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more speed for my 120ac?

Old 06-17-2008, 10:58 AM
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JohnMcGowan
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Default more speed for my 120ac?

Hi, I am currently running a YS 120AC with an APC 16x8 with 30% heli fuel on a Goldberg Ultimate bipe. It does quite nicely and is pretty darn quick thank you but I am wanting just a tad more high end top speed. Its taching appx 9200 on the ground now. Would you recommend that I jump to a APC 16x10 for just a little more top end speed? And I need to keep the rpms below what level?
Thanks much,
John
Old 06-17-2008, 12:33 PM
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dmccormick001
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Default RE: more speed for my 120ac?

You may not get any extra speed by increasing the pitch on your prop. That will only happen if you can turn the higher pitch prop at the same speed. However, increasing the pitch will add extra drag on the motor, and your motor may not be able to turn it at the same RPMs as the lesser pitch prop. Sometimes you'll actually increase your speed by dropping down in either diameter or pitch, because it will allow the engine to turn more RPMs, and that can translate into more speed, even with a smaller prop. You just have to experiment to find the best size for a given engine/plane combo. You probably don't want to allow a 4-stroke to turn more than about 10K, but if you can find a prop that will get those remaining 800 or so RPMs you may see a little more speed. Try a 16x6, or maybe go the other way and try a 14x8 or 14x10 and see what happens. Just don't make it so fast you fold the wings up on that bipe. They're made to be acrobatic, not fast!
Old 06-17-2008, 12:44 PM
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Jimmy Skids
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Default RE: more speed for my 120ac?

I'm running a APC 16x10 on my 120AC with 30% and pulling down about 8900 on the ground. I've run APC 14x12N's on my 120SC's in the high 9's without any problems. Keep in mind everything I have read on the 120's suggests keeping the R's around 9200 and down on these motors. Yet another option would be to raise your nitro up to say a 45 nitro/20 oil blend while running something like the 16x10 APC. Just beware that nitro can be addicting and with the current shortage could become very expensive.

Jimmy Skids
Old 06-17-2008, 01:23 PM
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JohnMcGowan
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Default RE: more speed for my 120ac?

thanks guys. I also have a apc 15x10 I could try thats new in the wrapper? I guess I will just have to bolt them on and experiment with the tach huh?
Jimmy, does your plane seem to like the 16x10?
John
Old 06-17-2008, 03:44 PM
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Jimmy Skids
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Default RE: more speed for my 120ac?

John,
Plane and motor seem to like the 16X10 just fine. I have the motor on a 120 size Spitfire with a very big cowl and spinner and needed the prop length just to get some prop mass past the spinner and cowl combination. The thrust is strong and top end speed is good as well.
Skids
Old 06-17-2008, 04:14 PM
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JohnMcGowan
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Default RE: more speed for my 120ac?

Appx how fast do you think your spit is Jimmy? Do you have any video of it "all out"
Id like to see how fast it is. Sounds great!
Thanks again.
John
Old 06-17-2008, 05:23 PM
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Default RE: more speed for my 120ac?

John,
Moves at around 100 MPH by best guess wide open in a straight line, sorry I don't have any video of this particular bird, and haven't put the gun on it as it's mainly just for fun. We race 40 size Warbirds with YS engines on them moving at 150+ and burning 55% + nitro. Needless to say we are exceeding recommended RPM's in this type of application.
Skids
Old 06-17-2008, 05:30 PM
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Default RE: more speed for my 120ac?


..... I have the motor on a 120 size Spitfire with a very big cowl and spinner and needed the prop length just to get some prop mass past the spinner and cowl combination.
What do you mean by this? The prop bites into the air in front of it and screws itself into it, pulling the aircraft forward. The aircraft flys as a result of the air that flows over and around the control surfaces as the plane is pulled forward through the air, not as a result of air that is blown across it from the prop wash. The second number in a prop's markings, the pitch, is the number of inches that the prop will travel forward with each revolution at a given RPM. What's behind the prop doesn't prevent it from being able to propell the aircraft forward, since the prop doesn't act like a fan. As a matter of fact, the amount of air you feel being blown behind a prop while the plane is on the ground is much more than what occurs once the plane is moving through the air. While propwash is important on some 3D planes when hovering or doing aerobatics at or near stall speed, a warbird shouldn't be affected very much by the presence of or the lack of propwash. And the presence of a large cowling shouldn't affect the size prop necessary to fly your plane well. I'm not trying to argue with you or be difficult, but just trying to question what might be a misunderstanding of what is happening on your plane.

