PPRuNe Forums - helicopter design features

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Where is "Vertical Freedom" when we need him? supporting the merit of the B3 fantastic altitude performances...

Quote:

Originally Posted by GipsyMagpie (Post 9942842)

You want to go high altitude? You want less engines. Just look at the Lama and B3 Squirrel (altitude record for helis I think).

This should give best power to weight you can get (with sufficiently powerful engine obviously - efficient because you only have single gas generator to drive). Clearly you need to strip out any excess weight too (no redundancy like twin hyds for example).

I would also design the rotor for efficiency at high altitude (it'll fly like a pig at low level though). You would have to design control phasing for the altitude (Lock number effects) - you see this going wrong when flying a "sea level" helicopter at 20000ft - forwards back left and right all get skewed around. I would also increase control gearing/control power as everything gets a little sloppy on the controls.

I think a lower rotor speed would also be a good idea. The speed of sound is lower at altitude (ie lower temperature) so tip effects would kick in earlier. Longer blades might cause problems with tip effects so I think broader chord is the way to go.

Just some thoughts

Well, one of the Testpilots at Airbus said, that the 145T2 would be able to perform nicely at high altitude but they werenˋt allowed to extend the graphs to what is possible by Airbus France....
And thatˋs a twin and I ˋm willing to belive the guy beeing TQ limited all the time with both engines working with bleed valves open at MCP.....

FB-

That's a great point about twins being gearbox torque limited under most operating conditions (AEO). While engine performance will typically be adversely affected by lower pressure and/or higher temperature atmospheric conditions, the performance of the gearbox remains largely unaffected.

Consider the example of the EC145 twin. Each of its Arriel 1E2 engines has a T-O rating of 738shp and an MCP rating of 692shp at SL/ISA. The MRGB has a (AEO) T-O rating of 1040shp and an MCP rating of 848shp. You can see that the engines have plenty of power margin above the MRGB limit at SL/ISA, 42% at T-O and 63% at MCP. I don't have specific performance data for the engine, but it would require a reduction in power of 218shp per engine at T-O and 268shp per engine at MCP before matching the MRGB power limits (AEO).

In order to assist the threadstarter with his back yard construction project.

Make it a single. Unless you have someone to reliably and consistently pay the bills and want to scale the whole thing up to account for the lost payload.

As the target design altitude increases so will the tr thrust/mr thrust ratio, unless you really enjoy directional challenge.

Make the engine big. The higher the intended alt. The larger the "size"of the gas producer to the lonely power turbine(s). Make it torque out first, sooo much more intuitive and linear for judging your power situation. Even if it means pushing more fuel drums around.

As much as it is a safety feature it is a psychological salesmanship feature. That is...make the cabin small in relationship to the installed power and the size of the rotor disk. Everyone will be consistently amazed when it is loaded to the headliner with junk and full of fuel and "always flies away" Brings in more money with a second trip at lower elevations. Otherwise it gets really tiring explaining why only 30% of the seats can be filled. Hard on a girls reputation too especially if she is made by Bell.

Lock number / control cross coupling concerns, I am willing to bet aren't a problem this planet. We could ask VF about this. Fly by wire parts are found in the electrical section of the hardware store however.

On a side note, protecting the dignity of the Bell ladies. The 206L4 ha t/r, 417(eagle 407 hp) and 214b I am confident would happily keep up with the beloved ecureil b3. Though on a design note it is interesting to watch a spec. L4 walk away from a spec. 407 above 8000' pa ISA +20.

Hope this helps. Send pictures.

Quote:

Originally Posted by riff_raff (Post 9943932)

With a twin you will always have oversized engines to be able to fly away on one engine. No point making the Transmission larger (and heavier) as that is not the design goal for a category A helicopter. The twin is there for redundancy and not heavy weight lifting.

I reaaly appreatiate all your imputs
I am in no way trying to build anything, it just got my attention looking at a SuperPuma FM (AS332L2) that is limited for land/take off to 7000 ft DA ... for such a "powerfull" heli does not seem much altitud for a landing or take off...
That is way I wanted to know what differences are there is design that gives extra values when it gets hot&high.
Of course if you what to land above 20.000 ft you don`t take twins ... but that is not my curiosity. What I would like to learn is if such type of rotor performs better that others, type of compressors, blade design...and such.
I understand that is quite technical but I am sure there are some of us around that know somethings about all this.
Thanks to all of you in advance

Quote:

Originally Posted by Washeduprotorgypsy (Post 9944380)

Lock number / control cross coupling concerns, I am willing to bet aren't a problem this planet. We could ask VF about this.

Now that would be worth hearing. The cross coupling definitely does happen - I've seen it. It wasn't that bad though.

Great points about the size of the cabin though.