Pitch rates in OR simulations

The Rocketry Forum

Help Support The Rocketry Forum:

Dane Ronnow

Well-Known Member
TRF Supporter
Joined
Dec 16, 2020
Messages
219
Reaction score
104
Location
Las Vegas, NV
I've got a question about pitch rate in OR simulations. (I searched the forum for info on this, but couldn't find anything that addressed this specifically. Forgive me if I missed it somewhere.)

I'm building a 2.6 inch MPR, designed to fly on three different Aerotech composites—F67, G74 and G80. The rocket is 37.5 inches in length, weighs just over 21 oz. without motors, and has the following calibers of stability with those motors:

F67 - 1.65
G74 - 1.61
G80 - 1.41

The simulation pitch rates are essentially flat in zero wind. With 5 mph of wind, pitch rates are between 12 and 15 degrees/sec (depending on motor used) and twice that with 10 mph wind.

Are those pitch rates excessive? Or are they normal for those wind speeds?

Attached are screen captures of simulation plots for each motor, in 5 mph wind.

Thanks much for any advice I can get.

Pitch Rate F67.JPGPitch Rate G74.JPGPitch Rate G80.JPG
 

SpaceManMat

Space Nut
Joined
Dec 20, 2013
Messages
689
Reaction score
60
That doesn’t look good to me. One thing to be careful of is that stability is dynamic over the flight. Suggest you post the graph stability vs time
 

Dane Ronnow

Well-Known Member
TRF Supporter
Joined
Dec 16, 2020
Messages
219
Reaction score
104
Location
Las Vegas, NV
Below are plots for Stability vs. Time in zero, 5 and 10 mph wind, all with the G80 motor. The fourth plot is 5 mph with pitch rate plotted.

Stability - 0 mph wind - G80.JPGStability - 5 mph wind - G80.JPGStability - 10 mph wind - G80.JPG

Stability - 5 mph wind with pitch plot - G80.JPG
For the sake of comparison, I increased the fin area to see if the pitch rate decreased. It didn't. It increased from 12 degrees/sec (at the first deviation) to a little over 15 degrees. This tells me the oscillations in pitch is due to the wind acting against the fins, not any instability of the rocket.

I'm thinking now that what appears to be wildly fluctuating pitch deviations on the graph is simply the angle of attack changing with the wind, followed by the restorative force bringing it back past zero to the opposite side, and so on, dampening the change in AOA as the velocity increases.
 
Last edited:

gtg738w

FlightSketch - flightsketch.com
TRF Sponsor
Joined
Dec 27, 2018
Messages
305
Reaction score
290
I'm thinking now that what appears to be wildly fluctuating pitch deviations on the graph is simply the angle of attack changing with the wind, followed by the restorative force bringing it back past zero to the opposite side, and so on, dampening the change in AOA as the velocity increases.
Exactly, when it leaves the rod, it is at an angle of attack relative to the wind. The pitch rate is the fins trying to correct that. You can see it in real data too:

tilt.jpg
 

Dane Ronnow

Well-Known Member
TRF Supporter
Joined
Dec 16, 2020
Messages
219
Reaction score
104
Location
Las Vegas, NV
Exactly, when it leaves the rod, it is at an angle of attack relative to the wind. The pitch rate is the fins trying to correct that.
Thanks, Russ. I appreciate your input. Does FlightSketch give you that tilt data?
 

Latest posts

Top