Helicopters: Unstable? B**locks!
 

Well whaddya know, something we *do* agree on! That is, the definition of "trim."

Okay, first things first, Nick:I do disgree. Nick, it must be a long time since you flew anything smaller than an S-76. (Say, how does the S-76 fly at 140 knots with all the SCAS stuff turned off? Do you even remember?) Not all helos fly alike. General statements like "helicopters exhibit positive dihedral effect" are just misleading. I do not wish to be mean, but maybe we need Shawn to weigh-in here on how small helicopters fly, because he seems to have more "street cred" than a lowly line pilot who's spent a lifetime at the controls wondering how to make the machine do the things I need it to.

Small helos. Those are the ones I fly. They do not have artificial rotor or yaw stability added. And they are perfectly capable of flying along on a straight track with the rotor unbanked and the trim ball slammed hard against one or the other side of the race, especially on approach. Don't believe me? Ask any of my early instructors. Fly sideways? That's what helicopters do!

Contrary to your contention that if the ball is not centered then the aircraft must be turning, in the helicopters I fly (206's and the like at 105 - 110 kts), the turn needle is the primary indicator of a rate of turn, not the yaw trim ball. In fact, a 206 flies a little better with the ball displaced slightly out to the left (ask Prouty, he'll explain it). This does not mean that the rotor is banked. In fact, it is level and it is the fuselage that is slightly banked. But you know that. You even know that we don't even have any way of knowing the actual pitch or bank angle of the rotor; only that of the fuselage. And in a helicopter with a teetering rotor, those angles can be very different. Yes, an airplane can skid it's way around a turn with the wings level. The rudder, firmly connected to the fuselage and hence to the wings will see to that. A helicopter won't. In fact, I'd love to see a helicopter that could turn with the rotor unbanked. That'd be a neat trick!

If you contend that helicopters exhibit a positive yaw/roll coupling, why did you say that bit about the 212 shark fin? And I quote:If I were silly enough to climb into my 206, start out from a hover and translate into protracted left sideward flight, I would find that the helicopter would tend to bank more to the left as the speed increased, even though I was holding (probably full) right pedal. It's not hard to see why (the drag of the fuselage). At some point, the flapping limit of the hub would be reached.

If what you said was true, the helicopter would quickly tend to roll back to the right, in the direction of the pedal I was pushing. In reality, this simply does not happen. In fact, it would not happen with even as little as a 45 degree out-of-yaw-trim condition, or even a small out-of-yaw-trim condition in cruise flight. This is very easy to demonstrate. Most pilots do unconsciously tend to counteract the small rolling moments, but if they held the cyclic extremely steady (through friction in the 206) they would see what I mean. Anyone can go up and do it for themselves. I'd love to be proven wrong.

(Good idea, chaps. Post your results here on this forum...who's going to be the first to make a fool of PPRUNE FAN#1! -Ed.)

What I have observed in 11,000 hours of driving these little buggers around is that many helicopters exhibit little or no positive dihedral effect at all. You seem to disagree, although your various thoughts on the subject are confusing at best. Oh well.

PPRUNE Fan #1,

The ball goes out in level flight because of the Centrifugal force in the skidding turn, and the lack of a bank to make that force point downward relative to the helicopter. Sorry! Aircraft can and do turn without banking, that's why we have the turn needle and ball, to prevent this from confusing us.

One of the Wright Bros many inventions is the banked turn, prior to them, all turns were skidding. The below explanation is fairly simple, don't take offense, I know with 11,000 hours you are way beyond it, but sometimes it helps to get back to basics.

Let me explain - the sideslip of a helicopter which has too much pedal applied (either more or less than that needed for anti-torque) creates an side angle for the relative wind against the body. This angle is called sideslip, and it creates a side force that points left or right, and is a turning force. No bank, but a lateral force makes the helo turn. In such a turn, the ball slides outward because there is no bank, so you are 'skidding". A car turns this way, because the wheels make this side force when they are turned.

Try this next time: keep the wings dead level at 100 knot cruise while you kick the ball out 1 ball, either direction. Spot the gyro heading. Hold it for several seconds. Note the heading drift away from the ball as you skid in a turn.

As far as remembering how an unstabilized helo flys, I flew the S-76 for almost 2 years with no stability (we hadn't invented the sas yet) during all its structural shakedown and certification, including all the handling qualities flights, full autos and everything. I have a sas built into my right hand that would stop minor earth quakes! ;-)

Regarding the direction of roll with yaw, when you do the skid turn maneuver from above, note which way you have to move the cyclic to maintain wings level with the ball out. It will be opposite the pedal (toward the ball) because the helo has positive dihedral. Unless you are flying on the Planet Mongo, where the AFF regulates all helos to fly opposite to how they do on the Earth.

