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Old Jan 26, 2012, 09:32 AM
DS will change your life
SpeedsterDEN's Avatar
Danmark, Nordjylland, Sæby
Joined May 2010
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Quote:
Originally Posted by Herb View Post
Yes I have been thinking about the dolly too - no work on it yet though.

S thanks for the post, I have the same EDF motor and that ESC as well, both are top notch (they run in my CMP T-45 with a WeMoTec Midifan). The ESC is very well made, Hobbywing, and easy to program with the box. The HK EDF motor runs cool in EDF at 1500-2000W .

What is you CG, measured at where the wing meets the fuse? I see your wingspan is 135cm = 53'', whereas ours is 154cm = 63'' - so ours is just 14% slightly larger.

It is very interesting that you run such a small prop, I think most outrunners will fly apart and/or destroy the bearings past 15k, hence the need for an inrunner...

The video is impressive
Hi
The CG is 140 mm, measured at where the wing meets the fuse.
And it feel perfekt.

I also think of a dolly, but I dont have rudder, so it can be a challenge.
Motor and prop test will come soon

Cheers
S
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Old Jan 30, 2012, 11:02 AM
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Herb's Avatar
Irvine, Calif USA
Joined Feb 1999
12,158 Posts
Yes that fits perfectly !

140mm (your CG) x 1.14 (scale ratio) = 160mm = 6.38``

Thanks

Btw I have that SK3 1900kv EDF motor in the T-45 EDF and it's a very good motor (even more so given the price) !
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Old Jan 31, 2012, 11:25 AM
DS will change your life
SpeedsterDEN's Avatar
Danmark, Nordjylland, Sæby
Joined May 2010
1,523 Posts
SK3 1900kv EDF motor

ME 163, SK3 1900kv EDF, first motor test.

Turnigy SK3 Fandrive 3659-1900kv Motor test. (2 min 44 sec)


This will kick some a..
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Old Feb 04, 2012, 12:11 PM
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Irvine, Calif USA
Joined Feb 1999
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Nice !

I wish I had a semi-scientific criterion to decide what prop size & pitch to put on this thing.

My feeling is that many Me-163 setups use a prop that is WAY too large, at least for my taste. It seems people often just take over what worked well / flew well in a P-51 or Corsair and transfer it to the Me-163 ... Coming from the EDF world, where "props" are tiny in diameter, one would be inclined to think small(er). The same would follow if one were to think of an F5D type setup.

A simple back-of-the-envelope calculation (for my Me-163B, other sizes would have to be recalculated) would lead me to the following considerations:

Let's say I want ca. 300W/lb which will give a good performance envelope. Since the weight rtf is in the 8lbs (3.6kg) region with some good sized batteries, that would limit the setup to ca. 2400W. Since I don't want to use overly exotic ESC's, I will use (2400W/100A) = 24V or let's say conservatively an 8S = 4S+4S setup (6S is a bit marginal).

Also, let's say that I want the (max, level) flying speed to be in the 150mph (240km/h) region.

http://www.ecalc.ch/motorcalc_e.htm

The above requirements then give a number of constraints, such as a prop pitch speed of ca. 240km/h + 10% (?) = 265 km/h .

Also the prop pitch can't (normally) be higher than the diameter, which limits to commercial apc-style props such as 12x12, 11x11 etc.

Then from the above calculator I get the following approximate table :

prop - motor kv - Thrust (kg) - Pitch speed (mph)

13x13 470 3.8 141

12x12 530 3.5 148

11x11 610 3.3 156

10x10 720 3.1 167

9x9 850 2.9 178

From this exercise I would conclude that

a) 13x13 and 12x12 props presumably won't spin fast enough to give me the desired speed of 150mph.

b) the 9x9 pitch speed (178mph) is probably excessive, and a lot of prop air will juts dissipate in heat and turbulent wake.

c) that 11x11 and 10x10 should work.

Now, there's another variable in the equation, and that's the airframe drag. It will increase as velocity^2, and at flying speed (the desired 150mph) thrust=drag.

Since the Me-163 airframe is a bit draggy (it definitely does not look like an F5D airframe!) at some point without sufficient prop thrust
it will stop to accelerate.

I don't have a good way of computing the Me-163's airframe drag, but I am guessing that the drag could make the choice swing towards the larger, 11x11 prop. Since drag in these models is largely proportional to the total surface area, I could compare the Me-163 to some other airframe where the drag is known, and then scale the results by the ratio of surface areas ...

