Rotor Inertia

heliflyknow · October 13, 2015

I'm trying to find a list of the weights of common helicopters' rotor systems, but I'm not having much luck. I can find their diameter/speed but the blade weights or entire rotor system weights are hard or impossible to find.

Does anyone know of a resource I can get technical data from? Preferably in PDF format. I want to try to extrapolate the centrifugal force and inertia of different systems.

Gomer Pylot · October 13, 2015

No two blades are exactly the same weight, before or after tracking. After the head is assembled, weights are added or subtracted as necessary to track the system. They're close, but not exact. I've never seen any data for weights on any model. It may exist for some, perhaps for all, I don't know, but I've never seen it. Good luck with tracking that down.

October 13, 2015

The weights of the different rotor systems can be found in the equipment lists. you will have to add it all up to get the combined weight of all the components tho.

Gomer? the weights aren't added to track the system, they are there to balance it. Tracking is accomplished via pitch links (normally for ground and hover track) and trim tabs (for forward high speed flight track). Track and balance is a fine combination of working all 3 of these to an acceptable level. Track has a large effect on balance, where as balance has little effect on track.

example of 500 equipment list:

http://download.mdhelicopters.com/Pubs/Archive%20files/TCDS%20Reports/369-E-5004%20369HS%2006-08-1971.pdf

Gomer Pylot · October 14, 2015

They can have an effect on both. That's why it's called "tracking and balancing". The weights are added to balance the system, but balance certainly has an effect on track. An out of balance system won't track, and I've seen weeks spent on trying to do it. Bell wound up putting ~300lb of extra weights on the 412 head just to get reliable tracking. Weight and balance are inextricably related.

October 14, 2015

I don't think you read my post Gomer, & if so? Which part didn't you understand?

300 POUNDS? to get it to track? where did they add all this weight? I'm sure you misunderstood the amount of weight, as a few grams added to the tip makes quite a noticeable change on a smaller rotor system. Something is way wrong for that much weight to be added, even to a rotor as large as the 412.

adam32 · October 15, 2015

I don't think the entire head on a 412 even weighs 300 lbs???

Gomer Pylot · October 15, 2015

It's mostly in the "Chinese weights", pendulums that are around the head. They swing out when the blades start turning. The problem with the 412's head is that the blades aren't in the same plane. It's like two two-bladed rotors, stacked on on top of the other, at 90 degrees to each other. That doesn't track easily, or well. You can never get a 412 in perfect track, you just try to get an acceptable ride. Before adding the weights, an acceptable ride wasn't always possible. I watched as they tried to track an early model, with a Bell tech rep on hand, for over a week, all day every day. In the end, Bell added the weights, which eventually gave a track good enough, although not perfect. Look closely at a 412 head sometime, and you'll see all the stuff they added, all just to get an acceptable track. I flew 412s for many years across the GOM, and enjoyed not one minute of it. And if you don't believe the entire head on a 412 weighs 300 lb, try lifting one. You obviously have never been near one.

Edited October 15, 2015 by Gomer Pylot

adam32 · October 15, 2015

Near one, flown one, worked on one and I still don't think the bare head weighs over 300 lbs, definitely don't think they added another 300 lbs on it either. Maybe but I don't believe so.

heliflyknow · October 17, 2015

Pokey, do you have a link for an R-22 and Huey equipment list?

iChris · November 11, 2015

I'm trying to find a list of the weights of common helicopters' rotor systems, but I'm not having much luck. I can find their diameter/speed but the blade weights or entire rotor system weights are hard or impossible to find.

Does anyone know of a resource I can get technical data from? Preferably in PDF format.

I want to try to extrapolate the centrifugal force and inertia of different systems.

Those calculations have already been made. It was done in the design of each helicopter and documented in the engineering data. However, that data is not readily available or apparent in most of the current documentation you’re familiar with. In most cases, you’ll need to go direct to the manufacturer to obtain those types of data sheets.

The rotor inertia for each helicopter was calculated and recorded under the nomenclature, “moment of inertia”, “rotational moment of inertia”, “mass moment of inertia”, or “polar moment of inertia”. This is a measure of the rotors resistance to any change in its state of rotation; moreover, to our interest, the rotors tendency to preserve its rotation after the engine quits.

The result of these calculations were used to quantify the autorotation characteristics with a simple index number that would indicate how a given helicopter compares to other helicopters in this regard. The common indices used are, Equivalent Hover Time, Usable Kinetic Energy, Autorotation Index, Energy Factor, and Flare Index, each with its own corresponding simple equation and resulting index. Usage of a given index also varies between manufacturer.

Flight test with a number of single-engine helicopters concluded that each rotor design should provide for enough stored kinetic energy to provide the equivalent of at least 1.5 seconds hover time before rotor stall following an engine failure. Test also showed that if the time was increased to 3.5 seconds, the H/V curve can be eliminated. Since there’s a lower percentage risk of multiple engines quitting at the same time, equivalent hover times as low as of .75 seconds are considered sufficient for a twin-engine helicopter.

So, its been a trade-off between high inertia, for good authoritative performance, and low inertia, for minimum blade and hub weight. As always, the decay rate of rotor speed when the engine quits is a function of the power required and of the rotor’s level of kinetic energy.

Engineering data sheets also include centripetal (centrifugal if you prefer) loading. The rotating blades of a helicopter produce high centrifugal loads on the rotor head and the blade attachment points. Centrifugal loads range from 6 – 12 tons at the blade root of light helicopters and larger helicopters may develop up to 40 tons of Centrifugal load on each blade root. Again, you’ll need to go direct to the manufacturer to obtain those types of data sheets.

See following technical papers for more information:

High Energy Rotor System; T.L. Wood; Bell Helicopter Textron

Flight Test Evaluation of the High Inertia Rotor System; L.W. Dooley, R.D. Year; Bell Helicopter Textron

An Evaluation of Helicopter Autorotation Assist Concepts; A.H. Logan & R.D. Graves, Hughes Helicopters, Inc. and G.T. White, U.S. Army Applied Technology Laboratories

Edited November 11, 2015 by iChris

iChris · November 11, 2015

As stated previously, common indices used are, Equivalent Hover Time, Usable Kinetic Energy, Autorotation Index, Energy Factor, and Flare Index, each with its own corresponding equation and resulting index.

Manufacturers include factor such as, rotor disk loading, which affects the descent rate, the stored kinetic energy in the rotor system, which influences the probability of a successful entry and completion of the autorotational, as well as subjective factors "difficulty rating" flight assessments by test pilots.

Usage of a given index and now it’s calculated varies between manufacturer; however, when compared with other indices the trends are similar. Below is an example of an AI index sometime used by Sikorsky: