What effect does cold weather have on helicopter performance?

Recently I was flying around my RC helicopter on a 40 $^\circ$F (4$^\circ$C).

At some point in the flight the rotor just stopped moving in mid-air and then I regained control back at ground level. I never had any issues before in warmer weather, so I assume it's a cold-weather related issue with this model.

This leaves me with two questions:

• Could something like this happen in a full-size helicopter like a Robinson R22?
• What effect does temperature have on helicopter performance in general?
• The rotors stopping abruptly is NOT something regular helicopters will suffer from normally, I'm pretty sure of that... You may have a problem with battery performance at low temperature, or if you were outdoors and the controller is infra-red, the signal may have been interrupted. Or there could be any sort of random glitch in the electronics.
– Andy
Oct 23 '15 at 12:08
• @Ethan If I were you, I would ask this questions on Electronics.SE since I'm pretty sure it's a battery- or electronics-related problem. Oct 23 '15 at 12:25
• @Ethan, I cleaned up the question to make it more to the point, and aviation related. Feel free to rolll back the changes if you're not satisfied with the result Oct 23 '15 at 12:31
• First you should try to understand if the result was the rotor being blocked, or the engine that had stopped. I doubt reasons could be compared, a RC motor is electric (I assume) and R22 is a piston engine or a turbine. What could stop one will likely no affect the other. If this is the rotor itself, due to a lubrication or thermal contraction cause, the two mechanisms are not at all similarly built and are not affected the same way (my assumption). Don't you have the temperature operating range in the manual, by chance?
– mins
Oct 23 '15 at 19:48
• I think (also given the two closevotes on this question), you might want to ask these questions in a different manner: instead of "I played with my quadcopter and then X happened", your question might be better received by the community when you ask "I was wondering whether X could happen; here's what I found so far in my own research". To me, it sounds like you post whatever your quadcopter does without any prior research (whether this is true or not). Oct 24 '15 at 23:31

Cold weather definitely affects the performance of helicopters (as it does all aircraft), because it affects the density of the air they move through.

I will ignore icing so let's assume dry air.

The lift equation is:

$$L = \frac{1}{2} \rho V^2 S C_L$$

• $L$ = Lift
• $\rho$ = density of the air.
• $V$ = velocity of the aerofoil (rotor)
• $S$ = the wing area of the rotor
• $C_L$ = Coefficient of lift , which is determined by the type of rotor and angle of attack.

As you can see, lift is proportional to the density of the air and since density at any given altitude is inversely proportional to the temperature, the colder the air, the more dense it is. So, for a given rotor, spinning at a given speed at a given altitude, the colder the air, the more lift is produced.

Engines also operate more efficiently when the air is colder since the density is higher and the charge of air/fuel introduced into the engine is greater. (See here for more information.
Of course, this does not apply to your RCH apart from some effects on the efficiency of the batteries and motors (which may perform more poorly at lower temperatures).

So in general, the colder it is, the higher the performance. In most helicopters, the difference in performance on a cold, dry winter day is very noticeable and much better than on a humid, hot summer day.

It's also worth understanding the properties of air. The colder the air, the lower the dewpoint so the less moisture it can contain. Moisture in the air also lowers peformance.

The problems you encountered with your RC model aren't problems with helicopters in general.

Colder weather does not make you more susceptible to vortex ring state (aside from some minor side effects): You would still only descend into the vortex if you did not pilot the helicopter correctly. (Note that you cannot enter vortex ring state with the rotors stopped since they are not producing a vortex. Also note that on a real helicopter, if the rotors ever stop, they will not start again no matter what you do.)

BTW, I'm being pedantic, but helicopters do not have propellors. Technically, they are the same thing, but asking a helicopter pilot about the propellor annoys them, even if they don't show it. It's as bad a calling a helicopter a "chopper". Only Arnie and Chuck get to call helicopters "choppers". And that's only because helicopter pilots are universally weedy and would get stomped by Arnie or Chuck if they argued.

• I tweaked this a little to line it up with the edits to the question, if I butchered any of the helicopter-specific stuff feel free to roll it back :) Oct 23 '15 at 16:37
• What is vortex ring state. Is that the wake vortex coming from the propellers. The helicopter started to shake like crazy when I descended it into its own downwash too fast Oct 23 '15 at 18:48
• @Ethan Shaking is one of the symptoms of VRS. See this Oct 23 '15 at 20:52

First, I don't think cold weather had anything to do the stopping of rotor. Probably a malfunction.

However, Helicopters do face a number of problems during flight in cold conditions

• Ice accumulation in leading edges- Very few helicopters have leading edge anti-icing systems and are certified for flying in cold conditions (like North Sea).

• The possibility of misting in windshield.

• Whiteout conditions, usually while taking off and landing.

• Ice/Snow ingestion in engines.

• Icing and blockage of fuel lines etc.

Most of these have to do with icing and not with cold conditions as such. In fact, as Lift is directly proportional to density (L $\propto$ $\rho$), helicopters have better performance in cold weather (as air density is higher) and rotors produce more lift. Engine power is also usually increased. It is in hot-high conditions that helicopter lift production is critically affected.

I'm adding this as the question has been cleaned up and changed a bit (effect of temperature in helicopter performance).

Temperature does affect the performance of the helicopter, as the density varies with temperature. The performance of the helicopter depends on three main atmospheric parameters (assuming all other helicopter parameters are equal):

• Density altitude (air density)

• Gross weight

• Wind velocity (during takeoff, hovering, and landing)

The lift produced by the Helicopter blades is given by,

$L \ = \ \frac{1}{2} C_{L} \rho V^{2} S$

Now, in helicopters, $S$, the blade area, $V$, due to rotor speed are fixed. So, as the density varies (due to altitude or weather), the lift coefficient $C_{L}$ has to be adjusted by varying the angle of attack of the blade.

As the density decreases (due to hotter temperature or higher altitude), the required angle of attack (for producing same lift) increases. However, the maximum angle is limited by the blade stall angle. The net result is that the helicopter's ability to hover (or fly) at a particular altitude (or weight) is reduced due to change in temperature.

The following image shows the hover chart for CH-47D. It is obvious from the chart that the hover performance is significantly improved as the temperatures fall, at least till 0$^{\circ}$

Source: globalsecurity.org

• On a model, yes, cold weather might have something to do with propellers stopping: batteries don't like cold. Though +4°C is not that cold. Oct 23 '15 at 12:20
• @JanHudec I'm assuming that the batteries are LiPo or Li-ion. They should usually operate at 4°C, though I can't be sure. Also, Ethan says that he regained control in a few seconds. I'm not sure if its battery problem Oct 23 '15 at 12:30
• Indeed, 4°C does not sound like the batteries should already have problems. Oct 23 '15 at 12:39

Besides the aerodynamic aspects of cold temperatues, there is another aspect concerning the thermodynamics of the engine. Power and efficacy are better the colder the air is.

However, in your RC model, maybe the clearance between some moving components is too small so they gall as the material expands at higher temperatures.