CG Effect on Range/Airspeed
 

In main wing/tailplane configured FW aircraft, CG at rearward edge of envelope is a factor which decreasing total drag and therefore increasing range. I am interested to know if a given CG position would do the same in a helicopter configured with main rotor and tail rotor.
Is this so? If so, which CG position would yield the least drag and therefore increase fuel efficiency (and thus range): Forward CG? Mid-Range CG? or Aft CG? Please cite any references or data sources. Thank You.

In most cases, an aft CG reduces the equivalent front plate drag area and should result in reduced power for level flight. For example, the UH-60 has ~30 sq. ft. area and a 10% change of the flat plate area from the baseline value changes the required power by a maximum of 6.5%.
YMMV.

And aft CG is another limiting factor of max forward speed in a helicopter as you may run out of forward cyclic

I think Blackhawk might be a special case with a moveable tail plane but I also think aft CG is preferred. When flying faster, the pitch attitude tends to be negative, presenting the aircraft obliquely to the flow (more draggy). Aft CG tends to raise the nose at equivalent speed. I've flown at least one type where you managed the location of the fuel towards the rear tanks to raise the nose a little in the cruise. The disadvantage might be that the aircraft is less stable at cruise speed meaning it's more work for you/AFCS possibly sapping energy/causing drag.

The 139 is designed to fly at an attitude that presents the underside of the aircraft to the airflow - 5 degrees nose up is accelerative! C of G in this respect becomes irrelevant.

If memory serves me correctly, 99% of the AW139 CG envelope is aft of the mast. Oddball.

The UH-60 Stabilator doesn't really enter into it. Like you said, reducing pitch attitude is the key. To over-simplify it, think of the fuselage like a 4x8 piece of plywood. Lying flat, flying edgewise produces ~24 sq. in. of front plate area. At just 1-degree of tilt, the equivalent area becomes ~30.6 sq. in., which is a 21% increase!

In fixed-wing, the horizontal tailplane is producing a downforce, because the centre of lift on the main wing is aft of the cg and tries to push the nose down. The tailplane balances that force. A rearward cg would allow the tailplane to present less downward force and less drag, extending the range.

In helicopters, some of the newer, bigger birds have tail rotors angled upwards to provide some lift and to extend the cg range, instead of having all the lift coming from a single point. In seeming opposition to this, the synch elevator produces a downward force, but mainly in an attempt to keep the fuselage more level. This has 2 advantages - it makes the cabin more comfortable for the pax, and it helps to minimise the drag from presenting the roof of the aircraft to the airflow. There is a bit of a battle between the upwards tail rotor and the movable elevator, but luckily the bigger machines have a computer to sort it out, not just a mechanical linkage like an old Huey.

In that case you are already WAY outside the legal, certified CG envelope!

My understanding is that the SK76 was also originally fitted with a movable horizontal stabiliser but it was deleted on production versions.

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And aft CG is another limiting factor of max forward speedin a helicopter as you may reach the forward cyclic limit

"The Bell 47 was an example of a manufacturer dealing with this issue.
They were modified by the addition of a controllable elevator coupled to the fore and aft cyclic.
I seem to recollect that post mod there was an increase in max forward speed in the order of 5-10 mph?"

It also made a difference to cabin load increaseespecially in the 3B1 etc. A Nobar 3B1 then also became a lot of fun on feral animal ops.

A problem these days is that there is an STC for a fixed elevator on the 47G and the angle of incidence has to be manually changed for external loads such as cargo rack and hook - and some pilots don't remember to read their POH as well as they should!

FlingW47 - If I recall - Far 27 certified machines such as the Robinson 22/44/66 etc VNE is a speed determined by a cyclic stick plot which allows enough cyclic to bring the nose back to level flight at most rear C of G should the machine encounter a wind gust of 15 kts or so (not sure of the actual figure) at the established VNE.

