I hear lots of people say this maneuver is a high stress maneuver, but I have to question that. Most maneuvers that are considered high stress I can understand, but in my mind the airframe is merely spinning on the axis of the wing tube during a knife-edge spin, sort of like a sideways waterfall. I'd like to hear more thoughts/opinions or explanations on exactly what is happening and how the airframe is being stressed. Not being a "nay sayer" here, just trying to gain a better understanding.

It is going to be more stress on your airframe than circle flying, but a GOOD quality airframe should not have any issues with a knife edge spin. I have been doing knife edge spins for years, on multiple airframes and I have not seen any ill effects on my airframes.

One thing I can tell you, is that you can control the speed of the spin with your throttle. If the stress worries you then do not do knife edge spins at WOT.

A tight, knife dge spin at WOT just makes me giggle, it looks pretty neat!

Jeff

Probably the biggest stress is via gyroscopic loads from the prop being channeled through the engine box/fuse structure. And of course the higher the rpm, the higher the resistance of the gyroscope (prop).

I don't think there's a lot of high aerodynamic loads in this maneuver.

Earle

Earle hit the nail on the head.

Basically, the spinning mass of the propeller resists the downward pitching of the airplane and produces a yawing force to the left. Thus, the firewall is stressed as if you were pulling the prop hub up and left. That load can be a lot higher than you might think, and as Earle said, the higher the throttle, the worse it gets, both due to the higher RPM and often faster pitching of the airplane.

Pick up a box-fan and try to turn the case while the fan is spinning, you'll see gyroscopic precession at work.

It also heavily stresses both the engine and prop, but those don't typically show much stress in models from these maneuvers.

I imagine the loads on the fuselage longerons have to be pretty high, but I don't think I've ever seen one crack even a little on my planes (which get plenty of KE spins in). Agrred that the engine mount/box gets it the worst in these maneuvers when the throttle is pushed up. Even then, most planes today seem to take that abuse with no issues either. Sure looks violent though.

Only time to get stressed is when the pull out occurs too low. Hard to have any higher stress than that moment.

make sure all the electronics have good tight connections.

I've had 4 issues while doing knife edge spins, 2 times it pulled the elevator servo out of the reciever.
2 times pulled the motor wires apart from the esc.

I cracked and bent about 2" of the inboard trailing edge of the wing last week doing knife-edge spins. The plane was my EF 78" Extra 300.

Originally Posted by Randidly

make sure all the electronics have good tight connections.

I've had 4 issues while doing knife edge spins, 2 times it pulled the elevator servo out of the reciever.
2 times pulled the motor wires apart from the esc.

A big +1 to that... I separated all 3 bullett connections between motor and ESC the first time I KE-spun my EE60. Now all motor connections get heatshrinked along with servo connections.

Another possible issue - depending on how gear is mounted and where - is slinging an Rx or battery loose... I've seen canopies lost on fatties, and wing hatches slung off (followed closely by the battery pack) of profiles.

-Case

My big Edge had the canisters come loose twice early on, both times doing knife edge spins! I had to use 28-30mm stepped couplers, and until I put a 6-32 screw thru the coupler and header, the canisters would slip back towards the tail. Lots of centrifugal force in that direction apparently

I dont find it any more stressful than any other spin or maneuver for that matter.

In a traditional spin, there should be very little gyroscopic force since you'd be at or close to idle. This is a pretty low-stress maneuver all around. Even in an accelerated (flat/power on) spin, the power is there, but the yaw rate can't come close to the pitch rate seen in a knifeedge spin, so the gyroscopic forces on the motorbox and the air loads on the tail should be much lower than in the knifeedge spin.

There are some airplanes that really wind up in a flat spin, but they're typically not scale-ish or traditional aerobatic/3D airplanes.

However, some of these failures definitely sound like a call for better securing of components!

I had a Goldberg Ultimate that REALLY winded up in a spin.... I loved it and more and more I kept adding power but the Dang 4-40 push rods would actually bow out..... but that plane held together nicely.....but I am sure it was pretty stressed!

There are also fuselage stresses happening too. Think of holding a rock on a rubber band and spinning around. Essentially everything above and below the wing is trying to rip itself outwards.

True... the centrifugal loads are just relatively minor compared to the others, as far as the airframe is concerned. As far as components inside the airplane are concerned, the centrifugal loads are the biggest factor, and that's why it's important to properly secure everything... wires, ESC's, receivers, batteries.