That was sure a funny experience for me--I pondered it for a while, didn't see the correct relationship, wrote my note, hit return and within seconds saw it the right way round! Made me laugh....thanks.
Looks like someone's going to be buying the altimeter with the accelerometer :-)
--Steve On 02/24/2015 08:29 AM, Bill Kuker wrote:
In a vacuum, you have essentially 2 accelerometers, one is a little mems chip with a tiny little proof mass inside, and the other is the load cell, and the airframe is the proof mass. They will tell you the same thing.In atmospheric flight the little chip feels drag as a reduction in force, but the load cell feels drag as an increase in force.On Tue, Feb 24, 2015 at 11:12 AM, Steve Peterson <steve_peterson@xxxxxxxxxxxxx <mailto:steve_peterson@xxxxxxxxxxxxx>> wrote:Ohhhhhh....I think I just got it. You're going to use the load cell to measure drag force and the accelerometer will get you the rest of it. Is that it? --Steve On 02/24/2015 08:09 AM, Steve Peterson wrote:Hey Bill, I don't believe this removes drag from the equation. Here's my thinking: Consider the rocket sitting (before launch) on the launch pad with the motor itself being supported by the launch pad. The non-motor portion of the rocket is pressing down on the load cell with constant force due to the mass of the non-motor portion of the rocket being accelerated by gravity. Now consider what the reading would be if you pressed your hand down on the nose cone--the reading would increase and if you varied the pressure of your hand the reading would likewise vary. When the rocket is flying, the load cell is being pushed back against the motor by the mass of the non-motor portion of the rocket, but, now, instead of your hand adding to that force, drag is adding to that force. Since drag varies with the square of the velocity, so the reading will vary due to the varying drag force just as it did when you pressed with your hand. Thus, to establish the net force (thrust) due to the motor, I think we'd still need to know the drag force. --Steve On 02/24/2015 05:26 AM, Bill Kuker wrote:How about a load cell between the motor and airframe plus an accelerometer? That would just leave the changing mass of the fuel to account for. Bill On Tue, Feb 24, 2015 at 2:53 AM, Andrej Vrbec <dmarc-noreply@xxxxxxxxxxxxx <mailto:dmarc-noreply@xxxxxxxxxxxxx>> wrote: Air density was routinely measured in 50's and 60's by a falling sphere method. See here for example: http://deepblue.lib.umich.edu/bitstream/handle/2027.42/70398/JAPIAU-27-7-706-1.pdf?sequence=2&isAllowed=y Andrej ------------------------------------------------------------------------ *From:* "shawn.mchatten@xxxxxxxxxxx <mailto:shawn.mchatten@xxxxxxxxxxx>" <shawn.mchatten@xxxxxxxxxxx <mailto:shawn.mchatten@xxxxxxxxxxx>> *To:* sugpro@xxxxxxxxxxxxx <mailto:sugpro@xxxxxxxxxxxxx> *Sent:* Monday, February 23, 2015 11:47 PM *Subject:* [sugpro] Re: Verifying Motor Performance Through Flight Tests Hahaha! I think I got it. Although.... do we really need humidity, temp and pressure or are they all just variables to give us air density which is what we really want. And if there was some way to measure air density directly would that not be more helpful. One way to measure viscosity is to drop an object of "standard" mass and shape through the substance and measure the rate of descent. So if we flew a rocket to 500m and pitched a 10cm/500g cube out the side with a 50cm square chute (or whatever other "standard" object we can make up) and measure it's descent would that not be all the data we need to measure air resistance directly. It's just a matter of making an easily repeatable standard. Maybe a tube or sphere instead of cube and streamer instead of parachute. Shawn On 2015-02-23 16:27, Michael Monteith wrote: > Com on. Keep up. lol It's really just figuring out what it would > take to determine motor performance > through actually flight testing. I do like your idea though. Just > measure things like temperature, > humidity, and pressure on the way down so you can correct for air density. > > Thanks Shawn > Michael > -------------------------------------------- > On Mon, 2/23/15,shawn.mchatten@xxxxxxxxxxx <mailto:shawn.mchatten@xxxxxxxxxxx> <shawn.mchatten@xxxxxxxxxxx <mailto:shawn.mchatten@xxxxxxxxxxx>> wrote: > > Subject: [sugpro] Re: Verifying Motor Performance Through Flight Tests > To:sugpro@xxxxxxxxxxxxx <mailto:sugpro@xxxxxxxxxxxxx> > Date: Monday, February 23, 2015, 3:54 PM > > > Not sure I'm following all this conversation but > instead of a balloon can you gather data from a parachute > recovery on the way DOWN instead of on the way up to > establish air density etc. For that matter are there any > papers that show air density or viscosity based on a > specific parachute and mass configuration. If not that would > be a cool standard to create for the community. > > Shawn