Prouty discuses this subject in the chapter 'Spiral Dives and Dutch Rolls' in his book "Even More Helicopter Aerodynamics'. The following is a partial excerpt from this chapter.

"The U.S. military allows the airplanes it purchases to have slightly unstable spiral-stability characteristics. The military is satisfied if a fighter does not double its bank angle in more than 12 seconds or a transport in more than 20 seconds.

The FAA requires helicopters that are certified for instrument flight rules (IFR) to not be subjected to spiral dives. You can see the result of this policy on the Bell 212. Before it could be certified for instrument flight, it had to be modified to decrease its directional stability by the addition of a vertical 'destabilizer' ahead of the rotor."

Well Pfan, you did say:
so I am only helping you out here.....:}

Your side ways flight example does not occur the way you have stated. In summary, you basically said that:
from the hover, translate left. Hold very steady cyclic. The helicopter would not roll back to the right, it I hope that summarises your thoughts.

I think, however, that you will find the opposite. As you translate left, you have to continually feed in more left cyclic to keep translating left, and this occurs for two reasons. Flapback is the main culprit at these speeds (but we are not going to go into that) and the di-hedral effect as a result of the right yaw you have created (but this is insignificant at such low speeds (hence the 212 fin addition). When you try to fly this tomorrow, please do not friction the cyclic because I think you will need to feed in significant LEFT cyclic to maintain left drift (right yaw). Your comment that:
is ar5e about. You will only increase speed as a result of increasing the bank. The bank does not increase because of the speed increase.

Unfortunatley in the next two weeks I will only have access to a SCAS stabilised machine, thus I wont be able to convince you of a flught test result. So, I am awaiting your experiments.
:ok:

The fin on the Bell 212 was put on because one FAA test pilot considered that the machine had too much directional stability.
Hard to understand how anything could have too much directional stability, but there was some method to the thought process.
First of all, the FARs do not require much for helicopters in terms of directional stability or spiral stability.
Secondly, remember that the requirements have to be shown for all combinations of weight and CG.
So, at some combination of weight and CG, the directional stability was determined to be sufficiently strong that the helicopter exhibited a negative spiral mode (i.e. in a steady turn, the stick would have to held out of the turn to stop it rolling further into the turn).
The cause was thought to be the overly-strong tendency of the helicopter to line up with the relative airflow (i.e too much directional stability). The solution was the fin.
It was an interesting interpretation of the FARs.
Interesting to note that after the gentleman retired from the FAA, the fin was quietly removed...
At least that's the story as I heard it.

pprune fan #1 and helmetfire,

You can see the natural behavior of the rotor even on a stabilized helo if you wantch the sas actuator behavior on the little window most systems have. Select the axis you want to examine, and note what the sas has to do to keep things normal. After all, those little sas actuators are just small pilot input devices for the autopilot.

In pprune fan's sideward flight experiment (which helmetfire has diagnosed correctly) just see what direction the inner loop must input to keep the bank.

Most rotors try to "back flap"when a speed or wind increase is imposed on them. That is due to the fact that the blade that sweeps into the increasing wind gets more lift (from its increased speed) so it flaps upward. The full effect is felt approximately 90 degrees later, so the rotor disk tends to flap away from the speed increase. That is one of the contributers to the dihedral I mentioned for the last two posts.

For PPrune Fan #1:

It doesn't matter what size helicopter we are discussing here. (Size never matters... right?)

Even small helicopters possess positive dihedral effect.

Part 27 reguires no perceptible negative dihedral stability for IFR certification.
So does Part 29.

The US Navy and Army specifications require positive effective dihedral, even with stabiity augmentation off.

The good news for those designing a helicopter is that (for all the reasons Nick has stated above) most helicopters will naturally exhibit the positive dihedral effect. However, major changes to the shape of the aircraft, such as the addition of floats, may affect this issue.

Just curious how the fin works.

When the airframe is yawed, does the fin create a lift force (sideways) that causes the fin to act like a lever arm around the roll axis, to roll the airframe the opposite way? Is this how the tendency to roll a little too postively into the turn is countered?

Flight Safety,

I believe, Prouty's explanation is that in IFR conditions a slight roll, say to the right, and the subsequent sideslip to the right will cause the helicopter to start yawing to the right and pitching down. If it is gradual enough, the pilot may never notice the change in attitude until the spiral dive has become serious. The destabilizer is intended to resist the right yaw and subsequent downward pitch.

What the hell do I know. :rolleyes: My flight instructor said that rolling up the window in the driver's door did the same thing. :O Mind you, he was in the back seat with the receptionist at the time.

The worst part was that the receptionist's first name was 'Bruce'. We'd be flying along when the instructor would start yelling 'Bruce!, Bruce! Oh Bruce!'. Here I was trying to fly the helicopter while looking up in the manual what a 'Bruce' was.

