X-15 CG

[burkefj]

That's interesting you beat me to it, I was waiting for a windier day to test it to make sure it wasn't twitchy, I've got mine balanced at 1 and 7/8 in back from the chine joint, but this puts my receiver and my battery back as far as I can and still reach it with my fingers to plug and unplug it, and I'm not using any nose weight I don't think I could get the CG any further back without putting tail weight in which I hate to do... Or you'd have to work out a hatch mechanism.

This probably helps you a little bit more since you're running a cluster of three which means more tail weight than I have with a single e6 motor, with the d 24s or d13s I don't think you could get the balance back that far.

I think you've got about 72 or 73 g of plugged motors in the tail and I'm running about 51 or 52 g. For the e6 and 29 for the d24 and d13.

There are a few tenths of an ounce difference in weight of the nose cones I think mine might be on a little bit heavier side.

For your flights with stability on, it may seem ok, but for me, hand flying it in some wind today, it was starting to get twitchy on boost and I don't know if I'd personally run it further aft, especially on the long 7-8 second burn motor where it gets pretty fast and pretty small near the end of the flight, especially if there is any wind you have to react to.

I will say I'm not using any dual rate or expo settings, so it MAY be possible to tone down the twitchiness with less throw on boost, but I definitely need the larger throws on glide. For a kit recommendation I want to make sure 100% of people can have success with basic settings. I look forward to seeing how far you are able to get moved back with stability on.

I got 1:15 glide times consistently over 3 flights. I took out 28% of my up trim for glide and a little bit of down trim on boost.

 

[burkefj]

Here is the 1 7/8" CG flight today.

 

[sr205347d]

For a kit recommendation I want to make sure 100% of people can have success with basic settings. I look forward to seeing how far you are able to get moved back with stability on.

The use of a gyro rx might just increase the probability of success, as smaller models can be more twitchy than larger ones. The rate dampening helps me because my old man reactions aren't as quick as they used to be.

After that last flight, I removed the remaining nose weight. That puts the launch CG a smidge over 2", maybe 2 1/16", aft of the wing-strake joint. I will let you know how it works.

 

[burkefj]

The use of a gyro rx might just increase the probability of success, as smaller models can be more twitchy than larger ones. The rate dampening helps me because my old man reactions aren't as quick as they used to be.

After that last flight, I removed the remaining nose weight. That puts the launch CG a smidge over 2", maybe 2 1/16", aft of the wing-strake joint. I will let you know how it works.

Great!

 

[sr205347d]

Launch CG at about 2 1/8 inches.

View attachment X-15.m4v

I could easily move the CG further aft as my battery is still located in the nose cone. But I think I will just let it be.

I flew it all the way from apogee to landing in SAFE mode.

If you wanted to experiment with a further aft CG, you could put your RX and battery on a lightweight foamboard slide-out tray.

 

[burkefj]

Launch CG at about 2 1/8 inches.

View attachment 580194

I could easily move the CG further aft as my battery is still located in the nose cone. But I think I will just let it be.

I flew it all the way from apogee to landing in SAFE mode.

If you wanted to experiment with a further aft CG, you could put your RX and battery on a lightweight foamboard slide-out tray.

Looks good. If you build it with the e6 motor in mind and you have the battery and the RX back inside the body tube as far as you can reach it balances perfectly at about 1 and 7/8 in. You have more ability to move the CG back further because you're running 3 C motors which are heavier... I'm glad you were willing to experiment with this it helped make the kit better and redesign it for the 24 mm motor and still have a good glide profile with a lot better altitude.

You can definitely take advantage of a little more rearward CG since you're running about .4 oz more propellant tail weight so your glide CG is further forward than mine with the e motors.

 

[burkefj]

By the way for reference what is your boost takeoff weight and what total flight or glide time are you getting with this setup. I'm guessing you're at least three quarters of an ounce heavier than mine due to the extra weight of the three motor cluster versus a single reload.

 

[sr205347d]

By the way for reference what is your boost takeoff weight and what total flight or glide time are you getting with this setup. I'm guessing you're at least three quarters of an ounce heavier than mine due to the extra weight of the three motor cluster versus a single reload.

Liftoff weight is currently 10.5 oz which is significantly greater than yours, I’m sure. Besides the motors, my RX and battery are heavier. I calculated the packing tape to weigh 0.00413 oz/in^2 for a fin taped on both sides. I don’t know what paint on Depron weighs.

Flight duration was about 40 seconds, but my glide speed surely wasn’t optimal for max duration. I don’t really care about duration as long as I make it down on the runway!

 

[burkefj]

Liftoff weight is currently 10.5 oz which is significantly greater than yours, I’m sure. Besides the motors, my RX and battery are heavier. I calculated the packing tape to weigh 0.00413 oz/in^2 for a fin taped on both sides. I don’t know what paint on Depron weighs.

Flight duration was about 40 seconds, but my glide speed surely wasn’t optimal for max duration. I don’t really care about duration as long as I make it down on the runway!

For the X-15 completely painted it added about 0.3 oz. I'm below 8.5 Oz ready to fly now and total Newton seconds is higher for the e6 versus three C's so I'm probably getting a hundred ft higher or more and I'm getting about a minute 15 seconds of pure glide time after burnout, but 40 seconds for your setup isn't terrible. As you said, like me, we are trying to hit a runway so give up some glide time to do that

 

[burkefj]

A friend of mine was asking if you are using any safe or default mode after boost to maintain a light circle in case you lose sight or just using the gyro rx and what rx you were using? He's using ardupilot and trying to do some return to home type of thing.

 

[sr205347d]

I never lose sight, so no need for return to home, but it is difficult with the X-15 to see the attitude because of the small size and all black coloring. I use a Spektrum AR630 with AS3X and SAFE. AS3X is active in the boost mode to damp rates in roll primarily, and pitch. At the top, I select SAFE mode to keep it right side up while I steer it around. As I get near downwind for the landing approach, I select a third mode with AS3X and cruise trim. To use SAFE, you need a Spektrum TX with Forward Programming capability. There are YouTube videos with programming instructions.