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

Building second DD w/ FAI GRRT

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Old 09-30-2002, 03:09 AM
  #1  
FlooredCOBRA
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Default Building second DD w/ FAI GRRT

Well I finally got my DD in the mail along with my carb for my engine. Finally!!. The excitement of seeing a package after a long day at work on your doorstep is priceless!.

Last Time I flew my other DD i was running a OS .46 FX with a 9x7 pylon prop, stock muffler. It flew great but I really wanted to put on the GRRT but couldn't due to I didn't have a carb for my engine. I didn't want to use the WOT venturi it came with.

So anyways I'm really excited about getting my DD finished. I am going to do a few things different on this one. Mostly trying to get the fuel tanks as low as possible for less drag. I am going to try to run them in series. Seen a few folks do it that way and seems like a good ides to keep obstructions low and away from wind resistance.

I do have a question though. I see so may different suggestions for different prop combos for speed. I am kind of confused with everyone talking about rpms on ground and like using 7" props and all. It seems a 7" prop would be way to small to move the plane around even with alot of pitch. Does the pitch make up that much of a difference when going down in diameter?
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Old 09-30-2002, 03:49 AM
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ChuckAuger
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Default Well, I don't want to mislead you..

But I ran a 7.4" diameter prop on my DDust..

And if it was too small diameter to fly the plane, I must have missed it.. from launch to out of sight straight up in just a couple seconds..

Confused?? Don't be...RPM on the ground?? yes, that's where I stand with my tach. Just have to have enough experience to know how much your engine/prop will unload in the air and go from there! What is confusing you is the disk loading of the prop...a 7" diameter prop is plenty for a 3.5lb plane.. (it's one of those geometry type things, area of the disk divided by weight of plane..) but don't sweat all that!

The GRRT will not run worth a crap on props other than 7~8" diameter, so all this has been taken care of for you! If guys are flying DDusts with GRRT's, then surely they have props that will work. So if they work for them, maybe they'll work for you!

Does the pitch make that much difference?? Yeah..it's what makes you go fast!! So you have to run a prop with a 7" diameter, so you can sling enough pitch to go fast!! The 11X6 on your trainer and 40 sized engine ain't gonna go very fast..so you drop down to 9X6..engine speeds way up, but you ain't got no pitch!

So you go to a 9X8..now you have some pitch..you will go fast! But...your trainer engine won't turn it as fast as the 11X6..so yo ain't going no faster! Dang! You need a GRRT!

Now just put a 7" diameter x 9" pitch (or so) prop on the GRRT and see what happens!

What it all boils down to is run disk loading as high as you can with as much pitch as your engine will pull.. then you can go fast....as fast as your engine will pull that much pitch..

And a good rule of thumb..if guys are using it, then it might work for you..but a little change in pipe, plug, prop, fuel, might get you out there past them.
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Old 09-30-2002, 04:19 AM
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Default Building second DD w/ FAI GRRT

okay is disk run ground rpms? was trying to figure out what you was saying. Get the most rpms on the ground as possible and it will perform fast in air. Okay my thing is how do you know when your rpms become non efficient? meaning when do you know when you have more rpms than thrust? Just trying to get an idea on the theory behind high speed prop planes.
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Old 09-30-2002, 04:45 AM
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Default Disk loading..

Is just a static number, run on a calculator. a 7.4" diameter prop will have 43 in^sq. Don't worry about it too much, as long as you have a known airframe weight. For example, the DDust weighed 56oz/43 in^2 = 1.3 0z/in^2, so I guess 1.3oz/in^2 is pretty vertical. I don't have any published numbers, but I never saw anybody who thought this was underpowered

Run as many RPM's on the ground as you can, with as small diameter as possible, and as much pitch as your engine will pull.. it really is simple..if you run a lot of un-needed diameter, you can't turn any pitch. No pitch=no speed. So lose the 11" diameter props.., lose the 6" pitch..get you a prop that turns about 8~9" of pitch at about 1K~1.5K less than the maximum rated HP RPM of your engine..it will unload to right into the max rpm range of your engine..then you will be going as fast as your engine will allow..

