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  #131  
Old 09-20-2008, 07:10 PM
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chanstevens chanstevens is offline
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Bump and update--I got a somewhat successful flight today.

1--remounted motor pod with about 2 degrees of down thrust (nose end higher than aft end). Note that in a thread running in a rocket boost glider forum, there's some talk that thrust needs to be directed through the 3D CG, which would be a bit ABOVE the wing joint on this model, so this seems to be consistent with that wisdom now.

2--extended my pod length about an inch, making it easier to move my CG forwad at same time as reducing overall weight.

Did not add ballast to the bottom (i.e no keel on this ship).

Winds were barely 3 mph, boosted straight up on 1/2A6-2, though during delay burn it looped back in a pitch-up direction and started to level out a bit at about 20 feet. The ejection charge spit the motor as planned, but introduced some funky thrust that sent it looping again, but when it figured out it was supposed to be a glider, it flattened out and did fine, albeit upside down the whole glide portion, which was about 20 yards of travel.

This is definitely tempermental, and if I can eventually get a decent A and B motor flight out of it, I'll probably just retire it except for the occasional novelty launch. If you've got one, it's worth debugging just for the experience, but I would not wish this as a purchase on anyone I liked. Just too much work for too little "wow" compared to the various other designs out there. Heck, for a retro I'd go with Semroc's repro of the Space Plane...

--Chan Stevens
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  #132  
Old 12-27-2008, 08:10 PM
marslndr marslndr is offline
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I'm in the middle of my Invader build and decide to see what was posted. Wow, people are having problems! A couple of things come to mind, most have been mentioned. First we have 2 vehicles. First is a rocket during boost. For this the CG needs to be above the CP, where is the CP on a disc? Any weight added for finishing will move the CG aft. It sounds like the invader is marginal at best so any aft movement is BAD BAD BAD, as is any wind. Wind adds angle of attack, angle of attack adds lift, lift cause's a loop under power. The invader has a flat wing, it can generate lift in either direction depending on the angle of attack. Any deviation from zero angle of attack during boost will cause trouble with a marginally stable rocket.

I think I will go with one coat of dope and be done with it.

Second is a glider. For this it needs to be trimmed for best glide speed. Best glide speed is NOT the slowest glide speed. If you get a good glide during a hand toss it will stall after boost. You want a controlled, steep descent during hand tossing. The speed it will glide at after ejection is much higher than hand tossing.

This is of course all a thought exercise for me so far. Hope it helps.

Mark
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  #133  
Old 12-27-2008, 08:47 PM
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CPMcGraw CPMcGraw is offline
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Quote:
Originally Posted by marslndr
I'm in the middle of my Invader build and decide to see what was posted. Wow, people are having problems! A couple of things come to mind, most have been mentioned. First we have 2 vehicles. First is a rocket during boost. For this the CG needs to be above the CP, where is the CP on a disc? Any weight added for finishing will move the CG aft. It sounds like the invader is marginal at best so any aft movement is BAD BAD BAD, as is any wind. Wind adds angle of attack, angle of attack adds lift, lift cause's a loop under power. The invader has a flat wing, it can generate lift in either direction depending on the angle of attack. Any deviation from zero angle of attack during boost will cause trouble with a marginally stable rocket.

I think I will go with one coat of dope and be done with it.

Second is a glider. For this it needs to be trimmed for best glide speed. Best glide speed is NOT the slowest glide speed. If you get a good glide during a hand toss it will stall after boost. You want a controlled, steep descent during hand tossing. The speed it will glide at after ejection is much higher than hand tossing.

This is of course all a thought exercise for me so far. Hope it helps.

Mark


Trim as a glider first, then add the motor. Remember to build the downthrust in the motor tube, about 2 degrees. Nose down, nozzle up.
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  #134  
Old 12-28-2008, 07:53 AM
marslndr marslndr is offline
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Quote:
Originally Posted by CPMcGraw
Trim as a glider first, then add the motor. Remember to build the downthrust in the motor tube, about 2 degrees. Nose down, nozzle up.


Hmm... any nose down angle should make things worse I would think. The motor is already above the CG, that should give it a nose down moment. Angling the motor tube nose down should increase boost problems.

