DIY Telemetry and Control

[Inq]

You

The first sentence of the Wikipedia article states "The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe."

The equation in the Wikipedia article also works off the principle of having constant stagnation pressure with a decrease in static pressure caused by an increase in the speed of the flow caused by a reduction in area.
Basic Equations:
P0a = 1/2 rho*u_a^2 + Pstatic_a
P0b = 1/2 rho*u_b^2 + Pstatic_b
P0a = P0b
Derivation:
1/2 rho*u_a^2 + Pstatic_a = 1/2 rho*u_b^2 + Pstatic_b
Pstatic_a - Pstatic_b = 1/2 rho*u_b^2 - 1/2 rho*u_a^2
Pstatic_a - Pstatic_b = 1/2 rho*(u_b^2 - u_a^2)
Which is this equation from the article.
View attachment 235401
You are correct that the flow crosses the port, but what the decrease in pressure on the port measures is the change in static pressure caused by the reduction in flow area. The same decrease in pressure would happen regardless of if the tube at 90 degrees was there or not.

The waterbed pump also works by the same principle of a constriction in the tube causing an increased flow speed. The water exiting the tube will have a similar static pressure to the ambient air. Since the venturi has a smaller cross-sectional area, the water flows faster and has a lower static pressure. Since the water bed is exposed to the air, the water in it also has a similar static pressure to the air. Therefore, the water in the venturi has a lower static pressure than the water in the waterbed and it draws water from the waterbed into it.

I don't know precisely how the vacuum pumps you built were designed, but I suspect they also had a flow restriction that decreased the static or had the flow expand after a restricted section.

I'm sure you're right. I was recollecting a science demonstration in 8th grade and I've never cut one of the water bed vacuum pumps open. All my degrees were in structural mechanics. I'll stay in my lane . This will be great. So, there is no fundamental reason to using a differential pressure sensor as long as the static sensor is in a free field with tangent placement of orifices. Got it.

 

[telnar1236]

You


I'm sure you're right. I was recollecting a science demonstration in 8th grade and I've never cut one of the water bed vacuum pumps open. All my degrees were in structural mechanics. I'll stay in my lane . This will be great. So, there is no fundamental reason to using a differential pressure sensor as long as the static sensor is in a free field with tangent placement of orifices. Got it.

Sorry if my previous post came across a bit strong. I'm currently working on a mating system for a modular plane, and I guarantee I'll be asking for your help with how to get it to print in ABS and even for that matter on getting the electronics on my own data logger to work when I get around to building it. Yeah, the only reason for a differential pressure sensor as opposed to one for stagnation and one for static is minimizing the error if using tangent inlets for the static pressure.

 

[Power_Broker]

Sorry if my previous post came across a bit strong. I'm currently working on a mating system for a modular plane, and I guarantee I'll be asking for your help with how to get it to print in ABS and even for that matter on getting the electronics on my own data logger to work when I get around to building it. Yeah, the only reason for a differential pressure sensor as opposed to one for stagnation and one for static is minimizing the error if using tangent inlets for the static pressure.

I don't mean to hijack the thread, but I made a modular ABS ASA design. You can see it here.

I also have another (much larger) modular 3D printed plane in the works that has more robust mating. Below are some pics. The mating system for the larger plane design is very strong and sturdy, plus it only needs 4 M3 screws/square nuts. If it's of interest, I might agree to post an example .stl.

 

[Inq]

Sorry if my previous post came across a bit strong. I'm currently working on a mating system for a modular plane, and I guarantee I'll be asking for your help with how to get it to print in ABS and even for that matter on getting the electronics on my own data logger to work when I get around to building it. Yeah, the only reason for a differential pressure sensor as opposed to one for stagnation and one for static is minimizing the error if using tangent inlets for the static pressure.

No, not at all. I woke up in the middle of the night thinking that... I might be all wrong and that mine was too strong of a negative. I realized that the venturi was showing lower pressure because it necked down and I realized just because the water bed vacuum was a T-shape on the outside, didn't mean it didn't have a venture or even a 90 degree bend inside pointing downstream. I felt kind of foolish. I think my aerodynamics stopped about the time I blew across a Coke bottle in 8th grade.

 

[Inq]

I don't mean to hijack the thread, but I made a modular ABS ASA design. You can see it here.

I don't feel hijacked. It seems quite appropriate to the thread at hand.

Cool design(s). I'm sure someone will be interested, but for me... I'm tending the opposite directions with my planes. I want to go as scale as possible and I'm going for as light as possible pushing the limits of spiralized mode.

 

[telnar1236]

I don't mean to hijack the thread, but I made a modular ABS ASA design. You can see it here.

