Chinook question

[joe nelson]

In an aircraft like a chinook, where does the C/G fall? Does it fall half way between the rotor masts or farther forward due to the rear engines. I have read that it has a much wider c/g envelope that a single rotor helicopter. Just need some enlightenment here.

[brett s]

You want where the empty cg tends to end up, or what's allowable for flight?

Empty is usually close to the rear limit, further back than halfway between the rotors. The allowable range is pretty wide but I can't remember the exact limits, I can look them up tonight if you're interested.

[joe nelson]

Brett,

That would be great! I saw a video of a model H-21 with a single engine and free turning rotors. Needless to say, it whetted my interest. I ordered the plans and did a lot of napkin engineering at breakfast...I have "rotor envy"!:peace:

PS What I'm looking for is the normal c/g range for flight.

[brett s]

Ok - at light weights (below 28,550 lbs - empty weight is around 23,000 lbs) the allowable range is just over 39" from the fwd to the aft limit. Fwd limit is station 309.7, aft is 349.0.

At 33,000 lbs it's just over 33" (309.7-343.0), at 44,800 lbs it's 17" (319.0-336.0), and at 50,000 lbs (max gross on a CH-47D) it's 9" (322.0-331.0) - straight line variation between those weights.

For reference, the fwd rotor centerline is at station 85.845, the aft rotor is station 552.530 - the center main cargo hook is at station 331 with the fwd & aft hooks being at stations 249 & 409. Hope this all helps :)

[joe nelson]

Brett,

A BIG THANKS! That tells me alot. The c/g is controlled more by rotor authority than where the load is applied in the helicopter. Lighter loads has a larger envelope and can be placed anywhere in a large c/g range. A heavier load, which requires more authority, is far more restricted to a very small envelope. Is this right?:usa2:

[brett s]

That's true for just about all helicopters that I've seen other than light 2 or 3 seaters. Most aren't quite as large a range as the Chinook though :)

[joe nelson]

Brett,

Honestly, I didn't know what to think about tandem rotor helicopters. I didn't know if the dynamics were the same or what. Hey, I still believe in the rotor blade fairy... you need one and it magically appears at the hangar in a box....A poor attempt at humor!

Here's another dumb question. How well does it fly in an autorotation?

PS I've always got a kick out of watching the early models taxi. Had an old CWO (W-4) in my unit who jokingly called it a moment of indecision.

[Hognose]

Tandem History
Frank Piasecki got the idea for the tandem-rotor helicopter after observing that lateral-rotor Platt-LePage helicopters flew more stably sideways than forward. Obvious as it seems, nobody else got that idea. The reason Platt-LePage (and Focke-Achgelis) had gone with a lateral configuration instead of a tandem one, is that they were concerned about interference from the downwash of the front rotor in the rear rotor. Piasecki's innovation was to raise the rear rotor high, something which all subsequent tandems have done. No more interference!

He made an experimental ship and got the wide CG range as a bonus. Piasecki/Vertol/Boeing helicopters are all descended from that experiment. There have been a few overseas designs... the Russian YAK firm designed a monster several times the size of the 'hook, which was not built. The British Army operated a couple dozen piston-powered Belvederes in the 1960s, they were the equivalent of the H-21 but they were one of the last British indigenous helicopter designs. Britain "rationalized" its aerospace industry to a nullity and is reduced to building American or French machines under licence.

cheers

-=K=-

[brett s]

The only unusual things about a Chinook from a pilot's standpoint would be crappy yaw stability with AFCS turned off (without due attention I've seen cabin windows get blown in from being so far out of trim) & being darn near impossible to enter VRS - autorotations are pretty calm given it's got relatively low disk loading compared to some large helicopters.

[joe nelson]

Do you think a tandem gyro would work? Would it be possible? I saw the small model fly OK!

[brett s]

I'm trying to think of any advantage that layout would have for a gyro & I'm not coming up with anything - but it seems to me it'd definitely weigh more & be more difficult to build than a single rotor one.

If you don't plan on overlapping the rotors you end up with a very long fuselage, if you do then you need to tie them together with shafting to keep them synchronized. Flight controls could get pretty complicated, they definitely are on a Chinook.

[joe nelson]

Brett,

Let me explain why I'm asking these questions. I am designing a gyro and I'm looking at every configuration to help me achieve my performance goals. I've looked at large diameter rotors, intermeshing and now tandem.

The goals are as follows; 100mph cruise, 200 mile range and 1000 pound useful load. My thinking is a tandem would give me greater load carrying and c/g flexibility. A larger load equates to more gas therefore longer range. A 34% overlap like the Chinook would make for a more compact airframe.

Let me do some more napkin engineering to see if I can make a control system! MANY THANKS:usa2:

[karlbamforth]

Joe,
Take a look here.

http://www.braggroup.com/

The Brag group may already have researched what you are looking for.

[joe nelson]

Karl,

I've seen the bragg group aircraft and it is very interesting. It reminds me of the Rotodyne of the 1960's. Maybe I need to look into compound gyros as well. They have many positive attributes like wing to unload the rotor at high speeds and multi-engines. Another characteristic is the rear loading airframe like the Chinook.

The major "con" of the bragg concept is the lateral rotor arrangement doesn't give the same longitudinal c/g envelope as does the tandem rotor. This would allow you to use much of the airframe for cargo carrying and fuel storeage.

My main concern is whether I should use a large diameter rotor or two smaller ones...both have complexities of their own. It is so much easier to just fly these things and leave the designing to someone else!

[Rotor Rooter]

Facts and facts :help:


Hognose has mentioned someone's previous comment; "Frank Piasecki got the idea for the tandem-rotor helicopter after observing that lateral-rotor Platt-LePage helicopters flew more stably sideways than forward."

Here are a couple more opposing statements about the forerunner of the Platt-LePage XR-1, XR-1A Stability of Side-by-side (http://www.unicopter.com/B326.html#PPRuNe)

In addition, it has been said that the early tandems flew sideways to minimize the vibration when flying alongside an aircraft carrier during fixed-wing takeoffs and landings.

:noidea:


Dave

[brett s]

One important thing you may not have considered - the Chinook doesn't use traditional fwd/aft cyclic inputs.

It uses differential collective pitch between the two rotors - it's got gobs of control authority in that axis, which is why the cg range is so large.

It uses differential lateral cyclic for yaw, lateral cyclic on both for roll - control mixing is "interesting" to say the least.


It also has a few nasty habits that are dealt with electronically - how about a negative stick gradient at higher speeds for starters, The trimmed fwd/aft stick position would start moving backwards (quite a bit) at higher speeds, there's an electric actuator that programs based on airspeed to correct the stick position ...

[joe nelson]

Morning Brett,

This morning over my coffee I did some "napkin engineering or gyrineering". This is my attempt at the mixing of control inputs... gyro style. Assume both rotors are co-planar and teetering hinge. Both rotors are configured 90 degrees apart, interconnected to maintain that separation. They are counter-rotating.

A collective will raise and lower the pitch of both rotors' angle of attack. The cyclic's forw/aft movement will move the rotor aoa differentially...forward cyclic moves the front rotor aoa down and the rear rotor up and vice-versa. Lateral cyclic moves the front rotor side to side only and the pedals move the rear rotor. Ah, it's probably too simple to work or maybe I'm simple for trying to work it out.

[joe nelson]

Hognose,

I read your blog/joke! Does this qualify as a good S.F. watch?

[joe nelson]

My next question is can c/v joints be used to connect both rotors together to maintain 180 degree separation? My thoughts are to save weight by not using gearboxes.:wave: