I am a beginner to the hobby and I've been constructing chuck gliders out of foam board and card for a few years. I'd like to start making models that I can control, but I don't have much of a budget. I do, however, have a hobby-grade RC car with a receiver/esc/servo combo and matching transmitter. I am planning on removing this unit and using it to control the one channel (pitch) of a foam board glider. Is this a feasible idea? What are some major problems I might come across?
Let's take a look at what parts from the RC car we might be able to reuse for an RC plane. You haven't shared any specific details about the RC car you're looking to salvage parts from or the RC plane you're trying to build, so I'm not able to make a specific analysis.
RC Transmitter and Receiver
One major concern is how many analog channels the RC car control system has. analog channels allow you to vary the control signal through its range of motion as opposed to having the channel just be on or off. AFAIK most RC cars only have two channels (throttle and steering) and you'll need 4-6 channels to control an RC plane (ailerons, elevator, rudder, throttle, any aux channels you may need). You probably won't be able to reuse the RC TX/RX if it doesn't have >=4 analog channels.
The RC transmitter might also be less than ideal for RC aircraft control. If you have one of the pistol-grip type devices, like this one from Radiolink, you might find that operating the plane becomes cumbersome even if it has enough analog channels.
Most RC aircraft controllers are shaped quite differently for ergonomic reasons, like this one:
The major factor to consider here is the current rating of the RC car ESC. Depending on the size plane you plan to pair it with, you may need an ESC capable of supplying >20-30 amps of current to the propeller's driving motor.
Assuming that the current rating for the ESC is adequate, the chances are good you'll be able to reuse the RC car ESC.
This is something you didn't ask about in your question, but I thought it would be good to mention anyway. It's possible you may be able to reuse the drive motor to power the propeller, although it's difficult to give a more specific answer without knowing more about the RC car and airplane.
As @JacobB notes:
Most RC cars use comparatively low voltage batteries (2-3s) and have a spur and pinion gear to create the necessary torque.
And because many RC cars use brushed motors (and the ones that are brushless have high Kv, like 4000 or 5000), having the motor attached directly to a propeller would likely result in either a severe lack of power and/or extremely inefficient performance.
Servos are rated for how much deflection they can provide (e.g. 45°, 90°, 180°, 360°, continuous/infinite rotation) and how much force/torque they can deliver. Again depending on the size of the aircraft you want to control, the servos you find in your RC car may be too small to control the aerodynamic surfaces of your plane.
The aerodynamic surfaces on a plane can experience decently strong forces in-flight, so it's important to make sure your servos are appropriately sized and can maintain control.
Back in the early days of RC, people only had single channel radios, and vintage models were designed to be flown with only the rudder (and a small amount of fuel in the tank to limit the maximum height).
Vintage models were typically high-wing with lots of dihedral and a forward CG to make them very stable. Car RC gear should be fine in this sort of model, and they also make good trainers.
You will need to find the right size model plan for your motor, as most vintage designs were for IC engines. Assuming you have a brushed motor, measure the length of the can in mm and add a zero to get the size code - if it's 30mm long, you have a '300' size motor. If it's 40mm long, you have a '400' size motor. Then look for a model that was designed for that size motor. You don't have to build that exact model, but that gives you a target size and weight.
You can buy prop adapters that will fit the shaft of the motor; the shaft sizes are a fairly standard range of sizes.
Also, check the frequency of your radio is legal for air use in your country. In particular, 40MHz is for ground use only in the UK. In the US 72 was for air use and 75MHz for ground use. 2.4GHz can be used for air or ground in most countries.
This is an interesting idea and I think it could work. There are, however, a few things that are worth considering. First of all, a controller for an rc car will usually have only 2 or 3 channels. This is only a problem if you want more controls such as rudder, elevator, elevons, etc but if you only want to control pitch you will have enough channels. Another thing to consider is that the electronics in rc cars are usually a lot heavier that those in drones/planes. This is especially the case with ESCs as rc car ESCs will have heat-sinks and sometimes fans built in. If the ESC that you have is too heavy you can get around this by replacing it with a cheap, lightweight ESC (with an adequate BEC to power the servo) that has no heat-sink/fan.
These are just a few suggestions that I thought were worth noting. Hopefully they prove helpful.