggest trying a 10x7 and you should gain a little more speed.

We race those engines locally on quickie airframes and hit 125 mph. As you mentioned it doesn't seem possible based on pitch and rpm, but its been well substantiated.

They are nice motors, with the 10x7 you should be able to peak in the low 14's, back it off about 400 rpm and fly.
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Old 05-16-2008, 02:15 PM
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Jburry
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Default RE: .46AX behavior without baffle

Yup, in time I'll be propping it up. The plane's too new to this relatively new pilot so far. She is my first low wing and first aileron ship, I need practice! 100+mph on your second plane is a bit of a jump from the 3 channel .25 size ship I trained on. Once I've "gotten my money" out of the plane (say mid this season), I'm gonna start propping up. I'll betrying 10x7, 10x8, 9x9, 9x10 if I can find it or cut a 10x10 down.

I knew she unloaded like a bat outta hell in the air, but when I got doppler numbers from it, I was sure surprised to see a 4600 rpm unloading! I hear (in sport and beginners forums mostly, admittedly) to expect 1000 rpm or so unloading, but my ear never agreed with that on this plane.

For now I just gotta keep her faster than my 'leccy buddy. His motor and speed controller let out the magic smoke last weekend trying to break 100mph, so I've got some cushion right now. But soon, new parts will arrive, LOL.

We've got a very liberal club, and noise isn't a concern (nearest dwelling 2+ mi away), but few others in the club fly fast planes. So far, I've seen none faster than our pair, the Little Red Sled and the P51D BUG

J
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Old 05-16-2008, 03:26 PM
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ChrisAttebery
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Default RE: .46AX behavior without baffle

In my area we race WM T34s with the 46AX. The two most popular props are the 10x5 and 9.5x6 APC. Both of these props hit 16k+ on the ground and the planes will get into the 95-100mph range. Not bad for a draggy airframe and a motor that's supposed to make its peak power at 14k rpm. I'd try a 10x6 or a 10x7 for your plane.


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Old 05-16-2008, 08:25 PM
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Jburry
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Default RE: .46AX behavior without baffle

Hmmmm... I'm gonna have to add some prop sizes to my next order, I guess.

When first I started flying this plane last spring, we took it pretty easy. Lots of putting around at part throttle, just learning it's handling. I'd never intended it to be a particularly fast plane. That just happened, I guess.

After a couple weeks getting comfy with her, we started leaving the throttle open longer and longer. Then, while pulling out of a full throttle dive, there was a noise. The elevator joiner had failed, and the non-driven elevator half was fluttering wildly. It broke that side of the stab completely, it was held on only by the covering. Apparantly my penchant for slightly excessive elevator throws paid off, we were able to land her gracefully.

I rebuilt and strenghtened the tail, and since then I've just flown the snot outta her. 95mph, full throttle pushed snap rolls look particularly violent. If there was a fleshy pilot in there, his brains came out his nose.

My biggest worry has always been airframe integrity. I guess I should stop worrying and just prop her for mach, LOL. If it can take full throttle snap rolls now, surely it can stand circuits at 130 or so, right? [8D]

J
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Old 05-16-2008, 11:32 PM
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Mike Connor
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Default RE: .46AX behavior without baffle


ORIGINAL: Jburry

11x6@17.6KRPM = 100.0mph pitch speed. I know I shouldn't be achieving pitch speed, but it seems I am.

Does anyone have any observations or thoughts on this?
As Dave pointed out, "its been well substantiated". There was a long thread not long ago about this. When you consider the camber of the prop it can be equivalent to 2" additional pitch. If the prop is a good match for the plane and the plane is clean enough it is possible to have as little as 10% slippage. Your 100 mph is very possible.

Do you have a video? I like playing with Doppler and your 17,600 rpm is impressive.
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Old 05-17-2008, 11:41 AM
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Default RE: .46AX behavior without baffle

We only recorded audio, and I don't know how to upload that for you.... The actual doppler numbers I got were:

HSE Dopplers

9 14 = 84mph, 85.5mph, 21850rpm 85 avg
34 43 = 85.8mph, 84.7, 84.3, 21810rpm 85 avg
106 116 = 93mph, 88mph, 91mph, 22270rpm 90 avg
142 150 = 89.9mph, 92.7mph, 91.9mph, 22220rpm 92 avg

Mustang Dopplers

27 35 = 121.5, 17240; 122.1, 17299; 119.9, 17347; 123.5, 17363 121.75 avg
55 104 = 117.8, 17213; 115.6, 17194; 118.1, 17174 117.17 avg
123 132 = 109.2, 17047; 109.7, 17080; 108.1, 17048; 108.7, 17015 108.925 avg

There are 4 runs from the little red electric plane there on top, and 3 runs of the mustang below. The different numbers (2 3 or 4 measurements per pass) were done on each of the octaves that displayed clearly between 200 and 2000 hz.

