X-15 CG

[sr205347d]

My X-15 from Dynasoar is ready to fly, except for balancing.



Frank says the CG should be 5/8" aft of the wing-strake joint, but I wanted to see where the CP is, so I modeled it in Rocksim and Open Rocket. (Disregard the CG in the diagram, as I did not model any internal components.)



OR puts the CP 2" aft of the wing-strake joint.
Barrowman (in Rocksim) puts it at 1.75".
The Rocksim stability equations put it at 2.54".
And, the cardboard cutout method in Rocksim puts it 4.4" aft of the same spot.

With the CG at Frank's recommended 5/8", its stability margin (in calibers) is: 0.43 per Barrowman; 0.53 per OR; 0.74 per Rocksim; and, 1.45 per cardboard cutout.

If I put the CG at 1.8" aft of the wing-strake joint, it would be unstable per Barrowman, marginally stable per OR and Rocksim, but have a 1.0 caliber stability margin per the cutout method.

What would the actual flight characteristics be in boost?

 

[burkefj]

I've got well over 200 flights on this particular kit design and I've got many videos of me flying it on YouTube at the exact cg location specified in the kit.

I'd suggest following the CG and the setup as per the instructions and if you're unhappy with the sensitivity on boost or you think it's too conservative then move it back slowly like you would with any other RC aircraft.

I see you reinforced the aircraft with tape, keep in mind this model has a much faster shorter glide than my other kits and if your ready to fly weight is significantly heavier than specified your flight performance won't be as good as that in my videos.

 

[sr205347d]

This is a thought experiment, to help me understand how the CP shifts with angle of attack and affects the flight characteristics. I know that Barrowman assumes a small AOA, and neglects the influence of the body tube. So, a long, slender rocket with small fins can be stable at low AOA, but at higher AOA, the CP shifts forward due to the influence of the body tube and the rocket can become completely unstable and swap ends (enabling back-slider recovery). This shift forward of the CP is shown by the cardboard cutout method.

The X-15, however (like many other winged aircraft), has the CP shift aft at higher AOA as shown by the cutout method. So back to my original question, if the CG were at or near the Barrowman derived position, there would be little or no moment about the CG to restore it to forward flight. Any perturbation would continue, until the AOA increased to the point where the CP shifted aft enough to provide a restorative moment. My expectation for the X-15 would be that there would be oscillations, but would not completely swap ends. (Still not desirable.)

I am wondering if anyone has observed this kind of flight characteristic in a rocket glider.

 

[burkefj]

This is a thought experiment, to help me understand how the CP shifts with angle of attack and affects the flight characteristics. I know that Barrowman assumes a small AOA, and neglects the influence of the body tube. So, a long, slender rocket with small fins can be stable at low AOA, but at higher AOA, the CP shifts forward due to the influence of the body tube and the rocket can become completely unstable and swap ends (enabling back-slider recovery). This shift forward of the CP is shown by the cardboard cutout method.

The X-15, however (like many other winged aircraft), has the CP shift aft at higher AOA as shown by the cutout method. So back to my original question, if the CG were at or near the Barrowman derived position, there would be little or no moment about the CG to restore it to forward flight. Any perturbation would continue, until the AOA increased to the point where the CP shifted aft enough to provide a restorative moment. My expectation for the X-15 would be that there would be oscillations, but would not completely swap ends. (Still not desirable.)

I am wondering if anyone has observed this kind of flight characteristic in a rocket glider.

I have personally never seen anything like this, but if the CG were near the barrowman point the model would most likely be so twitchy I would move the CG forward and fix it. I have not seen one be so twitchy that it got to near horizontal to line of flight and then restore it self in some sort of oscillation. I have seen things be so unstable it goes end over end, typically in rocket boost.

 

[sr205347d]

Maiden flight.

