Wing design

From my initial calculations, I need to get downforce on the front drive wheels in order to get enough traction to be able to accelerate at an acceptable rate. I am starting the wing design using a symmetric airfoil (NACA 0006). Ideally, I want to be able to create a significant amount of downforce at low speeds, then decrease downforce coefficient at higher speeds. I do not want to create too much downforce at high speeds. This will put unnecessary strain on the car and will increase drag significantly. The goal is to keep enough downforce on the drive wheels to create enough traction to get up to speed quickly. The idea is to have a flap that will start off at a large angle (60 degrees) then decrease angle as the speeds increase. This will lower the lift coefficient (downforce coefficient in my case), but maintain a certain downforce on the drive wheels throughout the speed range of the car.

This is a rough mock up of how the wing will be placed on the car. Because the wing is so close to the ground, ground effect will increase the wings effective downforce coefficient.

Profile view of the NACA 0006 airfoil showing different degrees of flap.

Series of pictures illustrating how the flap will decrease in angle as speed increases:

More wheels and transmitter modifications

Latest iteration of the "X2"

Wheels

Still working on more custom wheels. I want to try out a number of different wheels to see which ones will work best. I already have two types that I want to try. Each wheel has a different foam thickness.

Wheel being measured for roundness.

Transmitter

I had just about given up on my 3PM transmitter until I found out that increasing the range on my Futaba 3PM 2.4 ghz transmitter isn't very difficult. There are a couple of off the shelf components that I found to increase range. Since 2.4 ghz is widely used in wireless internet, there are many places that sell components such as pig tails and amplifiers. I was able to open up my 3PM and see what kind of connectors were used for the antenna. Then I was able to find a 500mW amplifier to significantly increase my range. This is what the transmitter looks like with the modification:

Futaba 3PM 2.4 Ghz modified to increase range. Range of the stock transmitter is a measly 300ft. With this 500mW amp I have increased the range by about 3x.

Parts used:

Pig-Tail (U.FL to RP-SMA):

500mW Amplifier (RP-SMA):
Gyro:

I included a gryo into my setup. I found out that after I corrected the steering, the car had become much more sensitive with steering input. I tried all the adjustments to minimize this and nothing seemed to work. A number of people have said a gyro helps significantly. This gyro is nice because I can use the third channel on my transmitter to increase or decrease the sensitivity of the gyro without stopping the car to adjust the gyro itself. I can adjust it remotely.

Matt Shumaker described how a gyro affected his car. He has noted that at low speeds the gryo helps significantly by keeping the car on a straight trajectory with much less steering input from the driver. But at high speeds, the gyro makes the car feel numb. This is especially true if the gyro is too slow to compensate for movements in the steering.

Futaba GY401 next to the right rear wheel in the "X2"
Front end displaying the Thunder Power batteries and Castle Creations HV-110 performing perfectly during all of my testing.

I am still working on a wing to create downforce. The tight spaces on my car limit where I can mount a wing. I am now sizing the wing and airfoil section to create the desired amount of downforce at a certain speed.

New wheels and more street testing

Last time out testing at the Houston Dragway, I had problems with my tires holding together at speeds from 60-70mph. I have been working with my father to make some custom aluminum wheels using 1/8 scale on road tires. The wheels I have been using so far are 1/10 scale nitro drag tires:
These tires are very very soft and they are the correct outside diameter (3.25") and width (1.5") for the X2. Since these tires weren't working, I had to go a different route. The 1/8 scale tires are approximately the correct outside diameter (about 75-80mm) but they are much harder foam. The only problem is they are very wide (3"). What I did was to make my own aluminum rims and remove the foam from a 1/8 scale rim. This way I could use the same method of mounting (1/10th scale clamping hub). This is what I started with:


I removed the foam from the 1/8 scale wheel and glued it onto the aluminum rim. Here's what the new wheel looks like (left) compared to the old one(right). The new wheel is much much stronger and should hold up to the extreme forces.

