Stability Help (Open Rocket)

[BLKKROW]

So I have been stumped lately, I am trying to determine if my Blackhawk 38 is stable or not.

Here are the steps I followed, I loaded everything up with laundry/av-sled/Kevlar cords. I weighed the bird and got a mass of 983.8 grams.

So with everything still inside, I went out to the garage to figure out the CG. So I had it hanging from the roof with a shock cord, and got a CG of 78.6cm from the tip of the nose cone.

I input all this information into Open Rocket then selected the CTI H143 motor, and am getting a stability margin of 4.42 cal.

Something is not adding up, all the components have been measured and are accurate on the OR file. The only addition I made to the kit is instead of gluing the shock cord to the nose cone/coupler in the lower half, I used two washers and a bolt to sandwich the nosecone coupler, and the same with the other coupler. This does add some weight, but I don't see it adding that much more to make the rocket unstable.

Please help I am trying to get this bird to fly in the beginning of the year for my L1. This is the only thing holding me back.

Could it be this rocket needs a motor that is bigger then 2 grains?

View attachment Blackhawk 38.ork

 

[Rex R]

one could wish that you had included as seperate components the shock cords and chutes. but looks like the av bay is what is throwing your cg forward, seems to be inherent w/ 'long skinny' rockets.

 

[T-Rex]

I input all this information into Open Rocket then selected the CTI H143 motor, and am getting a stability margin of 4.42 cal.

Something is not adding up, all the components have been measured and are accurate on the OR file. The only addition I made to the kit is instead of gluing the shock cord to the nose cone/coupler in the lower half, I used two washers and a bolt to sandwich the nosecone coupler, and the same with the other coupler. This does add some weight, but I don't see it adding that much more to make the rocket unstable.

You believe that with a stability margin of 4.42 your ship is unstable? I question your ability to be an L1 flyer. Said my piece, I'll shut up now.

 

[BLKKROW]

You believe that with a stability margin of 4.42 your ship is unstable? I question your ability to be an L1 flyer. Said my piece, I'll shut up now.

I understand that its over the rule of thumb of 2-3 cal. I also know that it is not unstable, I just want it to be closer to 3 cal of stability.

 

[Rex R]

well Tim L. & CJ would tell you 'stick a bigger motor up the pipe' , doesn't look bad...if you stick an I600 in there. course you might not see it again once it passes 9k feet. unless you were to move the AV bay aft, I think you'll just have to live with it sry.
rex

 

[BLKKROW]

well Tim L. & CJ would tell you 'stick a bigger motor up the pipe' , doesn't look bad...if you stick an I600 in there. course you might not see it again once it passes 9k feet. unless you were to move the AV bay aft, I think you'll just have to live with it sry.
rex

Thanks for looking I was thinking of making the lower section shorter by like 1.5 inches. This would bring it to around 3, plus I will not be flying anything to large into it. Seeing how the waiver will barely handle an H.

Just to make things clear, I was typing the original post in my garage and in a hurry. I really just wanted to make sure I understood what I was doing in OR. Now that I know what I was doing was correct, I can work on shortening the rocket slightly.

 

[T-Rex]

I understand that its over the rule of thumb of 2-3 cal. I also know that it is not unstable, I just want it to be closer to 3 cal of stability.

My apologies, you had me very concerned.

 

[BLKKROW]

My apologies, you had me very concerned.

I apologize, again I was typing the OP in a hurry.

If I shorten the length of the lower tube, then I am getting around 4.0, this does not account for the minor change in mass. So I am figuring around 3.9cal, this makes me feel better but still concerned.

I just want to remove all issues with my L1 attempt.

 

[CarVac]

Today I flew a rocket that was predicted to have 1 caliber of stability in both OpenRocket and RASAero, but it was definitely not stable at all.

Turns out neither program really likes extreme boattails, like 3.1 down to 1.1 inches. Be forewarned.

But for you that's not an issue. A stability margin of 4.2 is perfectly fine, but indeed the 'proper' solution is a bigger motor.

 

[BLKKROW]

Today I flew a rocket that was predicted to have 1 caliber of stability in both OpenRocket and RASAero, but it was definitely not stable at all.

Turns out neither program really likes extreme boattails, like 3.1 down to 1.1 inches. Be forewarned.

But for you that's not an issue. A stability margin of 4.2 is perfectly fine, but indeed the 'proper' solution is a bigger motor.

I am thinking of shortening the lower section by 1.5 inches. This would bring the margin below 4.0 would that be acceptable? Or should I keep the length, I don't plan on flying this on anything Higher then a 4 Grain. Cause I will have to fly it out of state to do that.

 

[CCotner]

What exactly is the problem with being dramatically overstable? Except for weathercocking, isn't overstable a good thing?