I expected that this one would be difficult to see the attitude and keep orientation, so I used a Spektrum AR630 RX, and set up SAFE mode. The rate gyros were active for launch, then at apogee, I selected SAFE so it would keep itself upright while I drove it. When it got close enough, I de-selected SAFE (rate gyros still active) and flew it in for landing. Worked great.

 

[burkefj]

Maiden flight.

I expected that this one would be difficult to see the attitude and keep orientation, so I used a Spektrum AR630 RX, and set up SAFE mode. The rate gyros were active for launch, then at apogee, I selected SAFE so it would keep itself upright while I drove it. When it got close enough, I de-selected SAFE (rate gyros still active) and flew it in for landing. Worked great.

Was that a D-13?

 

[sr205347d]

No. It uses a cluster of three C6 motors. Very cool!

 

[burkefj]

No. It uses a cluster of three C6 motors. Very cool!

Interesting if you were to build another one you could just go with a single 24 mm Mount and use the E6 long burn you'll get 10 more Newton seconds of total thrust at least your CG shift will be slightly less than 3 motors and your tail weight will be about 15 g less so reduced flight and glide weight as well. You also won't have to plug your motors. I have one set up like this but it goes fairly high and glides quite a bit faster probably similar to what yours is like. Yeah I know Tyler the comment on worrying about orientation is doubly true for one that goes as high on the E6 motors.

 

[sr205347d]

Yeah, but I can get a three pack of C6-0s for $8 at Hobby Lobby.

 

[burkefj]

Yeah, but I can get a three pack of C6-0s for $8 at Hobby Lobby.

E-6 motors are $8.50 per flight if you don't count shipping, and the chance of cato in C-6's is not equal to zero And all three need to light

I'm just giving you crap, you know I mean well.... I like your experimenting..

Frank

 

[burkefj]

I'm curious what your altitude is with this setup as to a single 18 mm d24 or d13 which are close to 20 Newton seconds compared to three C's which are 27 Newton seconds but with the single 18mm reload you only have 30 g of tail weight instead of 75 and the extra nose weight required for it, I would think that extra mass would offset any altitude gain you would get and the 18mm d motors are under $20 for a three pack

 

[sr205347d]

I think Frank is trying to get me to buy another X-15 kit.

 

[burkefj]

I think Frank is trying to get me to buy another X-15 kit.

Moi?

 

[burkefj]

Maiden flight.

I expected that this one would be difficult to see the attitude and keep orientation, so I used a Spektrum AR630 RX, and set up SAFE mode. The rate gyros were active for launch, then at apogee, I selected SAFE so it would keep itself upright while I drove it. When it got close enough, I de-selected SAFE (rate gyros still active) and flew it in for landing. Worked great.

What CG location did you wind up using for this setup? IE distance to the rear of the chine/wing intersection.
Frank

 

[sr205347d]

I packed enough adhesive putty into the nose cone to move the CG to your recommended 5/8” point.

CAUTION: When using the SAFE mode, please do not use any reduced “dual” rates. Set the rates to 100% for the SAFE mode, or else you won’t be able to command to the pitch and bank angle limits that you have set for SAFE.

 

[burkefj]

I packed enough adhesive putty into the nose cone to move the CG to your recommended 5/8” point.

CAUTION: When using the SAFE mode, please do not use any reduced “dual” rates. Set the rates to 100% for the SAFE mode, or else you won’t be able to command to the pitch and bank angle limits that you have set for SAFE.

I finally had some time to do some more testing, I've moved the CG back to 1" now and it is boosting fine and I'm getting improved glide times with less required up trim.

 

[BigMacDaddy]

I bet that this rocket is a good candidate for the base drag hack in Rocksim or OpenRocket. Between the pods and the wide top and bottom fins not to mention a relatively large body tube.

 

[sr205347d]

I finally had some time to do some more testing, I've moved the CG back to 1" now and it is boosting fine and I'm getting improved glide times with less required up trim.

That is great to know.

Did you try moving the CG even further aft? If so, did it get pitchy? I wonder if a gyro would smooth that out, and allow even better glide performance.