I also modified the front suspension. I moved the tie rods from up above the upper arms to very low to maintain correct front end geometry:

Data from the last street run. Link to the FDR file from my run on 10-11-07. You can view this after downloading and installing the Eagle tree software.

I have also updated my spreadsheet. I added the ability to choose a motor based on what Kv and Kt requirements are calculated. I also added a section where you can calculate the amount of downforce created by a wing added to the car. And based on the traction created by this wing, you can see at what speed your total traction will intersect with your traction needed to accelerate at your desired rate.


Here is the link to download the spreadsheet for use on your own calculations.

Graph showing traction available vs traction needed. The point where the two lines intersect is the speed at which enough downforce will be created to give you the traction to accelerate without spinning the wheels.

I am still working on the wing and how to attach it to my car. Once the wing is completed and attached, I want to get out to the Houston Dragway as soon as I can to do some more high speed testing.

Road test with modifications for the "X2"

This is the modified steering. The orientation of the servo is unorthodox. I had trouble making sure that I had enough room for the antenna, receiver and switch after the steering had been rotated 180 degrees. Rather than place the servo laying flat on the chassis behind the steering suspension, I stood up the servo and placed it between the steering mounts to create more room for the electronics all the way in the rear of the car.
This is the front end of the car. This is where the pitot tube ports are located. A pitot tube is used on full size aircraft to measure airspeed. The dynamic pressure port is on the bottom. This port measure the forward airspeed as the car is running. The tube on top is for the static port. This measures the ambient pressure. The difference in the dynamic pressure and static pressure is speed. More info on pitot tubes can be found here.


This is where the air travels down the brass tubes to the silicone tubes to the speed sensor. This is where the dynamic and static pressures are measured.
This is data from a short test run I did on the street in front of my house. There is a lot of noise in the data from 0-8 seconds. The car wasn't even running on the ground between 1-8 seconds. After that, you can see from the spikes in the speed data that I made 4 passes. My fastest pass was about 18 mph.

Currently I am working on getting new wheels put together and making a wing for some front end downforce. I will post more about these parts soon.

Update: Modifications

I have been working on changing a couple things for the X2. Since I am driving the car backwards, the front steering (now the back) needs to be turned around to keep the correct steering geometry. I turned the steering around 180 degrees. I also got some trailing front axles to keep excessive twitching of the steering to a minimum. I also changed the caster to -4 degrees.

I got an Eagle Tree Micrologger data acquisition system. This plugs in-between the Thunder Power batteries and Castle HV-110 speed control and can accurately measure a multitude of things. Currently it is measuring battery voltage, current, watts, mAh used, motor rpm and will include the airspeed sensor in the future.

I am also working on machining wheels out of aluminum. As you can see from my previous tests, I am having a hard time keeping the foam on the standard plastic rims. I want to experiment with some machined rims and different foam types to see if I can make a wheel that will hold up to the extreme performance of these speeds.

I am also working on a setup to provide downforce on the front wheels. My lack of acceleration is due to the fact that the coefficient of friction is lower than I expected. One way to increase traction without changing foam compound and increasing the weight of the car is to increase the downforce.
Here is a shot of the front suspension. It has been reversed to obtain the correct geometry. I installed trailing axle steering blocks to minimize the steering reaction. I also removed 1 set of washer from underneath the suspension arm mounts to increase the rear ride height to approximately 8mm. The upper arm mount shims were moved to provide -4 degrees of caster.
This is the Eagle Tree data acquisition micrologger. This will help me accuratley monitor voltage, speed, motor rpm and amp draw.
This is a sample graph of what the Micrologger can do. This example was from a quick bench test to see if everything was working. The car was freewheeling on the car stand. There seems to be some noise in the data. The only time the wheels were running is when you can see the rpm move into the 2 "humps" on the bottom of the graph. I was throttling up slowly to neutral then slowing down again. I did this twice. The data right before and after These rpm "humps" shouldnt be there since the motor wasnt even rotating. This could be caused by cogging of the brushless motor. You can see the rpm shoots up to 50,000 then goes to 0, which shouldn't happen.
Wheels being machined