Oh yeah..you need to set the pipe to run at this unloaded RPM...and some prop/plane/engine applications will unload more than 1~1.5K..the vrdf was probably unloading 2~2.5K, so my pipe was a little shorter than calculated, to anticipate the unloaded RPM's..

It's all just numbers..and fuel..and props..You'll get it pretty fast, or know that it ain't working.
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Old 09-30-2002, 06:22 AM
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Default Building second DD w/ FAI GRRT

okay i see what you mean now. Thanks Chuck!

I was a bit confused about a few things but now i see what your talking about. I'll stock up on a few diff variations on props till I get that sweet spot.

As for the pipe length i cant really do to much with that. My pipe clips directly onto my engine. So the guess work of pipes will be easy for me. I'll just have to experiment with props now. And get my DD completed! I'm really anxious to get it into the air now!
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Old 10-01-2002, 04:20 AM
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Default Building second DD w/ FAI GRRT

Here's what you need:
  • APC 7.2x8.6 racing props (model# LP07286) for best speed, OR
    APC 7.4x8.0 Q40 racing props (model# LP07480C) for a more solid take-off and still good speed.
  • 0%-nitro FAI fuel (or headshims to run 5%)
  • McCoy MC-9 or Hanger9 #9 Ducted-Fan Glow Plugs (buy several)
These will give you a good starting point for your GRRT. Mine loves them. Turns the 7.2x8.6 over 19k on the ground and really unloads in the air (guesstimate ~22k).

This engine is not designed for nitro, and basically hates it (blew expensive glow-plugs every flight on my buddies fairly new GRRT on Wildcat Jet-A (5% nitro 23% oil)). You can add head-shims to lower the compression ratio and allow it to run some nitro... but even then I wouldn't recommend more than 5% without a lot of experimentation.

One thing about this engine... it is sensitive to over-propping and won't come "up on the pipe" if you have too big of a prop (or have it running too rich). You can really tell when this engine "comes up on the pipe"... as the RPM's will jump from ~14k to ~19k with basically nothing in between. Kinda difficult to tune it for bigger props by moving the pipe out, there is no silicon-coupler as in most tuned-pipe installs.

When ready to fly it, you will want to richen the high-end till it just barely comes up on the pipe when on the ground. If you don't, it will lean out when it unloads in the air and your first flight will be powered for only the first 100' or so. If you richen it too much and it doesn't come up on the pipe at all, the 7.2x8.6 will not be enough thrust for take-off. Don't ask me how I know these two things.

I hope this proves useful to you!
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Old 10-01-2002, 03:34 PM
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Default Building second DD w/ FAI GRRT

low drag.... cut servo sized slots in your ribs and install the servos there horns pointing outwards, gets them out of air flow.
i've got a GRRT here, hoping to use a fan, 26K RPMs on this engine should be enough we did have a 8x10 on it, damn fast, but not the RPM it should do. we just didnt know back then
theres no such thing as wasted RPM. infact, a dickie bird told me dont go over 7-7.5" in pitch, its slower, go with the extra RPM you get past that figure, and that dosnt mean running a 11x7 on it.
8x8's seam good
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Old 10-01-2002, 03:47 PM
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Default Building second DD w/ FAI GRRT

FlooredCOBRA,

Think of it like this: to reach your target airspeed, you absolutely need to have enough pitch. If you had all the relevant data, you could estimate the maximum speed at which your engine could pull your airframe, and choose pitch to give you that speed at your engine's max power RPM. Then, given that pitch and your engine's power, you will be limited in the diameter prop you can turn, because increasing a prop's diameter dramatically increases the power needed to turn it a certain speed. Fortunately, a bunch of speed freaks have done a lot of experimenting for you, and have found a solution which is in the right ball park. Short version: don't worry about the small diameter of the prop, it is necessary. For a more conventional plane, not being driven to max airspeed, a larger prop would generally be more efficient, and have other benefits.

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Old 10-02-2002, 12:00 AM
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Default Building second DD w/ FAI GRRT

Well i just looked at the bundle of different props i already had and I think they are way off. Here is what I have, all APC

9X7.5 (racing)
9X8.5 (racing prop)
8.75X7.5 (pylon prop)
8X8 (looks to be a regular prop, no bag & label with it)

I can see a difference in the design between the racing and pylon prop. The racing ones are curved and seem a like they have more meat on them. The pylon is much skinnier and is more pointed. As for the 8x8 it just looks like a regular prop, it may be a racing but kinda hard to tell since its so small.