Simple answer : to enhance Roll with yaw characteristics in early model IFR B212

If you want to see negative dihedral, try a Jet Ranger with fixed floats in autorotation. The huge area under the CG (the vertical position of the CG) will give you negative dihedral - apply left pedal, and you'd expect to roll left in most helicopters. In the 206 with fixed floats in autorotation, adding left pedal will cause the helicopter to yaw left, and after pausing for two marching paces (as my drill instructors used to say), it will roll right. Negative dihedral.
Seen one other time in a very light (i.e nothing in the machine expect a slave radio, brand new off the production line) Bell 206L3 with high skid gear and fairings on the crosstubes. Fairings are not used on the crosstubes any longer if I remember correctly.

B212 - external load
 

My company is considering getting a B212 for external load work in order to carry more per trip than we presently do, using an AS350. We’d use the aircraft in a warm, low-altitude environment (35ºC, below 2,000 ft MSL); our loads are heavy and compact, and stable when carried on a line. Other than a hook limit of 5,000 lbs. and a max gross weight of 11,900 lbs., I don’t know any weights/performance limits on this type of aircraft. If any of you have used or are using the B212 in external load operations, please help me with the following questions:

- What is a typical empty weight for a B212 in VFR, external load configuration (no autopilot or co-pilot instruments, seats removed or web seats only)?
- What is the max gross weight of the B212 with external load, in other words, is there a difference with the internal load-only MGW?
- What’s the average fuel burn per hour doing external load work?
- What is the published VNE with door(s) removed, bubble door for the pilot, and/or with a load on the hook?
- In the environment I’ve described (ISA + 20-25ºC, 0-2,000 ft MSL), does the B212’s performance (hover OGE) limit it to less than its structural limits?
- How would a B205-A1 rate in comparison to a B212 on these points and also price?

Any help with these questions will be highly appreciated.

The 205 will outperform the 212 due to its lighter weight...burns less fuel....but will not perform nearly as well following an engine failure as the 212....but only marginally worse when heavy.

Depend on a whole lot of variables. But they are essentially the same aircraft and have very similar figures, and all your questions will be effected by which engine you have fitted to each.

The 205 A1 is essentially a 205 fitted with the 212 running gear, therefore they have the same speed limitations: normal cruise at 100 kias, vne at 120. Fuel burn depends on donk fitted as does lifting performance. Both can be fitted with the same range extenders, etc.

205 A1 (MAUW 10,500lbs) is often fitted with a -13 or -17 donk. Both will allow MAUW hover IGE at the figures you quote, but the -13 struggles OGE and is obviously outperformed at HDA. The -17 is good to quite high DAs. Fuel burn is 600lbs/hr (as low as 580 in the cruise) for the -13. I cannot recall the -17 burn.

212 (MAUW 11,200 lbs) has the PT6 in two versions, -3, and -3B. Same story as above, both will do IGE at MAUW at the DAs you mention. Same as OGE, the -3 will start to strugggle OGE, and the -3B begins to out perform at higher DAs. Fuel burn for the -3s is 640lbs/hr, but can be up to 700 - 740 for the -3Bs and has a noticeable affect on range.

for lifting, the body weight differences actually gives the A1 the advantage, generally by about 100 to 200kg despite a lower MAUW. But generally, they are pretty similar unless you compare say the -17 A1 at altidude to the -3 212, or vice versa.

Our old chief engineer used to say that due to the expense of the Lycomings, the 212 was only about 10-15% higher costs even though it is a twin. But I believe that with the increase in ex mil UH-1s, the Lycoming costs may be getting better.

My prefernce is always for the twin, especially doing ext load ops, because once the load is gone, the 212 will not come down and mess with your ground crews - or your pilot's next of kin.

Be aware of the proposed AD that will affect a lot of Lycoming engines....to the tune of about 150,000 USD per engine....do some research to see if the engine you are considering falls within the AD.

Contact Erickson Malaysia, as they use 205's for most of the long lining, in the same conditions u described.

Since we are on the subject. I have always been curious why companies prefer the 212 over the 205. If all things being the same or close and expense for the 212 being more. In the states I always see more 212s on firework than 205s. Is it contract requirements or as someone above states in regards to engine failures.

You might look on the Bell web site for the specs; on the B H210 It is an updated BH205.

Bert,

Imagine yourself at 6000 feet msl, temp about 90F, at the end of a longline hovering OGE trying to dump water on a snag or someother point drop....and you have an engine failure or low side governor failure in your trusty 212...does it really matter the other engine is running? The cost of operation for a 212 exceeds that of a 205....but how many of the things got retired from Offshore Flying by competition from newer faster machines thus are now available for fires or other utility work.

Thanks very much for all replies, your info has been quite valuable. There is an embarrassing story behind this, but it's a bit too embarrassing...