Stability of Spinning Rockets?

[dedleytedley]

I'm planning an upscale clone of Estes Mosquito based on a BT-80. I have some balsa stock that is 1/4x2x12 to use for the fins. I had planned to shape them as a symmetrical airfoil but then I thought why not make each fin with one side flat and the other side an airfoil giving lift and causing the airframe to rotate. Is it likely to be stable or will the rotation combine with the pitch/yaw motions and cause it to cone?
I don't have a plan for the Mosquito or it's big brother the Mozzie so I'm building this by eye. Any hard numbers for the dimensions will be appreciated. Thanx Ted

 

[AndyJ]

Before I say anything, let me just say that this is purely based on intuition. So here's my :2:. If you get any significant rotation, the rocket should act like a gyroscope, and would have a tendency to keep rotating despite external forces (like a bike wheel). Now, this being said, it would be hard to generate a significant amount of spin (though I'm not saying its impossible or that you shouldn't try). What motor are you planning on using? because I would guess (again, just a hunch) that you would need a fairly significant amount of speed to get enough of a rotation to get that gyroscopic effect. I would at least make sure the rocket was stable in its own right (you probably had that covered anyways). I hope this somewhat covers your question, though I freely invite anyone to correct me.

 

[andytherocketeer]

It's not quite as simple as that. A non-rigid body rotating around an axis will tend to want to rotate about the axis of least angular momentum. In the case of the bike wheel or an art applewhite saucer, that axis happens to be the intended direction of flight. In the case of a normal shape rocket, that axis is perpendicular to the longitudinal axis. So my intuition would be that if you want to make a spinny rocket, make it as stumpy as possible, and have a ring of mass around the front end of the body tube.

 

[Micromeister]

For more years then I'd like to remember it's been a somewhat common practice to use symmetric Airfoiled fins to help spin stabilze many model rockets, as this was/is a common practice with most research sounding Rockets.
Back in the day I personally did this with my Cinaroc/Omaga 2-staged model. The Cinaroc film at normal speed will almost make you sick it spins so fast. The flight however was pretty much arrow straight. Two staged Flight profile was the same, with staging looking really neat as the booster followed, slowed down still spinning before it began to tumble. Somewhere I have a couple still pics of the smoke trail but apparently they didn't seem important enough to have scanned them already
I've semi-symmetric airfoiled fins on other models to induce slower rates of spin to help with marginally stable competition models. The result is Always a greatly reduce achived altitiude but the models maintain arrow straight flights.
Hope this helps a bit.

 

[shreadvector]

A long rocket with canted (or asymmetric airfoiled) fins compared to the same fins on a short rocket: The long rocket will fly rifle-bullet-straight.

Rockets that cone tend to be those with huge masses up near the nose, slow moving (wrong motor for the mass) and fins that are too small and do not extend out into the free airstream.

 

[dedleytedley]

Thanks for the input! I plan to use 24mm mmt, flying on D12's mostly. I found a plan on jimz's site and the upscale gives a BT length of 242mm. It will use the Estes ellipse nose cone and the fins will be ttw set at 45 degrees to the long axis. It should be very light as configured so I'm considering going with tumble recovery like the original mosquito. I'd expect it to come in nose first,rotating to slow it. Any more comments? Ted

 

[Micromeister]

Ted:
Safety First always please.
The Rotation won't be enough to slow a model that size. It wouldn't be enough spin to slow standard size mosquito for that matter. Your BT-80 model needs a recovery system. A small 12 or 15" chute or 4" or so wide streamer will do the job nicely.
Do you remember the Estes Gyro? A two fin Helo "spin recovery" model that comes in two fast to be considered a safe recovery method, even the 3 fin modification if built a little on the heavy side comes in to fast. I stick the round BT-20 nosecone in the grass every time.

Used to fly a lot of 2.6" upscales and even though they can be pretty light they are big and heavy enough to hurt if it hits someone. More important however they comes in fast enough to damage the model or in this case the BT-80BB styrene cone. If your worried about fin damage attach a teather from the shockline to the motor hook and bring the model down sideways under chute being sure to use a spill hole in the canopy to reduce canopy oscillation. I use this method alot on models I don't want landing tail first.

 

[shreadvector]

Marc McReynolds has a giant Gyroc that he flies occasionally. It is huge a fluffy and the large elevons/spinnerons create a lot of drag. The model descends pretty slowly and very safely. I'm pretty sure he has flown it at one of the recent NARAMs in one of the fun or fantasy events.

To do so requires that it be fluffy (light and low density) and have large draggy fins and spinning surfaces.

https://www.wsu.edu:8080/~brians/errors/to.html

Ted:
Safety First always please.
The Rotation won't be enough to slow a model that size. It wouldn't be enough spin to slow standard size mosquito for that matter. Your BT-80 model needs a recovery system. A small 12 or 15" chute or 4" or so wide streamer will do the job nicely.
Do you remember the Estes Gyro? A two fin Helo "spin recovery" model that comes in two fast to be considered a safe recovery method, even the 3 fin modification if built a little on the heavy side comes in to fast. I stick the round BT-20 nosecone in the grass every time.

