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Ground Effect and Translational Lift at the same time?

BleedGreen

Well-Known Member
pilot
Do we have any aerodynamic experts that can explain whether or not you can have the benefits of ground effect and translational lift at the same time?
So my understanding of ground effect is that it provides a more efficient rotor system by dissipating rotor tip vortices that would normally flow back into the rotor system, which in turn reduces induced drag.

Translational lift on the other hand, is when the aircraft reaches a speed where its outrunning its rotor tip vortices, preventing them from recirculating into the rotor, again reducing induced drag.

Once you hit effective translational lift, flying ahead of rotor tip vortices, how is ground effect still beneficial?
Two reasons I'm nuking these two concepts on a saturday evening...First, a couple of our ground school instructors mentioned that you can not have benefits from both phenomena at the same time.
The other reason I'm scratching my head on this one is how the maneuver description guide mentions a running takeoff. It basically says accelerate thru effective translational lift, fly off the deck while maintaining ground effect until reaching 50KIAS, then execute a shallow climb out.
Who's wrong here the MDG or the ground school instructors?
 

BleedGreen

Well-Known Member
pilot
Thanks for the resource but I didn't see anything that references how the two phenomena relate, i.e. both being possible at the same time, unless I'm missing something.
 

ChuckM

Well-Known Member
pilot
I will take a stab at this...

Ground effect is a function of dissipating the vortices that are fully formed when out of ground effect, increasing the efficiency of the rotor system. Translational lift is a function of overcoming blowback (or an inherent stability of the rotor system in a hover) and thusly the induced flow over and through the rotor system results in more air hitting the blades, from a more advantageous angle of attack (L/D ratio)

I would offer that the whole purpose of the standard maneuvers is to use both of these in a compound fashion. Take a max gross weight takeoff. You lift to a low crouch, utilizing ground effect, push through blowback to achieve translational lift and then maintain yourself in ground effect to minimize the amount of power it talks to accelerate until approaching a bucket airspeed.

I bounced this off another FRS IP as we sit here and he agrees. This is a very H-60, low inertia rotor system-centric take though.
 

busdriver

Well-Known Member
None
Ground effect is reduced in forward flight primarily because induced drag is a much smaller percentage of total drag in that regime, but it's still there.

As helo guys we tend to think of ETL as a defined moment in time but it isn't really a thing at all. I used to teach this stuff but it's been awhile, stuck on staff and all.

The burble feeling is transverse flow and differential induced drag front to back on the disc. That comes from the common "outrunning vortices" explanation and from geometry. As you tilt the disc forward and start the takeoff, the interaction with the ground changes the flow pattern at the front of the disc so that the leading vortex increases the downwash (induced drag) at the front of the disc. As a result the aircraft settles a bit and vibration increases, once that interaction moves aft of the tip path the transverse drag starts to reduce (outrunning the vortex). This doesn't exist if you're departing from an OGE hover. At the same time, with a coned disc you're introducing an induced flow variation front to back so that there's more drag on the aft portion of the disc thanks to an increased induced flow. This starts as soon as you start moving and doesn't really become a non-factor till much faster than "ETL" but they're additive, so we tend to associate the transverse vibrations as one thing, even if that moment in time is actually two things in combination.

I always taught staying low during a running takeoff is more a function of lift vector. If you're using thrust to climb, you're not using it to accelerate. Climbing at 50kias will keep you out of an avoid region of the HV diagram, but once you're above 50ft (25' really, but 50' gives you a buffer) get up to bucket speed as soon as you can.
 

IKE

Nerd Whirler
pilot
Rotary Wing Rules:
1. Helicopters are magic.
2. When you think you've figured something out, refer to rule #1.

For the TL;DR folks, I think it works thus: In a max-gross-weight takeoff, power required reduces as you inch forward. At some speed, the rotor downwash trails the helo so much that you start losing the benefit of being IGE. Whether this happens faster or slower than the benefit of "translational lift" probably depends on the helicopter. I think this explains the little altitude dip you tend to get in an H-60 during the maneuver. You just hope you add enough speed to get closer to bucket before hitting the ground.

For the nerds:
Seriously though, ground effect is a complicated phenomenon. USN TPS teaches an empirical method of determining the reduced power required (i.e., you hover the specific helicopter at different altitudes and draw a curve). As I understand it, no one can make solid theoretical predictions for IGE hovering.