But you did not see this on yours? Strange.

The Space plane solved the problem of residual lift during boost with spin tabs, that is not an option on this rocket without an extensive redesign.

One thing I find curious is that people have only had stability problems in the pitch axis, not the yaw axis. It looks like it does not have enough fin to correct yaw instability, the dihedral will help but it does not look like enough to me.

Anyway, I ready to attach my motor tube assembly so I need to decide soon.

Thanks

Mark
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  #135  
Old 12-28-2008, 08:18 AM
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CPMcGraw CPMcGraw is offline
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Quote:
Originally Posted by marslndr
Hmm... any nose down angle should make things worse I would think. The motor is already above the CG, that should give it a nose down moment. Angling the motor tube nose down should increase boost problems.

But you did not see this on yours? Strange.

The Space plane solved the problem of residual lift during boost with spin tabs, that is not an option on this rocket without an extensive redesign.

One thing I find curious is that people have only had stability problems in the pitch axis, not the yaw axis. It looks like it does not have enough fin to correct yaw instability, the dihedral will help but it does not look like enough to me.

Anyway, I ready to attach my motor tube assembly so I need to decide soon.

Thanks

Mark


Mark,

Been there, done that. At least three times. It's not written in any of the plans that I'm aware of, but this is the trick that makes the Invader fly right. I found this out the hard way. The Invader has plenty of yaw stability with those twin fins, but it's during the boost phase where you see the pitch instability. The downthrust in the motor tube is equal to the downthrust you'd build into the motor mount of a high-wing, flat-bottom RC trainer airplane. That big pie pan of a wing produces a lot of lift relative to its weight. The result is, if you mount the motor parallel to the wing root, you get the back-flipping power prang of death. Mount the motor with the 2-degree downthrust, and it counteracts that pitchover. The downthrust is only for the power phase of the flight, not the glide.
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  #136  
Old 12-28-2008, 11:35 AM
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LeeR LeeR is offline
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I built the Invader as a kid when it was first released. I seem to recall only lightly painting it with fluorescent spray paint. It flew great, and had perfect, circular glides back to the ground. Future builds of Invaders as an adult were not good. And I probably made them too heavy trying to put a nice finish on them.

A friend of mine built his with no dihedral. Flew fine, but glided back upside-down.

I guess the inexperience of youth was extremely beneficial ...
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  #137  
Old 12-28-2008, 09:23 PM
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Mark II Mark II is offline
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My Invader #1: Built according to the plans at JimZ's. Flight result - destroyed itself on the third consecutive power-prang.

My Invader #2: Micromaxx-sized, built with 1/64" plywood and a nose cone from FlisKits. Wing diameter = 3"; weight -~3 grams. Flight result: power prang.

My Invader #3: Normal-sized, built with lighter balsa than #1, and with the motor mount tipped downward by 2-3 degrees, as Craig described. Flight result - One semi-successful flight, with a glide time of 3-4 seconds. Destroyed itself after three subsequent power-prangs.

All of the three were bare wood, with no paint or coating of any kind. All three were extensively trim-tested until they had decent glides in multiple successive hand tosses (20 feet or more, even for the micro). In the one flight that didn't result in a power-prang, the "glide" was more like an angled death dive. If anyone can somehow manage to crack the code for making this design boost and glide, please let me know.

I read somewhere, maybe on TRF, that someone had downsized the wing to 2/3rds of the original pattern, which supposedly solved many problems. I may try that next, once I work myself up to deal with another bout of total failure.

Mark \\.
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  #138  
Old 12-28-2008, 10:01 PM
marslndr marslndr is offline
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I finished construction on mine today. I angled the aft end of the motor down about 1 deg. It will be awhile before I can test it. I also have it trimmed/balanced about 3/8s in forward of the 2 15/16 called for in the instructions. I figure I can sacrifice glide for a more stable boost, and moving the CG forward should not hurt. I have also thought about adding paper label elevons to the rear. That might help the boost and glide (depending on which way it arcs). I did add a second lug at the aft end, this might help it on the rod.

It glides real nice now with no trends toward a stall at all. We will see in the spring.

Mark
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