When I did my 135 course I was told they had tested a 5 blade system for that as well,that was 3 years ago:confused:

Please forgive my complete ignorance and I apologise if this is a stupid question but what are the advantages/disadvantages of a 5 bladed rotor system over a 4 bladed system???

PR

Roughly speaking more lift, hence higher MTWA

Frankly, there is no more lift or thrust in 5 vs 4 or 3 blades, as long as the total solidity of the rotor (total blade area divided by the total disk area) is the same. In other words, 4 skinny blades have no more lift than 3 somewhat wider blades. More blades usually weigh more, so that for the same solidity, a greater number of blades can reduce payload a bit, but the vibration is a bit smoother (especially when going from 2 to 3) with more blades.

4 to 5 seems to make quite a difference too!

There must also be cost and maintenance issue involved; by definition a 5 bladed head must be more complex, cost more and the blades themselves will cost more, relative to a 4 bladed design.

Very true, and also the cost to make 1 blade is the sum of a fixed cost (inspection, paperwork, tooling, etc) and a variable cost (materials and labor) so that a 5 bladed head with blades must cost at least 20% more than a 4 blades head.

The vibration goes down by a ratio of the 2/3 power of the number of blades, so that the difference between 1 and 2 blades is enormous, 2 to 3 is big, 3 to 4 is less so, and so on. By the time you get to 7 vs 8 the number is too small to count.

Nick
 

"so that the difference between 1 and 2 blades is enormous"


Yep!

If I am reading this right, if the 5 blade option is too expensive / complex and little to be gained, could this be why ECD have gone for a "CAT A" switch on the EC135T2? Increase the RRPM by 3% for class 1 take off / landings and get the benefit of increased lift with a greatly reduced cost implication.

Now the EC145 comes with an automatic increase if RRPM at low speed (below 55Kts I believe), so why keep going down the extra blades route?

Thanks nick, I should have gone a little more in depth with my answer, just trying to keep it simple.

I have not had a close look at the 145 head , is it essentially the same as the Bk117 ?.

Just checked EC45 site ,it looks identical to BK117.

I think a key sentance in the press release is "sponsored by the German Ministry of Economics and Technology" The easy blade fold feature looks useful .

Back to Nicks earlier replies , why would any Sikorsky have more than 4 blades then ?.

And just to nit pick Nick missed out one element which is Non Recurring cost ( engineering and tooling ) this unit element will be reduced ( by a small amount ) per blade if you are making 20% more blades.

widgeon,
You seem to think that I say 4 blades is "better" than 5 blades, which is not true, at all! The 5 blades are more expensive, but could be a better mix when all design factors are considered. Obviously, Sikorsky thinks so,too. I have hundreds of hours in a 7 bladed monster!

semirigid,
You hit the nail on the head! Increasing rotor rpm is a way to add extra equivilent blade area without adding any blages. In effect, the rpm makes up for the lesser blade area, so that thrust is increased without any rotor head/blade change (as long as the CF can be tolerated.)
The peak thrust of a rotor is basically determined by the Ct/sigma (coefficient of thrust/solidity), where all rotors are limited to a set value of max Ct/Sigma (usually about 0.20). Ct is thrust/ [(air density) x (disk area0 x (tip speed squared)] and solidity is [(number of blades) x (chord) x (radius)]/ rotor disk area.

Sounds complicated, but it is basically just like an airplane wing, where the wing stalls at a fixed coefficient of lift, so you need either more wing area (rotor solidity), or more forward speed (rotor tip speed).

Is an odd number of blades such as 5 better than 4?

slowrotor,
No not at all, the more the number of blades, the smaller the humps in lift each create, and the higher the frequency the N/rev vibration is. Both factors make the greater number of blades a better vibration case. If the rotor had infinite blades, there would be no vibration.

I think that the notion that EC use an increased Nr for their vertical Cat A profile to increase lift is a slight misconception. Although not denying the facts as stated by Nick (obviously they are correct) in the case of such profiles the weights are so restricted that lift, per se, is not the issue; it's rotor energy.

EC us the high Nr to increase the rotor energy for both the rejected take off and continued take off cases. In the former case, it allows the high rates of descent, that develop from a reject close to TDP, to be arrested, and in the latter case it helps minimise the drop down following a failure immediately after TDP.

In the case of the EC-155 it is the rate of descent in the rejected take off that is limiting the maximum TDP to 100 ft and the RTOW (1000 ft/min is common).

By using the increased Nr, they are also able to modify the HV curve and this allowed them to develop the 'increased slope' (short field in plain English!) procedure, whereby the a/c is allowed to start climbing sooner than would be the case with normal Nr. Similarly, it helps bridge the 15 kt gap between TDP and Vtoss. It is also a requirement for the offshore PC2e procedures they were developing last year.

By way of an example of the difference, I used to demonstrate failures in a 6 ft hover with normal Nr vesus at 10 ft with increased Nr, with no collective input, to illustrate the benefit. In the first case the low Nr audio would sound before ground contact, in the second case it would only go off once collective was applied to cushion the touchdown.

Perhaps they know that the "Old Lump of Titanium" has reached the end of its development life and there is a better way .... I think the present head design whilst great back in 1976 will not allow the A/C design to progress either due to retreating blade stall issues (read "cruise speed increases") or vibrations in flight ????

Cheers ;)

Thanks 212 man, I have been looking for an explanation for this modification to the T2 series for a while. Even the test pilot forum went very quiet when I mentioned it!

Having said all that, the RRPM increase is just 3%, does it really make that much difference? I suppose ECD thinks that it does, but it sure is a complicated method and not without controversy. I think we are back to Nick's old chestnut about "percieved risk" - take your hand off the collective to press a button, just incase you have an engine failure or leave you hand the collective just incase you mess up the approach.

If the engineering could take it, would it not have been better to have increased the mass of the blades and thereby slow down the rate of RRPM decay?

The stored enegry is proportional to the square of the rpm, so a 3% increase in rpm is actually a 6% increase in energy. It is not enormous, but it truly helps in Cat A and H-V engine cuts. For a hover cut, rotor energy is worth about 1/3 of the means to land, and engine power is worth about 2/3. So if we make a 6% increase on the 1/3, that would help increase Cat A weight by perhaps 2% total. For a 6000 lb machine, that is 120 lbs.

I think 212man has it right.

Nick,

Notwithstanding the weight gain, wouldn't the addition of a fifth blade allow all the blades to be made closer to the ideal shape for hovering (long and narrow), while preserving the solidity necessary for high-speed and maneuvering performance?

Additionally, wouldn't the addition of a fifth blade allow all blades to operate at a lower angle of attack, thus increasing the retreating blade stall margin?

As to the vibration, do additional blades actually reduce the energy of the vibration or just shift it to a frequency which is less objectionable?

-Stan-

I agree with 212man , we have the 102% switch on the collective for Cat A in the 139 and you are required to put 103% rrpm manually in the 412 for Cat A.

Stan, your basic premise is just a little off - remember, it is not the number of blades that determines the total thrust, it is the solidity. All the virtues you list are attainable by controlling solidity. An additional blade is one way to get more solidity, another path is to provide the original number but with somewhat wider blades.

Regarding the vibration, the basic energy (actually the root shear forces) are very much reduced when you have more blades, and the reduction is much faster than linear, so it really helps to reduce vibration by adding blades.