Nothing wrong with your memory Shy, the incidence angle of the tailplane on the prototype is rather obvious here..


https://cimg0.ibsrv.net/gimg/pprune....a60b617442.jpg

It also provided the CAA with an exam question for the engineers..

Describe the operation of a controllable elevator on a helicopter you have worked on.

Answer could well have been "I have never worked on a helicopter with a controllable elevator".

Probably a need for testicular fortitude to go down that route.

Effect of CG on Range in MR/TR Configured Helicopter
 

Gentlemen, I started this thread. With all due respect, discussion is beginning to drift to controllable elevators, tailplanes, etc.
That is not the question I have...to restate, does CG position have anything to do with extended range or increase airspeed in a main rotor/tail rotor configured helicopter and if so, what CG position is most favorable for decreased total drag / increased airspeed / increased fuel efficiency ("max range")?
Please address this question, with references to data if at all possible.
Thanks in advance to The Community for your consideration.

arismount, the lack of response is because it's an esoteric question to which no one has an answer, if there is any most advantageous CoG position for range the differences would be minute in the overall scheme of things.

It is also pretty hard to put the cg precisely where you want it to get the "best range" spot - people sit where they want to, though you can tell the fat slug to move into the back. Mostly the pilot simply confirms that the bird is in limits, and launches off. The range of a helicopter is affected to a very small amount by the cg, probably wouldn't notice it. We usually fly at fast cruise, rather than puttering along for range - unless fuel quantity is dire.

However, I did have a case where the range was affected by the cg. An engineer wanted to do a blade track in flight, using the strobe from the front seat. The engineer was obese, and when I picked up to the hover, the cyclic was almost back on the stops, so we didn't fly.
CG on forward limit, Range = zero.

References:

Filippone A., Flight Performance of Fixed and Rotary Wing Aircraft, pages 346-350, 2006.
Yeo, H and Johnson, W., Performance Analysis of a Utility Helicopter with Standard and Advanced Rotors, 2004.

To paraphrase the above,

On average, in forward flight, a helicopter will have about 30% of the total drag attributed to skin friction, 40% from the systems interference (main rotor, fuselage, tail rotor, hub), 10% from the landing gear, and the remaining drag due to all other causes. Airframe drag is proportional to the cube of the flight speed. The drag contributed by the fuselage increases to the point of being the dominant resistance at high speeds.

There exists a relationship between fuselage drag and pitch attitude. The drag polar of the helicopter fuselage generally increases as fuselage angles of attack increase and/or decrease from zero. As Crab noted with the AW-139, in some unique cases the drag coefficient decreases as the pitch angle becomes positive.

There is an equation given in the first reference. The equation is conceptually important, because as speed increases, the aircraft must change its pitch attitude to maintain trim (complicating matters, trim effects attitude in a seemingly unending circlular manner) . But, consider as the pitch attitude changes, the fuselage drag increases, There are many confounding factors. For example, rotor hub interference drag also increases considerably at higher speeds.

Typically, aerodynamists calculate total drag using the equivalent flat plate area method, sprinkled with well-educated guesses. The point being, there is no real hard and fast rule, and only very detailed flight testing of a particular model will yield anything definitive.

Some considerations:

Incorrect loading can also affect performance. The effects on power and range are mostly due to the angle of the fuselage, with some contribution from horizontal stabiliser download. Any range data in the flight manual is usually conservatively placed at the worst C of G position.

Aft C of G: The nose is up more, so the fuselage is pointed more squarely into the relative wind, for lower cruise drag. Also, the horizontal stabiliser is hardly working, its download is small, so the range is furthest. On the other hand, the main rotor must be flapped down to get the high speed thrust, with a risk of bending the main rotor shaft.

Neutral C of G: The nose is down about 3° further than it would be above. There is little change in performance, but the main rotors will not be down so much at the front.

