Frames
A drone frame is the rigid chassis that holds the four motors at a fixed spacing and carries everything else, and it is described by one number above all others: the wheelbase, measured diagonally from motor centre to motor centre in millimetres. That number decides the largest propeller the aircraft can turn, and the propeller decides the motors, and the motors decide the battery, and the battery decides the weight. The frame is chosen first because every other choice is downstream of it.
What a drone frame is
A drone frame is a set of arms and plates, usually cut from carbon fibre sheet and bolted together with M3 hardware and aluminium standoffs, that fixes the geometry of the aircraft and gives the electronics somewhere to live. On a folding camera drone the frame and the outer body are moulded as one thing and you never see it as a separate object. On a self built quadcopter it arrives as a flat pack of plates, arms, spacers and a bag of bolts, and it is the only part of the build with no electrical function at all.
What distinguishes the frame from the rest of the class is that it is passive and it is decisive. It draws no current and it can fail without any warning from the flight controller. It is also the part that sets the aircraft's class in the way flyers actually talk: a 220 mm frame is a 5 inch quadcopter, a 150 mm frame is a 3 inch, and a 65 mm ducted frame is a tiny indoor machine that fits in a hand. Say the wheelbase and an experienced flyer already knows roughly what the finished aircraft weighs, how long it flies and what it is for.
The numbers on a frame and what they mean
Frames are sold by wheelbase and propeller size, and then by four secondary figures that decide whether your electronics will actually fit. Read them in this order.
Wheelbase and propeller size go up together and everything else follows. A larger wheelbase clears a larger propeller, which needs a larger motor, which draws more current, which wants a larger pack, which adds mass, which needs the larger propeller you started with. Arm thickness is the number that decides how many crashes the frame survives, and it is the one place where a few grams of extra weight buys something real. Stack pattern is a compatibility question rather than a performance one: a 30.5 mm square is the standard for 5 inch builds, 20 mm for smaller ones, and a flight controller drilled for one will not bolt into the other.
Frame weight matters far beyond the build sheet, because the finished aircraft's total mass is what the Civil Aviation Authority registration scheme is organised around, and 250 grams is the boundary that scheme uses. A heavier frame is not merely a heavier aircraft, it is potentially an aircraft in a different category, and the requirements that follow from that are set by the Civil Aviation Authority, they change, and they must be checked there rather than inferred from a build guide.
How a frame fails
Frames fail in four ways, and the dangerous one is silent. An arm snapping clean off at the motor mount is obvious and is the usual outcome of a fast crash into something solid. A cracked plate around a bolt hole is visible if you look. Stripped standoff threads are found the moment you try to tighten anything. The fourth is delamination, where the layers of carbon separate internally after an impact, and from the outside the arm looks entirely sound.
- Snapped arm, from a direct impact at speed, usually at the narrowest point near the motor.
- Cracks radiating from bolt holes, from over tightened hardware or from repeated small knocks.
- Stripped or seized standoffs, from steel bolts turned too hard into aluminium threads.
- Delamination, invisible from outside, showing only as a pale line in the edge of the plate or as a dull sound when tapped.
In the air, a compromised frame feels like a tuning problem rather than a structural one, which is exactly why it catches people out. A delaminated arm flexes, the flight controller reads the flex as movement and corrects for it, and the aircraft develops an oscillation that gets worse the harder you fly. Video acquires a vibration that no amount of propeller balancing will remove. One motor runs hot because it is fighting a corner that will not hold still. A frame that has been through a heavy crash and looks fine but flies badly has almost certainly been damaged, and the check is to strip it and flex each arm by hand.
Caution
Carbon fibre dust is an irritant and carbon splinters from a broken arm are sharp and hard to see. Cut, file or sand a frame only outdoors or with extraction, wear gloves when handling a shattered arm, and vacuum the bench afterwards rather than blowing the dust into the air.
Choosing and replacing a frame
To choose a frame, decide what the aircraft is for and let the wheelbase follow, because the frame is the one component that cannot be changed later without changing everything else. Cinematic flying and long range flying want a larger wheelbase and a two blade propeller. Racing wants the smallest airframe that clears the propeller. Indoor flying wants ducts around the blades, which means a frame designed around ducts from the start.
- Choose the propeller size the flying calls for, from 2 inches indoors up to 7 inches for long range.
- Select a frame cut for that propeller, and check the stated clearance rather than assuming it.
- Confirm the stack pattern matches the flight controller and speed controller you intend to use.
- Check the camera mount width against the camera, since standard, micro and nano cameras are not interchangeable.
- Weigh the whole intended build on paper before ordering, especially if a weight threshold matters to you.
Replacing a frame is a full rebuild, and the order is fixed: strip the electronics off the old frame in the reverse of the order they went on, transfer the stack before the motors so you are not working around four sets of wires, then bolt the motors last. Use thread lock on the motor bolts and nothing else. Do not over tighten anything that passes through carbon, because the plate cracks long before the bolt strips. Replacing a single arm, where the frame allows it, is a twenty minute job and is the strongest argument for buying a frame whose arms are sold separately.
The alternatives
Frames differ in geometry and in material, and the two choices are independent of each other. Geometry comes in four common layouts. True X puts all four motors at equal distance from the centre and is the balanced default. Stretched X moves the rear motors further back, which settles the aircraft in fast forward flight and is the racing standard. Hybrid X sits between them. The deadcat layout sweeps the front arms forward and out to keep the propellers out of the camera's view, which is why it appears on cinematic machines and almost nowhere else. H frames put the arms on the corners of a rectangle and are used where a large flat battery has to sit inside the airframe.
Material is a choice between carbon fibre, injection moulded plastic and moulded ducted shells. Carbon is stiff, light and expensive, and it shatters rather than bending. Plastic is heavier and flexes, which sounds like a disadvantage until you fly indoors, where a plastic frame bounces off a wall that would break a carbon arm. Ducted whoop frames are moulded plastic with the propeller guards built in as part of the structure, and they are the only sensible answer for flying in a room with people in it.
Carbon does not win everywhere, and the case against it is a real one. For a first build that will be crashed weekly, for indoor flying, and for anything flown near other people, a moulded plastic or ducted frame is the better aircraft: it costs less, it survives more, and it does less damage when it arrives somewhere unplanned. FPV and racing drones sets out the flying that the stiff carbon frame was designed for, and if that is not your flying, the frame is not your frame.
Is a better frame worth buying?
Yes, more so than almost any other part, and for a reason that has nothing to do with performance. A frame does not make the aircraft faster or quieter, but it decides whether a crash costs you an evening with a screwdriver or a weekend and a new set of everything. Thicker arms, replaceable arms sold individually, and a design that puts the electronics inside the plates rather than on top of them are the three features that pay for themselves within a season of real flying.
Where the money changes only the receipt is in exotic materials and machined aluminium fittings on a machine that is flown gently. If the aircraft never goes fast and never leaves a park, the standard frame is doing everything the expensive one would. Spend the difference on the parts that fail more often: propellers, batteries, and FPV goggles if you are moving to first person flying at all. The parts of a drone shows how the frame relates to everything bolted to it, and Motors and speed controllers covers the components whose mounting pattern the frame has to match.