David
Old 06-17-2008, 05:57 PM
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Default RE: more speed for my 120ac?

David,
Understood. I have a 5 inch spinner on the plane and wanted more prop area exposed than with what I usually run on these engines (13x13.5N), with 5 inches of prop shielded by the spinner I assumed that there would be some loss of "performance". So I went with the 16 for performance and so that the plane didn't look like a big bird with a tiny beak.
Skids
Old 06-17-2008, 06:19 PM
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Default RE: more speed for my 120ac?

Looks like you've worked your way to the answer I would have given. The 16x10 is a common prop on the 140L engines, and might be a bit much for the 120. A 15x10 or 15x12 should be a good way to maintain RPMs while increasing speed.
Old 06-22-2008, 09:30 PM
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JohnMcGowan
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Default RE: more speed for my 120ac?

Ok guys, the jury is in with some results.I went to the field today, but before I arrived there I stopped at the local police station and asked if they could come up and clock my plane for me on their cruiser's radar. No problem, said the young officer.
So about 30 minutes later he arrived and I had a 16x8 SY fake carbon prop ( I say fake because it just has a covering on it that looks like a carbon fiber prop- to me its a copy of the APC with the overlay on it). Im flying an AT LEAST 15 year old Goldberg Ultimate biplane with no cowling or canopy that weighs 10.6 lbs with a 16 oz fuel tank. So I took off with the 16x8 and Mr Policeman pulled safely off to the side of the runway to try and clock me. I made a few circuits around and then I hollered to start as I would be wound up as I would be approaching his way. The first couple of runs yeilded 72mph, 78, there wa an 88 and then an 83. These speeds are all flying INTO the wind as he's clocking me, which the wind was appx 12 mph today. The engine tached 9200 on the ground using 30% Cool power Heli fuel. I then landed, changed to an APC 16x10 prop which gave me 8800 rpms on the ground, so I automatically am think this will be slower since the engine isnt turning quite as much. It didnt even look faster to me as I made several circuits to get some new readings- I guess due to the engine wasnt turning quite as many rpms as the previous prop is what threw me off? Well low and behold, when I landed he started reading off the newest speeds he had just clocked. I had one 91mph, a 99, a 96 and another 96. I would have never guessed this in a million years. Now Im just wondering once I add the cowling and the canopy to clean up the aerodynamics on the plane a little bit, how much more speed could I expect, if any?
Anywho..., this was all very interestiing today, to say the least. Moral of the story I guess,... dont be afraid to experiment with different props on your plane to get results
Thanks for stopping by [sm=lol.gif],
John

edit,.... I did not get a chance to try the 15x10 today.
Old 06-23-2008, 11:53 AM
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Default RE: more speed for my 120ac?

If the 16x10 gave you the most speed, and you were only turning 8800 on the ground with it, my bet is that going to a 15x10 will give you even more speed. You're still not getting all the RPMs out of that motor that you can safetly turn, and the 15" prop should allow you to get more RPMs than the 16" prop. Both props are supposed to move 10" forward with every rotation, so if you can turn the 15" more times per second than the 16", that should translate into more speed. Just be carefull and don't over stress that airframe! Check it after the first few flights and make sure it's still holding together OK.

David
Old 06-23-2008, 12:41 PM
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JohnMcGowan
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Default RE: more speed for my 120ac?

That sounds logical David. I'll give the 15x10 a go next time I go up to the field( next weekend).
The Ultimate bipe travelling at this rate of speed had the "corsair tail wiggle" a little bit. And thanks for the heads up on the airframe- I agree,.. Im watching and checking it as I go faster
John
Old 06-23-2008, 01:11 PM
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Default RE: more speed for my 120ac?

If you want a crash course on speed, check out the extreme speed forum here on RCU. I boils down to the getting max RPM out of max pitch. One thing you will see when going to speed props is takeoff roll will lengthen.

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