Forward C of G: The nose is down, so the fuselage has its top exposed to the free stream, requiring higher thrust, and there is a download from the horizontal stabiliser that looks like weight to the main rotor. This is usually the slowest and the worst for range, and the least comfortable for passengers. However, it is generally most stable.

I do not have any source to back this up, but with the NH90(at least the high cabin version), I think theres is a clear gain with being close to the aft limit, perhaps about 5 knots at MCP or so or maybe slightly more. This on a clean A/C withouth a lot of fun stuff mounted external.

When having the cargo in front, which in most cases do not put the CG even close to the forward limit but around the middle we clearly loose indicated airspeed.
Same, A/C same load but repositioned to get to aft CG, and stopping the automatic transfer to stay at that aft CG, there is a clear speed gain.
I also tried this with the Superpuma but the gain was not that high, as I remember it. Perhaps possbile to detect but not much more.

The NH90 also fly with the bottom of the fuselage quite flat, and any forward CG increase the (rear) stabilisator downforce, which in turn cost both induced drag and use up more of the main rotor power.

Paco, You trained me for my ATPL (Thank You Sir, for that!) so I will send any aerodynamic questions direct to You Paco :)

I've looked in my three "Prouty" helicopter aerodynamics manuals.

He does mention C of G a few times, but nowhere does he answer the OP's question directly. My own take is that it is probably more about the aerodynamics of the fuselage as presented to the oncoming airflow, i.e. mainly parasite drag related and the least "draggy" aircraft nose up/down attitude will vary from helicopter type to type.

The stabilizer is ment to push the tail down in most helos, to achieve a less nose down attitude.

Aft CG set the attitude less nose down to begin with and reduce the negative lift(downforce) comming from the stab.

Most helos should gain from this by reduced induced drag on the stabilizator and as the negative lift reduces the need for vertical positive lift from the main rotor reduces it can be tilted more forward thus giving more forward thrust from the same power.

There can be consequences - back in the day with the introduction of the MD500D version the MR TT Straps had a propensity to crack.

It was found in one environment that due to the FWD C of G being the more prevalent flight regime the straps did not crack as much or as often.

The fix by MD (Hughes at the time) was to remove some of the the horizontal stabiiser "nose up" trim and change the flapping angles between the mast and the head as evidenced by resultant cyclic displacement.

Granted AFT C of G changes the body angle on the cab which reduces drag in some cases but the poor old rotor has to deal with increased bending angles and flapping angles which can have knock on effects.

i.e. the Bell 212 can be flown fast but you better have deep pockets as they beat themselves to bits and eat flight control components.

Prouty, R., Helicopter Performance Stability, and Control, 1986.

On page 306, Prouty estimates the total equivalent flat plate area of an example UH-60 sized rotorcraft to be 19.3 ft-sq (zero angle of attack). The contribution of the horizontal stabilator to the total drag is estimated to be 0.2 ft-sq. or roughly 1% of the total area at 115 kts. So the contribution of the horizontal stabilizer is really insignificant.

I thought page 296 might be of interest:


https://cimg6.ibsrv.net/gimg/pprune....b279e073b1.jpg

Interesting that the 2 wind tunnel models show far greater changes in drag with positive fuselage AoA than the real world aircraft.

That 1% drag is at zero AoA. A forward CG will induce a noze down attitude that increases the iduced drag so the drag will be higher than this for a forward CG and closer to this value at aft CG.

I did a calculation of the difference in main rotor lift needed on a 10.6T NH90. Between max forward CG and max aft CG there is a reduced need for lift from the main rotor of about 327kg due to the reduced need for “negative” lift from the stabilizer.
At 3000’ +15C and at MCP the TAS increase is about 4kt between 10.5t and 10t gross weight, so for 327kg we gain about 2.6kt from the reduced load on the main rotor only.
There is no data to find the extra drag that max forward CG cost due to the induced drag from the stabilizer, but probably more than zero as lift seldom comes for free :)