Used to fly a lot of 2.6" upscales and even though they can be pretty light they are big and heavy enough to hurt if it hits someone. More important however they comes in fast enough to damage the model or in this case the BT-80BB styrene cone. If your worried about fin damage attach a teather from the shockline to the motor hook and bring the model down sideways under chute being sure to use a spill hole in the canopy to reduce canopy oscillation. I use this method alot on models I don't want landing tail first.

 

[DaveHein]

I think spinning rockets and other similar objects fly straighter because the aerodynamic forces are averaged out along the spin axis, and not because of gyroscopic stability. A good gyroscope needs to spin at a very high rate (greater than 1,000 RPM) and it needs to have a large moment of inertia. This generally means that it should be heavy and wide with the weight distributed toward the outer edges.

A football is a good example of an object that flies better with spin. The spin rate isn't very high -- maybe 100 to 200 RPM, and it doesn't weigh much. The Cp and Cg of a football are basically located at the same point in the center of the football. It is perfectly designed to tumble. However, a spinning football averages out the aerodynamic forces on each side, which causes it to fly straight and not tumble.

If you watch a slow motion replay of a perfect pass you'll see that the football changes its inertial orientation during the flight, and is constantly pointing in the direction of motion. A gyroscope would tend to maintain its inertial orientation and would always point in the same direction, or precess like a top.

A spinning football does have some rotational inertia. It can be spun on a table, and it will stand on its tip for a few seconds. But, like a rocket, it doesn't make a very good gyroscope.

Dave

 

[shreadvector]

Absolutely correct. I was going to say something similar earlier but I was distracted by a short meeting, several phone calls and an annoying number of e-mails (all work related). (Speaking of which, you'll poop your pants when you see the new paint job on the newest C-17 that just rolled out of the paint building....)

Anyway, the aerodynamic forces on the fins will keep the rocket stable (i.e. aft end to the rear) as long as there is airspeed. The rotation helps a bit because anything uneven will average out. That is why some kits have a built in spin tab - that only works IF the rocket leave the pad with a good air speed. Too slow and there is no spin and the rocket flops over to one side. Typical example: shuttle orbiter rocket kits and other space plane like kits.

A helicopter duration model will come down nice and stable IF and only if te blades deploy so that the descending cp is behind the cg. On my Rotorocs I have to make sure I have enough angle upward on the blades. if they deploy flat, the fins on the rocket will create enough disturbance to make the rocket flip upside down.

In space, with no aerodynamic forces, you can worry much more about the rotation flipping from axis to axis. What's that called again? My text books are packed away at home. Not 'presession' or 'nutation'.....?

I think spinning rockets and other similar objects fly straighter because the aerodynamic forces are averaged out along the spin axis, and not because of gyroscopic stability. A good gyroscope needs to spin at a very high rate (greater than 1,000 RPM) and it needs to have a large moment of inertia. This generally means that it should be heavy and wide with the weight distributed toward the outer edges.

A football is a good example of an object that flies better with spin. The spin rate isn't very high -- maybe 100 to 200 RPM, and it doesn't weigh much. The Cp and Cg of a football are basically located at the same point in the center of the football. It is perfectly designed to tumble. However, a spinning football averages out the aerodynamic forces on each side, which causes it to fly straight and not tumble.

If you watch a slow motion replay of a perfect pass you'll see that the football changes its inertial orientation during the flight, and is constantly pointing in the direction of motion. A gyroscope would tend to maintain its inertial orientation and would always point in the same direction, or precess like a top.

A spinning football does have some rotational inertia. It can be spun on a table, and it will stand on its tip for a few seconds. But, like a rocket, it doesn't make a very good gyroscope.

Dave

 

[dedleytedley]

Ted:
Safety First always please.
The Rotation won't be enough to slow a model that size. It wouldn't be enough spin to slow standard size mosquito for that matter. Your BT-80 model needs a recovery system. A small 12 or 15" chute or 4" or so wide streamer will do the job nicely.
Do you remember the Estes Gyro? A two fin Helo "spin recovery" model that comes in two fast to be considered a safe recovery method, even the 3 fin modification if built a little on the heavy side comes in to fast. I stick the round BT-20 nosecone in the grass every time.

Used to fly a lot of 2.6" upscales and even though they can be pretty light they are big and heavy enough to hurt if it hits someone. More important however they comes in fast enough to damage the model or in this case the BT-80BB styrene cone. If your worried about fin damage attach a teather from the shockline to the motor hook and bring the model down sideways under chute being sure to use a spill hole in the canopy to reduce canopy oscillation. I use this method alot on models I don't want landing tail first.

Hi John It's good to hear from you again. I didn't seriously consider tumble recovery for this bird as the original Mosquito is reputed to land hard by this method. It sure would look cool coming in ballistic though. I generally fly in empty areas with my van as the "blockhouse" and no spectators. That's good as I've had a few heads up flights.
There isn't enough room for a baffle, so I guess I'll go with a 24mm motor mount ring for a parachute stop.
It appears that the design will fly straight and spin from people's comments. I'd like to put mirror monokote on the flat sides of the fins to give a flashing effect as it spins. Ted