Helicopter Aerodynamics

Mjg2011

Mjg2011

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Mjg2011
I'm a fixed wing pilot and I am curious about helicopters. At my 141 university flight school, everyone can talk about airplanes and how they fly, but when a helicopter comes in, we are amazed by them and think they run off magic. So I guess my question is, do helicopters need less collective with an increase in airspeed and if so, why?

Thanks
 
It's like chickens are to 'birds'. We cursorily acknowledge the intersectionality in order to appease the pedants, but beyond that..... :D /TC
 
Mjg2011 said:
So I guess my question is, do helicopters need less collective with an increase in airspeed and if so, why?
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Are you asking that once a helicopter increases its forward speed does the pilot lower or reduce the collective pitch?
 
Rotor blades work just like fixed wings. When you’re climbing a fixed wing you will have both a greater angle of attack and fly slower than when you are cruising. In cruise your angle of attack is reduced and speed increases. Same is true with the rotor (mostly)

*this is the dumbed down, sorta true version of what velocity posted below me
 
You’d be surprised at how many helicopter pilots that can’t answer your question. Even professional ones. Below is a performance chart for a UH-60. As you can see, at a hover, the aircraft uses a lot of torque (collective). Reason for this is the increase in induced and profile drag on the rotor system. So, you’ve got large vortices on the tips (induced) and an increase in blade pitch (profile). This results in a lot of drag. As speed increases, torque goes down because of a decrease in tip vortices about ETL and a reduction in blade pitch because of an increase in AoA. As you can see TRQ starts to increase again about max endurance speed. This is because in order to overcome the increase in profile / parasitic drag on the entire airframe, you must increase collective.

Real world application. Just a few days ago I did a funeral flyover. We were put in holding prior to the flyover so to reduce drag (collective) and fuel flow to least amount, I went with a max endurance airspeed. Put this chart on its side and you essentially have a drag chart for helicopters.

873058DC-70C6-4FB5-B152-3EC33CE10ED5.jpeg
 
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I flew a CH-3C helicopter once. It was sufficient for my lifetime.

Cheers
 
For a very simplistic description, I’d look at the relationship between collective and airspeed in a helicopter the similar to the way you think of your power setting vs. airspeed in an airplane. More collective (power) is necessary at both the low speed end of the envelope and the high speed end, with a low point in between.
 
Hydraulic leaks and bumblebees is all I can say.
 
You know the saying…

“Helicopters don’t fly, they beat the air into submission.”
 
I’ll give you another example of the importance in knowing basic helicopter aerodynamics and the relationship with speed and torque (collective position) as they apply to twin engine ops.

In Afghanistan, we had a common training LZ that we used for pinnacles. It was at roughly 10,000 ft, very rocky and uneven. At some point once the pinnacle training was done, I’d pull back an engine on the “student.” Well in order to sustain flight with one engine on a Black Hawk at 10,000 ft up, you have to be within a SE speed. In this case, about 30-80 kts. Below that, there’s just too much overall rotor drag to overcome on one engine. You’ll over TRQ the engine (input trans) in a heart beat if you try and bring it to a hover. On the opposite end, once you get above your max SE speed (80 kts ish) the overall airframe drag, overcomes the engines ability to sustain flight. So, what do you do? You take the aircraft somewhere to do a roll on landing just above your min SE airspeed.

What’s funny is, some of the lessor experienced guys would rattle off the EP (rote memory) and just continue on to the pinnacle on one engine. In their minds, they hadn’t correlated yet the relationship of speed and drag. We’re flying on one engine, why can’t we simply land to a pinnacle? This would generally result in a debrief of the importance in maintaining a SE min / max (bucket) speed. ;)
 
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I always loved doing a "stuck/fixed collective" scenario. Watching folks work through a 1000 fpm climb at airspeed before realizing they could the on the back side of the power curve and it would descend was awesome.
 
k9medic said:
I always loved doing a "stuck/fixed collective" scenario. Watching folks work through a 1000 fpm climb at airspeed before realizing they could the on the back side of the power curve and it would descend was awesome.
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Stuck pedal obviously another good scenario about speed control and collective position. Left stuck pedal especially. Bring it in not too fast or the vert fin will have you flying sideways. Not too slow or the increase in collective (TRQ) might have you flying sideways the other way. Come in around 40-50 kts, arrive IGE around the same you’re getting below ETL. The increase in collective will slowly bring the nose back around to the right. Gets out hand, chop the throttle with a hovering auto.
 
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