 

[burkefj]

That is great to know.

Did you try moving the CG even further aft? If so, did it get pitchy? I wonder if a gyro would smooth that out, and allow even better glide performance.

No, I slowly moved it back over six flights, It was in 8-9mph boost winds and it did not appear twitchy, but I haven't had a gyro rx so am afraid to move it more and destroy it, it flies pretty nicely like this, could it be better? Maybe, feel free to try...The 18mm version did not have as much of a CG shift so it flew fine at the 5/8" setting, the 24mm version I'm doing now was running out of pitch authority so decided to take a risk and see if I could get it better, now I'm getting 55 second glide times out of it, even with an extra 3/4 ounce of glide weight.

Frank

 

[burkefj]

My best flights so far, two this morning just over a minute of glide with the more rearward CG, really nicely behaved model. My son had a bit of an issue tracking on boost because the model got into the smoke trail from where he was standing..

 

[sr205347d]

I have difficulty determining if I am looking at the top or bottom of the X-15, especially when high above. Either your eyes are better than mine, or you can keep track of the orientation it should be in.

 

[burkefj]

I have difficulty determining if I am looking at the top or bottom of the X-15, especially when high above. Either your eyes are better than mine, or you can keep track of the orientation it should be in.

Well I think part of it is I'm actively piloting the model at all times and and making fine corrections so I know exactly what direction it should be pointed in and I can see it react when I give the commands so my head's in the cockpit and I don't ever seem to get confused that way.. maybe it's an artifact you're having of using the stability augmentation and that you're somewhat of a spectator during the boost phase? I find that if I'm looking at a model and unclear what direction it should be pointed in it's much more confusing than if I'm actively moving the sticks all the time.

 

[sr205347d]

My son had a bit of an issue tracking on boost because the model got into the smoke trail from where he was standing..

That happens to me too, which makes it difficult to know what its attitude is.

 

[sr205347d]

I bet that this rocket is a good candidate for the base drag hack in Rocksim or OpenRocket. Between the pods and the wide top and bottom fins not to mention a relatively large body tube.

Disregard the mass and CG, as I did not model any internal components. The CP shown is Rocksim calculated, not Barrowman. This is three inches aft of the wing-strake joint. With Frank's originally recommended CG located 5/8" aft of that, the stability margin is 2.375" or 0.91 calibers.

He is now flying it with the CG 1" aft for a stability margin of 2" or 0.67 calibers.

I flew yesterday with the boost CG at about 1 3/8 inches aft for a margin of 0.63 calibers with no hint of instability.

 

[sr205347d]

I flew yesterday with the boost CG at about 1 3/8 inches aft for a margin of 0.63 calibers with no hint of instability.

View attachment X-15 Boost.m4v

 

[burkefj]

That happens to me too, which makes it difficult to know what its attitude is.

When I was boosting I was flying it out of the smoke trail but my son was standing a little bit away from me and his perspective is a little different so he didn't have the same advantage that I did, but yes every once in awhile on cold humid days you get a lot of thick smoke that can obscure the boost.

 

[sr205347d]

1/4 inch further aft, 1 5/8 inches from the wing-strake joint. No issues.

View attachment X-15 Launch.m4v

 

[BigMacDaddy]

FWIW - I have found a bunch more articles about the base drag of the X-15 - apparently those wedge fins and flat backed wing fairings create a ton of drag...

 

[Crawf56]

I have difficulty determining if I am looking at the top or bottom of the X-15, especially when high above. Either your eyes are better than mine, or you can keep track of the orientation it should be in.

I know what you mean. My "go to" color for the underside of my rocket gliders is fluorescent orange.

I have used both spray and brush on. On a scale model, I paint the underside of the wings and elevator orange.

Or I choose color schemes that have significant differences in top/bottom color.

 

[sr205347d]

Another 1/4 inch. 1 7/8" back. Still stable.

View attachment X-15 Launch.m4v