Seems the props I have are nothing what people been talking about, think I'm way off.....oops....

Also I was wondering about wooden props. What do you guys like better? what seems to work well? I know I did use a 9X7 pylon prop before with my OS .46 FX and it had an instant rev compared to APC. But with that engine I think its apples and oranges.
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Old 10-02-2002, 03:33 AM
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Default Building second DD w/ FAI GRRT

One difference you will note about racing props and standard props is the size of the hub. Especially on the smaller prop-diameters. As a general rule, the racing prop's hub is larger thicker stronger and rated for the RPM's you're going to turn it.

I have been warned by APC that if I'm turning more than 20k to use only the bigger hub size ("D1" I think), instead of the standard-hub ("C1" I think). They don't recommend the smaller hubs above 20K. The props I quoted you are the bigger hub. I have also heard stories about 7x9 and 7x10 sport props shedding blades at high RPM (a VERY bad thing).

Personally, I wouldn't use a wooden prop on this engine at all. There might be some wooden props out there that are designed for racing, but none of them will be as efficient (fast) as the APC.

Here is a picture of my GRRT (slightly older version pipe) with its APC 7.2x8.6. Note the diameter and thickness of the prop's hub:

(click for a larger version)


Now compare that with the APC 7x10 shown installed in these two images:

(click for a larger version)


(click for a larger version)


The 7x10 has a tiny hub in comparison. I haven't and wouldn't want to try it for fear of it breaking (especially after boring it out to fit the large crank of the GRRT).
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Old 10-02-2002, 03:51 AM
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Default Building second DD w/ FAI GRRT

yeah I see what you mean. I never did understand what the D1 and C1 ment on the props. Good thing i know now, I deffinetly wont take a chance. I can see how it would come apart after boring it out to fit the shaft.

Well as of now I have the DD complete as far as building process goes. Now its time to start some sanding and cutting out for my servos. I am going to install them in the main ribs horizontal and run the rods out. On this DD I am looking to keep the dragg as low as possible. My last DD I built exactly by the plans since it was my first one. It flew fast but I am now trying to keep any kind of drag to minimum as possible.

I now have about an Inch of CA glue on my fingers and I think its time to call it a night. I cant even feel the wood now....
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Old 10-02-2002, 03:57 PM
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Default Building second DD w/ FAI GRRT

this graph can help.... maybe..
im glad you said big shaft on the GRRT, i've been trying to find out what size it is., got M6 for 40's, then i found a nut last night that didnt wanna fit, did it up, past the dirty bit of thread and now its fine, think the nut says 8, so M8..

the old 8x8's we had shed blades, the new ones are thicker and dont. didnt know they had been changed till i found out here, yay RCU
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Old 10-04-2002, 12:32 AM
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Default Building second DD w/ FAI GRRT

Hey Andy!
Are you and Carl bringing your DD to the LORCS Fun Fly in Eldon on Sunday, October 6th?

We've got the Jett FIRE 50 DD ready to race with you guys!

(FYI: the Jett FIRE 50 is turning an APC 8.75x9 at 18,750)
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Old 10-04-2002, 01:19 AM
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Default Building second DD w/ FAI GRRT

Talked to the Gents at Morris Had a Grrt Set up with a r/c carb,also asked them there favorite prop,,was told 8x6 apc,,let this engine turn the "R's" dont load it up with a large prop. Running right it should be in the neightborhood of 3 hp.@ 31,000 with 15% Nitro,It will unload to say 35 to 37,000 rpm Plenty of rpm to go fast with no matter what you place it on.
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Old 10-04-2002, 10:13 PM
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Default Building second DD w/ FAI GRRT

unload at 37,000????????!!!!!!!!!!

holly mackrel. id like to hear the sound of that thing at that rpm. i thought the nelson and jet fai's only turn 30,000 in the air?

but i wonder if that is faster than a jett .50
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Old 10-04-2002, 10:21 PM
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Default Building second DD w/ FAI GRRT

Dave, my DD isn't flyable at the moment... needs to have the bottom recovered. I had designed a wheeled-cart for takeoff that didn't work out too well. Carl might have his ready... we'll see. We are planning on coming down though... unless those "who must be obeyed" say otherwise. The Jett50 is a great engine, a bit faster than the GRRT, easier to setup and run, but more expensive (I only paid $75 for my engine, NIB).