I have not taken further effects (such as prop unloading) into account as I did not know how to account for those (-10% rpm at
flying speed?), but it seems the above very simplistic arguments seem to suggest that a 11x11 or 10x10 prop should be in the right ballpark...

.
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Old Feb 04, 2012, 01:57 PM
Cant fly enough
Greg Beshouri's Avatar
USA, CA, Berkeley
Joined Dec 2003
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Hello Herb,

Nice analysis. The one thing that struck me about my K&A Komet is the speed with a relatively modest set-up. While the airframe is portly, I think the drag due to flow separation along the air frame is probably pretty low. Of course that is compared to other speed 400/480 sized aircraft which aren't all that streamlined.
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Old Feb 04, 2012, 06:06 PM
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Irvine, Calif USA
Joined Feb 1999
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Yes I agree ... Exactly as you say, it does not have the thinnest fuse cross section yet overall it is aerodynamically quite refined.

Nevertheless it cannot be compared to say F5D or other pylon racer types which have a much narrower fuse cross section ... and thinner wing.

Incidentally, on the original Me-163 they had to increase the fuse cross section form the A version to the B version so it would be able to carry enough fuel for useful operations use ...
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Old Feb 04, 2012, 11:03 PM
Cant fly enough
Greg Beshouri's Avatar
USA, CA, Berkeley
Joined Dec 2003
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Quote:
Originally Posted by Herb View Post
Yes I agree ... Exactly as you say, it does not have the thinnest fuse cross section ....

Incidentally, on the original Me-163 they had to increase the fuse cross section form the A version to the B version so it would be able to carry enough fuel for useful operations use ...
Didn't know that.

In some ways it reminds you of the Bee Bee R-1 Racer....
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Old Feb 07, 2012, 02:31 PM
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In the book I am reading (quoted above) by an Me-163 pilot he points out two interesting facts.

One is that Messerschmitt did not want to build any more Me-163B's in 1944, the ministry of air applied some pressure on them to continue; it seems they (M) thought much more highly of the Me-262.

The second interesting thing is that seasoned fighter pilots like the author found out that in a slow turn, like the one on landing approaches, the Me-163 would easily get into a unrecoverable deadly spin.

The reason is simple: To make a turn in a tail-less airfcraft, you give it aileron and then up elevator. The latter acts like spoilerons and dumps lifts, the result can be flow separation and a stalled wing.

The cure was to instruct all Me-163 pilots to use plenty of rudder to make a slow turn ...
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Old Feb 07, 2012, 02:56 PM
Cant fly enough
Greg Beshouri's Avatar
USA, CA, Berkeley
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Hi Herb,

I had certainly heard of the first. There was no love loss between Lippisch and Messerschmitt.

The second is very interesting.
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Old Feb 09, 2012, 04:32 PM
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Brookville, Ohio
Joined May 2000
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Herb,
Could you please tell what motor you are using in your Komet? Thank you, Paul
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Old Feb 09, 2012, 10:48 PM
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Irvine, Calif USA
Joined Feb 1999
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Hi,

I am using this motor, I hope the bearings will hold up for a while. kv=600, 300g.

http://www.hobbyking.com/hobbyking/s...dProduct=19617

I looked at various Heli motors and never found one that was right (they all seemed a bit too hot) and the mounting
seemed to a possible problem.

I will make sure the motor + prop setup are very well balanced (little or no vibration). That is one of the reasons my ply box motor mount is so sturdy.

.
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Old Feb 11, 2012, 01:29 PM
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The Me-163 is almost ready for the maiden

The esc I bolted to the firewall, there was no space left in the front under the hood.

To get the CG right (I am using 151mm) I had to move that battery pretty far back. I made a ply tray that at the back now sits on the wing spar carbon tube.

I also had to move my trusty JR Nicd 4cell receiver pack all the way back in the tail, just ahead of the vertical fin former.

.
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Old Feb 11, 2012, 01:31 PM
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The wings were glued in with silicone, and some transparent tape applied over all joints.

I did all the progamming and made sure I have twice as much "elevator" (+-16mm) throw as I have "aileron" throw (+-8mm), both with 30% expo.

The paint will come later ...
.
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Old Feb 11, 2012, 01:36 PM
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United States, CA, Salinas
Joined Oct 2006
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Wow ! Herb looks great, can't wait to here how the maiden goes.

Don
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Old Feb 11, 2012, 07:38 PM
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Hi Don,

Flew like a bat out of hell (actually looks a bit like a bat) !!

Two separate HD videos were taken ...

.
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