Info: Ground effect and translational lift

[darren_uk]

This is a cut-n-paste of a posting I've made on the FMS simulator board.

I've included it here as information for people new to helicopter flying about these two effects.

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The helicopter simulations [in FMS] seem to lack two things:

Ground effect
Translational lift

Can people confirm that these two are absent, and if the developers could let me know if they're in the pipeline to be implemented in a future release?

An explanation of the terms follows, and how I guessed they're not implemented at the moment:

Ground effect: This is where the helicopter sits on a cushion of air near the ground. The pilot sees this as follows: When he raises the collective, the helicopter will rise until a point where it will just sit there. To go higher the pilot will have to raise the collective a little more, and so on until ground effect has been cleared and there's no ground interference from the rotor downwash.

Translational lift: The highest power a helicopter pulls is in the hover. (More accurately, the highest power a helicopter pulls is in the hover out of ground effect - i.e., a high hover). As the helicopter begins to fly forward, it will enter clean air (not disturbed by rotor turbulence) and suddenly everything becomes more efficient.

The pilot sees this as: He begins his forward flight, and then at a certain point (around 15 knots in a real heli) he feels a slight rumble and then the helicopter rises further from the ground. Similarly in the model helicopter: the helicopter will begin to climb as his forward speed increases (in fact, any speed, forwards, sidewards, backwards).

I noticed the following cannot be done, which is possible in helicopters (and desirable in real helicopters which I'll explain in the next paragraph): Tilt the helicopter on the front skids, and sledge along the ground until the forward speed is enough for translational lift - then the helicopter will clear the ground and begin to climb.

Real helicopters: If the pilot is limited on power (hot day, high altitude, heavy) and he cannot lift into a hover (but he's calculated from his instruments that he has sufficient power to climb in translational lift) then he will tilt the helicopter onto its front skids and balance it there as the helicopter starts to sledge forward, getting faster and faster until about 15 knots when he'll pop off the ground. He then increases his forward speed to around 56 knots to gain maximum climb rate, and off he goes. Yes, the landing is the same: land on limited power and sledge along the ground on the skids.

[colin]

Interesting post....

Do you fly real helis? I don't know if it's a good idea to sledge a real heli. You might hit an obstacle and accidentally dig into the ground.

Actually, your point about translational lift is quite interesting for new pilots who have not setup their heli's for negative pitch.

If a new pilot has "mastered" hovering, and is now ready for forward flight.. it will be a good idea to setup at least an IDLEUP setting with Negative Blade Pitch.

If not, flying around in hover pitch curve may just see you flying higher and higher, and trying desperately to get the heli to descend.

Ha... I'm speaking from experience!

[darren_uk]

Hi Colin,

Yes, I fly real helicopters.

I built my Hirobo Shuttle before I had my first (trial) lesson, and I tell you I understood things like tail-rotor drift which impressed the instructor (thanks to me having to build the Hirobo).

Then I did my pilot's licence in 1999. The most ambitious thing I've done so far is "Around the UK in 4 days" (sounds like a novel!).

Translational Lift: Yeah - if one doesn't understand that it exists, then it's quite a surprise when the thing suddenly for no apparent reason goes into a climb - and probably even more disasterour when the learner then pulls back on the cyclic to stop his heli getting away from him, and before he knows it the thing's come out of translational lift and sinks into the ground. It's quite marked in a real helicopter too - and slight shudder of the airframe then "pop" you're in a climb - quite a marked lowering of the collective is needed, as well as the compensating rudder pedal adjustment.

You can pat your head, rub your stomach and juggle three eggs at the same time can't you? :-P

(and then when you're flying from one place to the other, not only do you have to fly the thing (which is actually easy in the cruise - just keep all the guages in the green otherwise the engine gets damaged, or the blades fly off :-P), keep an eye out for other traffic, keep an eye on the map and match the ground to what's on the map, listen out for radio calls, make appropriate radio calls, and keep any nervous passengers calm. That's why it's so fulfilling !


Sledging the helicopter (called "Limited Power Exercises"): It's part of the private pilot's learning. It's necessary. A couple of times I've had a crappy day (hot which means less dense air) and a heavy passenger: which means I can't do a regular takeoff. Not quite a sledging one, but close (the next one up is called "cushion creep" - it's where you lift the heli into the hover, but any normal forward cyclic makes it fall off the ground cushion and settle back into the ground, so you slowly ease the helicopter forward until 15 knots, then you get the additional lift from translational lift).

Re hitting obstacles? You're right, the surface has to be suitable (when flying in snow it's almost a requirement to sledge along, otherwise you're surrounded by flying snow from the turbulence, causing you to "white out", lose your sense of horizon, and crash". If a ground slide landing isn't possible, then you actually have to do a few practice approaches to blow as much of the loose snow away as you can!

Anyway, in a normal field (e.g., grass) the front of the skids have been designed to prevent a tip-over. It's a perfectly normal maneuver.

And if you think that's bad, how about this: The private pilot has to land without the use of the tail-rotor !! E.g.,, if he has a tail-rotor failure in the air, he can't bring the thing back into a hover because he'll just spin around and lose it. So again a ground slide is necessary, or a balancing act with the throttle at the end of the landing to reduce the torgue to prevent spinning.


I talk/write far too much when talking or writing about helicopters...sorry, it's just a fascinating subject for me and I get a buzz out of flying. And if I can't fly, then I need to get my buzz from reliving the experience.

[colin]

Hey Darren! Talk more!

You can ramble all you want about real helis. Nobody here would mind. In fact, I should start another section on it.

The only thing some people would get piss is that they would consider their RC helis as REAL helis as well. I won't argue with that.


For me, I fly real planks. I'm a PPL group A license since 1998. But I've not been flying for a while now, as it's really expensive flying here. I know what you mean by juggling act as PIC. When we're doing circuits at my local airfield, the chopper guys do their autorotation, and we have to keep out of their way. Sometimes they just appear out of no where. I also love to see the Bell 216s taxing from the Ramp to the center of the runway where they then lift off vertically. They are like just 1 feet off the ground, but they taxi just like us 3 wheeled planes.

I'm going to setup a different section in this board for Real Flight soon.
Maybe another for MS Flight Simulator as well.

I think the bottom line is that all of us share one main thing in common.... We just love aviation.

Hey guys,

Ive got a PPL too (theory exam passed in '96) but Im still doing the practical.. have 32 hours of flight time on cessna 150. But due to a lot of work lately I can hardly find the time to fly (takes me a while to get to my airfield, and now with my rc heli Im more busy with that in my spare time ) Another setback is that I have to take another full medical and here in Belgium its really a *****, cause I have to go to for or five different state hospitals for each test and I need to take like a week holiday to make it all happen.

Once had an introductory flight with a bell jetranger too. went out on 30 minute flight with an instructor to get 'an impression' of the controls. But since training on helis per hour costs around double/triple then training on cessna (heli is around 200 eur/hour on a robinson and cessna slightly over 1 eur/minute)
I decided to stick with cessnas. But I guess the choppervibe never left me My instructor told me once you get bitten by flying ... it doesnt let you go anymore