Hammbone2, while I agree with you completely about not over-propping this engine... if that stuff Morris told you were true, the GRRT would be blowing away the Nelson and Jett racing engines in the pylon world. (They are not.) According to ThrustHP, it takes almost NINE horsepower to turn an 8x6 at 37,000rpm. Subtract 10,000 RPM from your numbers... and the GRRT might do that. Wouldn't be as fast as an 7.2x8.6... but hey, that may not be important to everybody. One thing for sure... and 8x6 would take off like a rocket... where the 7.2x8.6 takes a few seconds to get up to speed.
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Old 10-04-2002, 11:00 PM
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Default Building second DD w/ FAI GRRT

Andy,
Sorry, to hear about your DD's accident. We were really hoping to be able to get both the Jett 50 DD and the GRRT DD up at the same time. One screaming Diamond Dust is impressive enough.....but two screaming Diamond Dusts would be awesome!
Originally posted by AndySteere
Carl might have his ready...
Hey, isn't this the same Carl who took way over two months to build a simple PBF?
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Old 10-04-2002, 11:22 PM
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Default Building second DD w/ FAI GRRT

Dave: Yeah, same Carl.

Fortunately, it's already built (by me last summer)... he just needs to add some head-shims to his GRRT and I think re-attach the muffler.
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Old 10-05-2002, 12:20 AM
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Default Building second DD w/ FAI GRRT

Originally posted by AndySteere
....he just needs to add some head-shims to his GRRT and I think re-attach the muffler.
BUMMER! With Sunday only two days away, it sounds like Carl may have waited too long to remove and then reinstall those four head bolts......not to mention the two muffler screws.....
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Old 10-07-2002, 01:05 AM
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Default Building second DD w/ FAI GRRT

Well I just got back in town from from a weekend trip. While out of town I went to my favorite hobby shop where I buy most of my goods from. I was looking to find a 7" prop of some sort to buy but could not find a single one. The owner and I searched through his distributor books and we could not find a company that sells that size props that I was looking for.

If y'all have the name of the company that makes those props (7") let me know so I know what to order. I was looking to get a few different 7" to half and so on props to have something to mess around with. Just looking for anything with 7" diameter props.
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Old 10-07-2002, 01:42 AM
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Default Building second DD w/ FAI GRRT

www.apcprop.com

APC makes several. I gave you the part numbers of two possibilities in my first post.
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Old 10-07-2002, 02:30 AM
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Default Building second DD w/ FAI GRRT

Thats right, I forgot about that.

Only thing is when we looked under APC it did not list any props that size. Odd that he did'nt have it under his listings. Well I got the parts numbers so thats good. I'll just see if he can order from what you listed. Thanks!
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Old 10-07-2002, 12:30 PM
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Default APC Props...

http://www.pspec.com/

Has them..you have to call and ask for them.
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Old 10-07-2002, 10:54 PM
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Default Building second DD w/ FAI GRRT

Out of curiosity I mailed Morris Hobbies where I purchased my engine from. I asked them what props worked for them and wanted their opinion about it.

They replied and said to use:
8 x 5, 8 x 6, 9 x 6, 9 x 7 are all good props for you to try. Have fun.

I think I am going to stick with what the guys on here have been saying to use. Since alot have the same set up as mine and know what works and does not work.
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Old 10-08-2002, 05:44 AM
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Default Building second DD w/ FAI GRRT

Their advice is not wrong... for the kinds of airplanes they may fly (probably heavier and higher-drag which needs more thrust and less speed).

If you want a diamond dust that will launch vertically out of a stand, but don't care how fast it goes... their prop suggestions might be worth a look (dump the 9x7 though).

For a fast diamond dust... you need something different. You did want a fast diamond dust, right?
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