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Cruise Flight Efficiency


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As I was ferrying the fuel guzzling venerable UH-1H home the other day I had fuel consumption on my mind like I am sure most of us do on cross country flights. My thoughts were on climbing for mountains and other obstacles. Is it more efficient to climb with the cyclic or collective? What I mean by this is to maintain cruise power (approximately 30-35 PSI for those familiar with the Huey) and climb with the cyclic only or to raise the collective and maintain cruise airspeed. I know that the Huey has fuel consumption charts, but this is a more generalized question for any helicopter. Basically speed versus power. Any thoughts?

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Seems like bringing the cyclic back to whatever that helicopter's "bucket" speed (best rate of climb/minimum rate of descent) would be significantly more efficient than pulling more pitch if you're climbing to clear obstacles.

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Power, airspeed and altitude are inextricably linked. There is a sweet spot for each regimen of flight. You just have to find it for each aircraft. Sometimes this information is listed in the POH, most often it's not. Best Rate of climb should be close to giving you the most bang for your buck as far as fuel efficiency I would think.

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If I can remember right, the Army says use the cyclic to adjust altitude of no more than 100ft. If its greater, use power. I do agree that climbing at bucket speed should be the most efficient since its the bottom of the lift/drag diagram.

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Quick answer: collective up, cyclic down.

 

It all depends on a lot of variables, how long can you cruise at altitude, how long will it take to get there, and is that altitude close to max efficiency, that is highest altitude/DA where engine(s) still make power limit, and does that altitude have a wind advantage?

A draggy air frame like a Huey, extra speed above max endurance plus 10% is going to be fuel-expensive. I'd probably climb at best climb speed and cruise power, aim for highest efficient DA, wind/terrain dependent, and then spend twice the climb time in descent at whatever airspeed cruise power gets inside VNE.

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If the fuel savings between a cyclic or collective climb is the difference between making it to your next fuel stop or not, you might want to think about planning fuel stops closer together.

 

I didn't say that I wasn't going to make my fuel stops. I am trying to be as efficient as possible, save my company money and be professional, ect. Simply a question! Try flying a Huey cross country and not have fuel stops that make you pucker a little.

 

 

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Without a detailed analysis, I would venture to say whatever favorable winds aloft you can take advantage of would mean more for fuel efficiency than anything else. Laterally as well as vertically within reason.

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I didn't say that I wasn't going to make my fuel stops. I am trying to be as efficient as possible, save my company money and be professional, ect. Simply a question! Try flying a Huey cross country and not have fuel stops that make you pucker a little.

 

I see that I forgot the little smiley face. Since I have no valid opinion on the matter, I go with what my parents taught me. "if you don't have anything nice to say, don't say anything nice at all"

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I tend to use cruise power most of the time. I set it and use the cyclic for altitude control. I have found, however, that there is a technique that works when dealing with winds. If you have a tailwind, reduce power somewhat in cruise, and increase it with a headwind. The tailwind allows you to go almost as fast with a lower fuel burn, and with a headwind you want it affecting you the shortest length of time. With a headwind, I usually use max cruise power, and with a tailwind I use several percent less, depending on what other factors are influencing things. If you're just flying along, and want the best fuel efficiency, use max cruise power into a headwind and cut back with a tailwind. For a long climb, you can use max rate of climb, but I don't think it helps much. I have a lot of time in a 412, as well as in a UH1, and IME you can't change the fuel burn a whole lot, but the headwind/tailwind technique does make some difference.

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Good point Gomer, I have actually found with the Huey in a tail wind that I do that automatically. The aircraft I fly are all utility so they are very light and cruise no where near MCP as it is, therefore I am always flying close to VNE with power to spare. With a tail wind the aircraft wants to exceed VNE constantly and therefore I tend to reduce power a little.

Edited by THE_COYOTE
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What Gomer Pylot said regarding wind.

Rule of thumb: I calculate my TAS vs GS and back off IAS halfway between calculated TAS & GS. Don't give up on the wind, especially with GPS. If a slight altitude change moves the wind a little further aft, it's a gain. Theonly time I don't even try it is after a strong deep cold front... unless the low altitude winds are on the right beam. Otherwise, I climb as high as I want, settle and watch the GPS until GS stabilizes, drop a couple hundred feet and check it again, until the descent stops producing improvements. Sometimes that's immediate.

Also, a cloud deck gives you a clue. If the tops are blowing over into the direction you're going the wind above is better than where you are. I have had tail winds both ways with a shallow cold front, as long as it's scattered to broken and there's a clear descent in reach. A solid cloud deck underneath a VFR only helo is a dangerous thing...

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Turbine engines give better fuel efficiency at higher altitudes, but the wind has more of an effect. The best case is flying high with a tail wind, and the worst is low with a headwind. Low with a tailwind is likely to be better than high with a headwind, though. Like Wally, I tend to find the altitude with the best wind advantage, whether it's high or low. I'm not trying to build time, and saving the company money by flying the least time reasonably possible is just the right thing to do.

 

Exceeding Vne in level flight isn't possible in most helicopters, but in some you have to watch it. In the S76 with Turbomeca engines, the first limit at altitudes near and above 5000 ft is usually Vne, and you have to back off on the power just to stay within Vne limits. In a 206, the only time you have to worry about Vne is in a steep dive, with max continuous torque, unless you get really high. Get high enough and you can exceed Vne in level flight in any aircraft. I don't fly that high, though.

 

Vne doesn't care about the wind, though. You are as likely to exceed Vne with a headwind as with a tailwind. Vne is a function of airspeed, not groundspeed. You could be making 300 knots groundspeed and still be well below Vne if you had enough tailwind. Or you could exceed Vne with zero groundspeed with enough headwind. The aircraft has no idea what the winds are doing, neither velocity nor direction.

Edited by Gomer Pylot
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Exceeding Vne in level flight isn't possible in most helicopters, but in some you have to watch it. In the S76 with Turbomeca engines, the first limit at altitudes near and above 5000 ft is usually Vne, and you have to back off on the power just to stay within Vne limits. In a 206, the only time you have to worry about Vne is in a steep dive, with max continuous torque, unless you get really high. Get high enough and you can exceed Vne in level flight in any aircraft. I don't fly that high, though.

 

Vne doesn't care about the wind, though. You are as likely to exceed Vne with a headwind as with a tailwind. Vne is a function of airspeed, not groundspeed. You could be making 300 knots groundspeed and still be well below Vne if you had enough tailwind. Or you could exceed Vne with zero groundspeed with enough headwind. The aircraft has no idea what the winds are doing, neither velocity nor direction.

 

I understand the difference between airspeed and ground speed, what I was referring to is the tendency of a nose low attitude with a tailwind resulting in a higher air speed. As far as aircraft capable of exceeding VNE; everything I fly can can do that in level flight all the way down to sea level (S-61, UH-1H, 206 BIII). I spend most of my time in the mountains and am constantly dealing with shifting winds, therefore it is quite difficult to pick a constant altitude to cruise at with favorable winds, mountains and all that stuff! :o

 

As far as turbine efficiency at altitude that is very true, when I co-piloted in the Skycrane for 2 years we spent a little time in New Mexico at higher altitudes (5000 +), and the normal burn rate of 500 gallons per hour was reduced sometimes by up to 20%.

Edited by THE_COYOTE
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