Delta wing CG / CP considerations

[meep]

I'm designing my first glider which is going to be swept wing delta style (see below for plans). Is there any things to look out for in such a design in terms of CG/CP?

• How does the lost weight from the burnt out motor affect flight performance? I guess i can check propellant weight of the motor and deduct it from the model to see where center of gravity ends up after the motor has burnt out?
• What i learnt in rc planes is that the CG should be about 1/3 from the front of the leading edge, so i try to plan accordingly, but i will also do some throw tests on the ground to see if i'm totally off with CG.

What do you think about the design below? I'm trying to get stability calculations > 0.5 but its hard with the design, and when the motors burns out, the CG will move even more forward, so not sure how well it will glide. I can move the CG back by placing the electronics (battery, receiver, etc.) more to the back of the rocket, but then stability will get worse because CG and CP move closer.

Maybe i should do the CG calculation for the gliding phase with a tool that is suited for calculating optimal CG location for rc planes?
Does anyone have a OpenRocket file for a similar styled design that flies well, so i can compare it?

 

[kjohnson]

Inferring from your post- this is a rear motored RC RG? You plan on keeping the motor case in the model? You will see a forward shift of the CG as the motor burns, but it won't be a lot. Set your CG so that it's controllable in boost and glide. Take a look at the info that @burkefj has posted here and at his website for some of his models with a similar planform.

kj

 

[meep]

yes thats correct, motor will be on the back. the issue i have with designing it is that it seems that i need to get the CP more to the back, which i'm not sure what adjustments i need to make.

i calculated the cg for the glider phase by replicating the wing shape in this cg calculation tool, so i will do that in parallel to try having a stable cg location for gliding.

 

[burkefj]

Use the winglib calc for approx starting point for CG location of the wing, the area of the tube/cone will contribute approx 1/2 of the eqivalent "flat" area so I'll include that in the model even though it may not be significant, sometimes it is, you can add that into the wing dimensions as extra wing panels 1 and 2 to get a better starting point. Note the width of panel 2 is .001 since that is just used to move the root of the actual wing back to where it will really be. Use 25-30% of MAC as the measure for stability, then build a light prototype without equipment, balance it and do some glide tests. I do these by pushing the model forward in about a 10 degree nose up attidude and give a hard push to approximate if it was in a "mush" if the model pitches over a bit or stays the same attitude, you are probably close for launch max rearward CG, if it pitches up further, you are too rearward. Once you have this then use openrocket to play with the equipment location by inputting ALL weights, wires, battery, pushrods etc and motor. Move things around including wing placement so you get a good natural cg with minimal or no nose weight required and tweak the simulation as you go along and add/subtract weight. That doesn't look like a huge amount of wing area for your flight weight, but I'm not sure what kind of materials you are using. I shoot for a 5-7 ounce/sq foot range and I'm using long burn motors. Plan for some up trim once the motor burns out and the cg shifts forward and be ready to react if your first flight are two are a bit twitchy and have some nose weight with you to tweak things through a few flights. I don't bother trying to do actual glide trimming ahead of time, just take a swag at the amount of trim needed and use the first flight or two to adjust as needed.

I only use openrocket for mass balancing and moving things around, and then build a separate model using only the vertical stabilizers in both the X and Y axis to check the lateral CP location and make sure my launch CG is not too rearward for the lateral CP placement. Since you are piloting the model you don't need a normal caliber of stability margin in openrocket, maybe only in the lateral yaw sense...

Photos below show using winglib, note the CG location is from the wing tip at the root edge, but the CG % you are specifiying is with respect to the % of the MAC not simply the leading edge of the wing....In this case since I've modeled the nose cone and body tube as 1/2 of the actual width the CG location they give you is with respect to the nose cone tip. Other than this you need to do some testing and figure things out.

I like to use long burn motors as it is less stress on the airframe and pilot and gives you more time to adjust if your boost trim is not good, fast burn motors won't give you that, but the model may be too heavy for those. I also don't like to use black powder motors in my rc rg's as their failure rate is too high and in a model where you have large $$ involved with rc gear I won't risk it.

 

[Ringdan]

I'm designing my first glider which is going to be swept wing delta style (see below for plans). Is there any things to look out for in such a design in terms of CG/CP?

• How does the lost weight from the burnt out motor affect flight performance? I guess i can check propellant weight of the motor and deduct it from the model to see where center of gravity ends up after the motor has burnt out?
• What i learnt in rc planes is that the CG should be about 1/3 from the front of the leading edge, so i try to plan accordingly, but i will also do some throw tests on the ground to see if i'm totally off with CG.

What do you think about the design below? I'm trying to get stability calculations > 0.5 but its hard with the design, and when the motors burns out, the CG will move even more forward, so not sure how well it will glide. I can move the CG back by placing the electronics (battery, receiver, etc.) more to the back of the rocket, but then stability will get worse because CG and CP move closer.

Maybe i should do the CG calculation for the gliding phase with a tool that is suited for calculating optimal CG location for rc planes?
Does anyone have a OpenRocket file for a similar styled design that flies well, so i can compare it?

Looking at your picture, I'm wondering. Does Openrocket calculate the lift that a wing creates? Cause when I try to add wings to make the rocket fly further, I'm only met with calculations and simulations that tell me that I'm only crashing earlier. Does anyone here know how to make working wings in Openrocket??

 

[MJ D]

I'm designing my first glider which is going to be swept wing delta style (see below for plans). Is there any things to look out for in such a design in terms of CG/CP?

• How does the lost weight from the burnt out motor affect flight performance? I guess i can check propellant weight of the motor and deduct it from the model to see where center of gravity ends up after the motor has burnt out?
• What i learnt in rc planes is that the CG should be about 1/3 from the front of the leading edge, so i try to plan accordingly, but i will also do some throw tests on the ground to see if i'm totally off with CG.

What do you think about the design below? I'm trying to get stability calculations > 0.5 but its hard with the design, and when the motors burns out, the CG will move even more forward, so not sure how well it will glide. I can move the CG back by placing the electronics (battery, receiver, etc.) more to the back of the rocket, but then stability will get worse because CG and CP move closer.

Maybe i should do the CG calculation for the gliding phase with a tool that is suited for calculating optimal CG location for rc planes?
Does anyone have a OpenRocket file for a similar styled design that flies well, so i can compare it?

Static stability margin can be determined fairly closely in a similar manner as for a conventional model, but there is no tail area to consider. You determine the mean average chord (MAC) of the wing using - if possible - simple graphical methods as shown. Then you assume the center of lift (which is the same as the neutral point in this case as it is single surface) is at 25% of that chord, and then assign a stability margin, normally 8-10% of the MAC, ahead of that point. That ought to get you started on CG location anyways.

 

[burkefj]

Static stability margin can be determined fairly closely in a similar manner as for a conventional model, but there is no tail area to consider. You determine the mean average chord (MAC) of the wing using - if possible - simple graphical methods as shown. Then you assume the center of lift (which is the same as the neutral point in this case as it is single surface) is at 25% of that chord, and then assign a stability margin, normally 8-10% of the MAC, ahead of that point. That ought to get you started on CG location anyways.

That's a good conservative start, on my Aurora Clipper Delta I'm actually using 23% Mac for boost. Not counting any forward fuselage lift in the calculation.

 

[MJ D]

Yeah it's just a start, and only deals with the wing not any forward area. But it's a good start for someone to get into NP estimates for new designs. Plus you can start ahead of that and work back if needed.