Translation lift (a terrible term) is, as stated above by others, a reduction of induced power required. The "reason" depends on how you are modeling power requirements. Hover and vertical climb are usually modeled with Momentum Theory, where power is thrust times induced velocity (Pi = T • vi). Induced velocity is the difference in air flow velocity between the area just above and just below the rotor disc. Thrust required equals weight (T = W in a hover). Thrust produced equals mass air flow times the difference in upstream and downstream velocities (T = dm/dt•(V2 - V1)). When you start moving forward, the rotor has to do less work on the air to produce the same thrust, because the velocity just above the rotor is slightly higher (the tip-path plane is tilted forward into the freestream flow). The actual math is a bit complicated, but the result is the drop in power required from hover to bucket airspeed. So, there is no extra (translational) lift, there's just a reduction in induced power required. [The exact same effect can be observed in a vertical climb. In an H-60, hover, pull in collective to a given torque and hold collective position. Your torque will drop as your climb rate increases.]

If anyone's interested, I can post some plots of H-60 HIGE/HOGE, vertical climb, and forward flight power required. I just have to scrub them of FOUO numbers.
 

BleedGreen

Well-Known Member
pilot
@busdriver staying low during a max gross weight takeoff until 50KIAS to stay out of the avoid range on the HV diagram made the "why we do this" click.

@IKE Thanks for the explanation, I felt the dismissal of one effect for the other, as explained by some of our ground school instructors, was an oversimplification of what's actually happening. I' assuming those charts are different than the performance charts in NATOPS? I'd be interested in checking them out if you wouldn't mind.
 

IKE

Nerd Whirler
pilot
@IKE Thanks for the explanation, I felt the dismissal of one effect for the other, as explained by some of our ground school instructors, was an oversimplification of what's actually happening. I' assuming those charts are different than the performance charts in NATOPS? I'd be interested in checking them out if you wouldn't mind.
These types of charts are built during flight and used to generate everything in NATOPS. The term "referred" means referred to standard day conditions (0 ft PA, 25 C), there's a bunch of math to "unrefer" the underlying curves to each of the pages in the Performance chapter of NATOPS.

This one shows how OGE altitude is determined (the minimum altitude at which the aircraft is OGE). The exact shape of the curve below OGE is what makes aerodynamicists' heads hurt.


This one shows how hover power depends on weight for both IGE and OGE. The data suck, because it's hard to test on a zero-wind day (anything over ~3 knots corrupts the data, due to "translational lift").


This one shows, in a roundabout way, how power reduces with an increase in climb rate. The power reduction is less than ideal, because download drag (air impacting the top of the helicopter) increases with climb rate.


This one shows level flight power required for various weights (referred). I'm sure you've seen this before, but the power reduction from hover through almost the 4th major division is the "translational lift" range (notice it may be different for high/low gross weights). After that, power still reduces a bit toward bucket airspeed, but power required due to drag starts to pick up. The left side of the figure is zero knots (hover) OGE. We don't make these charts for IGE. It would actually be interesting to see, since it wouldn't be as simple as just a factor of 0.8 or 0.9, etc. It's probably not worth the test money; easier to let the contract pilots or the first round of FRS IPs find a takeoff profile that balances power requirements and nugget-flyability.


/end nerdery
 

ChuckM

Well-Known Member
pilot
Thanks for posting those @IKE. Why does the OGE chart have gear height on the left side vs rotor height in agl?
Again, a stab... I know you are looking for expertise, but this is what I would tell an FRS student if I got this question:

...Probably because Radalt measures gear height. For instance your NATOPS defines HIGE to be anything less than ~45 feet using radalt. Add the ~10 feet for the height of the rotor system above the wheels and you'll notice that equals the approximate rotor diameter. Which more precisely is 53 ft.

Since ground effect occurs within one rotor diameter you can see why they make the number relatable to what you read as your altitude on he PFD. NATOPS, once upon a time defined HOGE at 42 or 43 feet, which made a little more sense when you added it up.

I am making an assumption that the radalt is also what is being used to plot the above curves by the pilots flying the profiles.
 
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BleedGreen

Well-Known Member
pilot
Again, a stab... I know you are looking for expertise, but this is what I would tell an FRS student if I got this question:

...Probably because Radalt measures gear height. For instance your NATOPS defines HIGE to be anything less than ~45 feet using radalt. Add the ~10 feet for the height of the rotor system above the wheels and you'll notice that equals the approximate rotor diameter. Which more precisely is 53 ft.

Since ground effect occurs within one rotor diameter you can see why they make the number relatable to what you read as your altitude on he PFD. NATOPS, once upon a time defined HOGE at 42 or 43 feet, which made a little more sense when you added it up.

I am making an assumption that the radalt is also what is being used to plot the above curves by the pilots flying the profiles.
Having a measurable value that you can read in the cockpit makes more sense...I guess one could argue that all you need to do is take the rad alt's reading +10. A favorite "gotcha" question the HT IP's liked to ask was "where is IGE measured from?" which is why that stuck out to me on the diagram.
 
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