minimum R/C rocket glider thrust to weight ratio

[mikec]

I've got an R&R Aircraft Gamma Star R/C rocket glider that I've flown many times on Estes D12 and E12 motors. I've always launched straight up.

The long burn times of the Aerotech E6 are tempting, but I'm wondering if the peak thrust is sufficient. The NAR R/C RG safety code only says "My R/C RBG will weigh no more than the motor manufacturer’s recommended maximum lift-off weight for the motors when used in R/C RBGs" but the E6 has no such recommendation AFAIK.

The Gamma Star weighs about 11 oz dry and the E6 weighs 50 grams and has an initial thrust is about 11.9N, so the E6 would have a thrust/weight of about 3.4. And the safety code allows me to launch at 45 degrees from the vertical. The Aerotech Phoenix on a G12 has a thrust/weight of between 3 and 4 depending on whose G12 thrust curve you believe, and I've seen that combo fly great with a 45-degree launch angle.

Any thoughts? The recommended motors for the Gamma Star didn't mention the E6.

 

[iter]

so the E6 would have a thrust/weight of about 3.4. And the safety code allows me to launch at 45 degrees from the vertical.

I think you've answered your own question :=)

Also, I wonder if you concern is legal (you want to make sure you follow the rules) or technical (you want to make sure you can conclude the flight safely). I have different observations that are relevant to different concerns.

Ari.

 

[mikec]

Also, I wonder if you concern is legal (you want to make sure you follow the rules) or technical (you want to make sure you can conclude the flight safely).

Mostly technical. I don't have much of a sense for how controllable the glider will be just off the rail with the E6. The TTW for the D12 is nearly 3x higher.
I know Frank Burke flies E6s but I expect his gliders are significantly lighter than the Gamma Star.

 

[burkefj]

I fly the E-6's in models up to about 13.5 ounces straight up. The peak thrust is 2.5 pounds but it drops to 1 pound pretty quickly. I use a 6' rail and have not had any issues. I think your speed off of a 6' rail/rod will be fine at 12.7 oz rtf and you can see if it works for you. Over 13.5 ounces and it is starting to push it. If it is too sluggish you can boost at a slight angle and zoom up as your speed increases, or you could try the E12 reload which is about a 3.5 second burn with more total impulse than the estes E-12..and launch straight up.

I'm not sure how big the control surfaces are on the delta star, I tend to use large surfaces with large throws which help at slow speed glide, and also help me on boost if it is a bit slow.

Frank

 

[iter]

Mostly technical. I don't have much of a sense for how controllable the glider will be just off the rail with the E6.

A thought that may ease your anxiety is the observation that most R/C airplanes have much lower TTW than 1, and manage to stay controllable. The ailerons on Stars are plenty big enough for roll control even at lower speeds. What you want to watch for is stall. In a sense, stalling the wing makes it uncontrollable. No amount of elevator can help keep a model flying if you slow it down past stall. So as long as you keep the climb angle (rail elevation) below stall, you can expect control.

I have flown rocket gliders with TTW as low as 2, and they do fine as long as I climb in a spiral instead of straight up.

Ari.

 

[burkefj]

What you are really concerned about is speed off the rod and pitch/roll control given that speed. It's not really stall issue which is related to angle of attack and airspeed. There is no angle of attack with a vertical launch. Even though rc planes can have less than 1:1 ttw, they aren't going vertical at takeoff. My only point is that if you have a slight wind that causes a pitch or roll that you want to correct right off the rod you want to have sufficient airspeed over the surfaces and enough authority to keep it going straight. Even if it looks on launch that the speed is slow, you can always pitch over slightly to increase airspeed and then pull up as it increases as long as your control authority for the airspeed is sufficient, you'll most likely be fine.

Frank

 

[mikec]

I'm not sure how big the control surfaces are on the delta star, I tend to use large surfaces with large throws...

The Gamma Star (which is a slight downscale of the larger Delta but otherwise very similar) has full-span but fairly small elevons. They're more designed for rigidity at high speed than control authority at low speed.

Thanks for all the feedback. At a minimum, if I try this I'll need a longer rail.

 

[mikec]

Thanks for all the feedback. At a minimum, if I try this I'll need a longer rail.

I finally ordered a case and some E6 reloads from https://dynasoarrocketry.com and got a chance to try this. It worked really well, and I didn't have any issues with controllability off the rail. I'm not sure why R&R didn't recommend the E6 in the first place.

BTW, shameless plug for Frank Burke at Dynasoar -- he deserves a lot of credit for pretty much single-handedly reviving interest in the low-thrust R/C Aerotech motors. If you have any interest in R/C at all you should give this a try, it's a lot of fun!

 

[burkefj]

Thanks very much, glad that worked out!

 

[JoeG]

Take Frank's advice.He won't steer you wrong.

If you have seen the video of my Phoenix on the new Aerotech G 8 you will notice it falling off a little at the end of the rods during the boost. A slight "push" on the elevator allowed it to recover and the speed built up exponentially it seemed. "Keep your head about you mate" and you should be fine.