The HSE dopplers are from the little red electric plane. That plane's RPMs seemed very, very consistant coming towards the speed trap, and leaving it, so we consider them to be OK.

Now, I gotta qualify those Mustang numbers. They are not believable (well, the third run is actually), and I think I know why. I was flying essentially standard orbits of our field, flying the downwind leg about 200m or more away, at 300' or so. I'd turn "base and final" in an almost knife edge attitude, allowing the nose to drop and plane to accelerate as it climbed thru the turn. Very little control input needed keeps the plane clean thru the dive. I'd end up on the C/L of the runway, between 1 and 200m from the threshold, at about 30-50', and do my best to keep her straight and level for at least a few seconds before and after the plane passed our pilot station. Beautiful roar. I think, though, for the entire straight run, RPM's are bleeding off slowly, sinking from a high of about 17.6 to a low at the far end of the run about a 1000 below that. Falling RPM's will falsely exaggerate the doppler numbers.

On each progressive run of the 3 the 'stang flew here, I made my approach longer and longer, so the third represents the least inflation. The mustang was clearly quicker than the HSE, but not by the 30mph these numbers would seem to indicate. more like 10mph, thus my best speed estimate of 100-105mph in the straights. I don't want to think about what it momentarily achieves in the dive.

Clearly I need a radar gun.

I've read about exceeding pitch speed by upto about 10% before too. The 'stang is a pretty slippery ship, but it does have fixed gear and the cylinder head is in the wind. Other than that, I did my best to keep it clean. The muffler is partially buried in the fuse too.

It really was the HUGE unloading and almost pipe-like performance of the lovely OS46AX that took me by surprise, though. I know some aren't big fans of the ax series for speed planes, but I couldn't be happier with my .46. Perhaps it's a little exceptional in the AX line with repect to achievable RPM's.

If you want the .wav (which includes one last dive/pullout pass at the end that dopplers at 146 mph or so, but was flown just for fun, not for doppler. It's simply nowhere near reality), let me know how I can get it to you. Email comes to mind

J
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Old 05-17-2008, 02:23 PM
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Default RE: .46AX behavior without baffle

I usually like to extract the audio from a video so I can see the exact attitude and position of the aircraft. I also like it when my #'s are confirmed using a second method like a radar gun. In most cases my Doppler #'s and radar are very close. It sounds like you have a good handle on things. It will be interesting to see what happens with the 10x7. It could help or hurt you depending on the airframe drag.

I wouldn't mind looking at one of your .wav files of a level pass close the recorder. You can send it to mdconnor@yahoo.com
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Old 05-18-2008, 09:49 AM
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Default RE: .46AX behavior without baffle

Jburry,
I received your sound files and our #'s are very close. It surprises me that engine can unload that much. Different props will be interesting.
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Old 05-18-2008, 03:38 PM
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Default RE: .46AX behavior without baffle

Yeah, I was really surprised when I saw the numbers. I'd expected maybe 15K. The larger AX series may not be revvers, but the .46 seems to really like winding out. Maybe their new 35 will be too. That'd help keep the peeps happy around here.... Sounds mighty nice, doesn't it?

Thanks for taking a look at those .wav's for me. My first thought when I looked at'em was "that can't be right", but now we've gotten similar results from 3 or 4 independant people.

J
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Old 05-19-2008, 05:18 AM
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Default RE: .46AX behavior without baffle

My 46 AX gets just on 12,000 rpm on the ground turning a MAS 11x7 prop. How much should it unload in the air? Also I have an APC 10x8 I thought this would be a bit better for speed?
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Old 05-19-2008, 10:32 AM
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Default RE: .46AX behavior without baffle

Your 11x7 is getting heavy for speed. The engine won't be able to get right up into the powerband. How much it unloads depends on the airframe. On a draggy plane, almost not at all. On something sleek, maybe a bunch!

The 10x8 would be better for speed.

Removing the baffle from the muffler helps the engine unload and is good for 400rpm on the ground, but it's louder
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