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Old Nov 18, 2012, 03:52 AM
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Quote:
Originally Posted by Libtekguy View Post
While this is a real world hazard called "Dissymmetry of lift", I doubt scale RCs are likely to suffer from this. RC Helis are so overpowered underweight that many hazards that full size helicopters have to be cautious of don't apply to RC. The rotor disc load just isn't anything close to full scale helicopters.

I'll spare the calculation, but at 1500 rpm, an 800 size heli as a blade tip speed of over 300 mph. Let's say you have the heli traveling at 100 mph, using the EDF thrusters, with only a small amount of blade pitch (1 degree), the advancing blade side would be traveling at 400 (300+100) mph, and the retreating side at 200 (300-100) mph (relative to the wind). So yes, the advancing side is producing much more lift. BUT the heli still has another 12 degrees or more of pitch it can use to properly compensate for blade speed, and what is even better, a FBL controller will automatically correct for this when it senses the heli want to to roll to the retreating blade side. Actually because of the 90 degree precession inherent in spinning masses (the rotor), the heli will want to roll to the retreating blade side and also pitch up.

As you lower the headspeed, and put on weight (scale fuselage), you will increase the risk of retreating blade stall, but I am completely confident that 100 mph is a safe speed for 800 size scale birds. In fact, without doing the calculations, I would even estimate that 150 mph would be just fine.
I'd say (no offense intended) that you are void of hands-on experience with unstabilized rigids of that size, otherwise you would not be making above statements.

If you were to use 1500 RPM headspeed on an 800 size, I'd say you are slightly out of sync.... Not that it is impossible, but 1000 RPM is more like it.
You are never going to get it up to 150 mph, because at the tip of the advancing blade you are getting close enough to the speed of sound, to start some serious problems, let alone the power required to drive the rotor at such speeds will seriousely increase due to that fact.

Any tilting tendency caused by advancing/retreating blades, is purely in the direction nose up, there is NO longitudinal roll tendency due to this.
In fact, the roll tendency that actually exists, is invariably towards the advancing blade. I have observed this from 450 size to 800 size to be true, regardless of number of blades or headspeed. This fact is caused by the air above the rotordisc being accelerated downward as the helicopter passes underneath it. Because of this, the air at the front of the disc has zero downward velocity, but the air at the back of the disc, has a noticeable downward velocity. This decreases the AoA of the blades in the rear half of the disc, thus reducing lift in the rear half, and due to the 90 degrees shift in precession, the advancing blades will sink, causing a roll towards that side. Since I was at first also convinced it would roll towards the retreating blade, I was not only very surprised it rolled exactly opposite, it also took me a long time to accept that and figure out why.

I do fly unstabilized 800 size multiblades, and I would say, that depending on blade profile and take off weight, at around 75 to 100 mph, roll and "nose-up" tendency become that strong, that there will be not enough steering deflection left in the stick (you cannot really go over 5 degree cyclic to each side, or you can provoke a high speed blade stall) to be able to control the helicopter in a safe manner.

An FBL system will not get you any further, because it corrects for you, but does not alter the aerodynamic properties of the rotor. In fact, it completely robs you of any warning of the oncoming bladestall, and worse, because of its autocorrecting function, once it is in a blad stall, the corrective action will make sure, it will STAY in that condition, so it robs you of the possibility to recover....

So despite nice theories and calculations, in reality, with a large scaler of 800 size you can consider yourself lucky if you manage 100 mph without problems, actually regardless or headspeed.

And that is based on actually flying such helicopters unstabilized, knowing first-hand what rotorsystems of such size will do in the real world.

The fact that a 3D heli will easily do 150 mph, is not a real good guide: the disc load and power to weight ratio are so much better compared to a scaler, that all direct comparisions for safe speeds are off from the start.

Brgds, Bert
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Last edited by Brutus1967; Nov 18, 2012 at 04:13 AM.
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