I also have another (much larger) modular 3D printed plane in the works that has more robust mating. Below are some pics. The mating system for the larger plane design is very strong and sturdy, plus it only needs 4 M3 screws/square nuts. If it's of interest, I might agree to post an example .stl.

View attachment 235418
View attachment 235419

That's a neat project. My modular design is taking a slightly different direction. The idea is to have something where you can use a variety of components to put together different geometries of plane. For those familiar with Kerbal Space Program, think something along the lines of that. I also want to design it so that I can use the same prop/EDF which will just lock into future scale plane designs, kind of like a swappable foam board plane. Finally, I want everything to twist and lock together without requiring tools for assembly to make it as easy as possible to rebuild. As you can see, just a bit ambitious so it's probably a ways off. I've been working on CAD for months on and off and I only just printed my first test parts this week.

 

[Power_Broker]

That's a neat project. My modular design is taking a slightly different direction. The idea is to have something where you can use a variety of components to put together different geometries of plane. For those familiar with Kerbal Space Program, think something along the lines of that.

I eventually (probably years at this rate) plan on making a library of parts that should allow you to do things like that. Have sections of different lengths/diameters, transition sections, section that branches into two "tubes", etc. The other cool thing is you can remix the parts to include whatever custom mounts you want since the interfaces are standardized.

 

[Inq]

Eventually the idea was to add gyro/accelerometer flight control features also. Although, the sensors are my first priority, I did want to explore the control side as it came up in another thread with @Foamforce and I wanted to be sure if I could get the information I need. Here it is all breadboarded up. This shows it with the static pressure sensor, but the dynamic one is removed for this test because it's all attached to stuff I don't need. I have FlySky IA10B receiver with all 10 channels configured. I also show one servo is connected to the Ailerons on the receiver. The data cable supplying the iBus protocol to the Microcontroller also powers the microcontroller. The video below shows the working, while the microcontroller:

1. Monitors all ten servo channels
2. Is using the Aileron values to also control the second servo. This shows that this provides a pass-through ability so things like the throttle can be controlled by the receiver, while active control can be programmed to control the control surfaces.
3. Hosts the Administrative website displayed in two windows on the screen behind
4. Pumps out real-time data of all 10 servo positions as well as the static pressure data.

Note the both instances of the aileron servos stay in sync and also the graph plotting the histogram data of the stick positions.

 

[JetCrafts]

heres an idea use a gps for speed and maybe altitube much more reliable for speed atleast

 

[JetCrafts]

heres an idea use a gps for speed and maybe altitube much more reliable for speed atleast

if you're confused y just ask and ill give lecture

 

[Power_Broker]

heres an idea use a gps for speed and maybe altitube much more reliable for speed atleast

A better approach would be to use a sensor fusion algorithm like an extended or unscented kalman filter to combine baro, pitot tube, IMU, and GPS data together to get higher 3D localization accuracy and estimation frequency.

This is especially important since many hobby GPS receivers are preconfigured for 1Hz navigation output rates. Even if you can configure the GPS receiver to have a higher nav output rate, you still will want the above sensor fusion to produce localization estimates at arbitrarily high frequencies. You'd also want the fusion to try and dead recon in case the GPS receiver fails or loses fix.

 

[telnar1236]

heres an idea use a gps for speed and maybe altitube much more reliable for speed atleast

The issue with GPS is that it cannot give you airspeed. If you just want to know how fast you've gone over the ground, GPS is great, but if you want to generate useful aerodynamic data, you need to record your airspeed.

 

[Inq]

My primary thing is to have fun with RC and my electronics hobby.

My #1 goal is to get LiPo voltage to stay up longer and come down when I need. $3 micro gets me that. Yes, I know they have some module to do that, but I like messing with the hobby/DIY aspects.
#2 Altitude and Airspeed. + $2 - Merely because I want something to tell me how high my sailplane is and how fast it was doing in lawn dart mode.

After that, Gyro/Accel control of the plane is just for fun and because I think I can. Yes, I can afford a $50-$100 gyro stabilizer (have one) but if I can make one with a $3 sensor... I consider that a fun self imposed challenge.

If there is enough (any) interest, I'll publish everything so someone else can get LiPo V, airspeed, altitude, maybe basic flight control... maybe more. Depends on when the interest dies down... others and mine.

 

[JetCrafts]

The issue with GPS is that it cannot give you airspeed. If you just want to know how fast you've gone over the ground, GPS is great, but if you want to generate useful aerodynamic data, you need to record your airspeed.

I don't know the Aero you're doing, but I manage to use ground speed for mine
prob because there's nearly any wind when I fly