 

[Rex R]

overstable just means you have to walk as far . go ahead and sim it, then plot side view.
rex

 

[Peter Olivola]

What's the goal for this rocket? If you're building for a small field and want to fly big motors the drag inherent in an overstable design might be appropriate. OTOH, as Rex noted, you'll be limited to low wind days.

What exactly is the problem with being dramatically overstable? Except for weathercocking, isn't overstable a good thing?

 

[bobkrech]

1. If you build your Blackhawk 38 as per the instructions, it will be stable.

2. To verify this use OR or RASAERO to determne where the CP which only depends on the shape of the rocket is located and mark it on the rocket, and ignore everything else.

3. It better to measure the CG directly since the rocket data base can have errors and you might not have built the rocket exactly the way you planned. Load the rocket with everything you need to launch it including the motor. Suspend the rocket on your finger and determine the balance point and mark it on the rocket as that is the actual CG. If the actual CG is forward of the calculated CP by 2 diameters or more you are set to go. As you burn out the motor the CG will move forward which is fine.

4. Being overstable is not necessarily a bad thing. In a crosswind, the CP moves forward and initially reduces the stability margin, but the rocket will turn into the wind and reduce the angle of attack. You will have a shorter recovery walk because your rocket will have turned up wind under power however the apogee will be lower.

Bob

 

[BLKKROW]

What's the goal for this rocket? If you're building for a small field and want to fly big motors the drag inherent in an overstable design might be appropriate. OTOH, as Rex noted, you'll be limited to low wind days.

1. If you build your Blackhawk 38 as per the instructions, it will be stable.

2. To verify this use OR or RASAERO to determne where the CP which only depends on the shape of the rocket is located and mark it on the rocket, and ignore everything else.

3. It better to measure the CG directly since the rocket data base can have errors and you might not have built the rocket exactly the way you planned. Load the rocket with everything you need to launch it including the motor. Suspend the rocket on your finger and determine the balance point and mark it on the rocket as that is the actual CG. If the actual CG is forward of the calculated CP by 2 diameters or more you are set to go. As you burn out the motor the CG will move forward which is fine.

4. Being overstable is not necessarily a bad thing. In a crosswind, the CP moves forward and initially reduces the stability margin, but the rocket will turn into the wind and reduce the angle of attack. You will have a shorter recovery walk because your rocket will have turned up wind under power however the apogee will be lower.

Bob

Thanks for the info, the field I plan on flying at is not small, but certainly is not an open range for miles.

 

[blackjack2564]

Could it be this rocket needs a motor that is bigger then 2 grains?

Nope...you are over thinking all this.
It has ALL already been done. Long before kit is released, it was simmed, then prototyped, then flown. There are no issues.

PS I fly mine all the time on 2 grain motors. An H-123 [AT] will fly it to 1800-2200 depending on conditions. In fact I just flew it yesterday on an old H-180 [29mm AT] to 2207 ft. Nice little fun flight. I have flown mine on every size from 1 grain 38mm [Motor eject 7sec delay, with NC stuck on fincan] 2-3-4-5-6-6xl's dual deploy with absolutely NO issues.

It's my "go to" rocket, for flying all my old motors that would normally fly smaller rockets motor eject. After 6-7yrs I don't trust them do to chuffing, etc. that can throw off the delay time, so I stick 'em in the BH38 and fly dual deploy.

So go fly it,have fun & no worries, mate!

 

[Sampo]

Hi,

A stability margin of 4 cal is not necessarily a bad thing for a long, skinny rocket. Skinny rockets lose a significant amount of stability even at small angles of attack. If you open the component analysis dialog and tune the angle of attack, you'll see that the Blackhawk drops to 1 cal stability at 5° AOA and unstable at 10° AOA.

The rule of thumb applies to "normal proportioned" rockets, skinny ones need more and stubby ones less margin. You can also use a more powerful motor to get the rocket up to speed faster so that the AOA stays small.

Cheers,
Sampo N.

 

[BLKKROW]

Hi,

A stability margin of 4 cal is not necessarily a bad thing for a long, skinny rocket. Skinny rockets lose a significant amount of stability even at small angles of attack. If you open the component analysis dialog and tune the angle of attack, you'll see that the Blackhawk drops to 1 cal stability at 5° AOA and unstable at 10° AOA.

The rule of thumb applies to "normal proportioned" rockets, skinny ones need more and stubby ones less margin. You can also use a more powerful motor to get the rocket up to speed faster so that the AOA stays small.

Cheers,
Sampo N.

Thanks for that info.

This thread has really helped me calm my nerves, I just want my L1 cert to go smoothly with no issues I can only hope.

 

[Sampo]

Hi,

This is the reason why OpenRocket doesn't state a rocket as being unstable/stable/over-stable, since I don't want to use a rule of thumb that I know is wrong. I've been trying to come up something that would be more generally valid, but haven't decided on anything. For example testing whether the rocket is stable at 0 and 5 degree AOA. But even that depends on the motor and launch conditions...

Cheers,
Sampo N.