Stupid questions about aviation

[Griffin]

The same applies to a fixed wing, right? How come the effect on efficiency is completely the opposite then?

I'm clearly missing something obvious here. :D

[sobek]

Rotor RPM is fixed. IAS is not. ;)

[Luigi Gorgonzola]

The same applies to a fixed wing, right? How come the effect on efficiency is completely the opposite then?

I'm clearly missing something obvious here. :D

Us rotorheads only have lift, while fixed-wings have lift and thrust. We have to spend a reasonable amount of lift to generate thrust and since lift becomes more expensive (in terms of fuel efficiency) the higher we fly, fuel efficiency decreases as air density decreases.

 

At least that's what I understood from (e.g.) the Rotorcraft Flying Handbook (FAA-H-8083-21, 8-1 ff.).

[Grimes]

Why has grey become the main color used on seemingly most modern aircraft? Obviously there are some countries out there that have regional colors based camouflage painted on their aircraft but for the US military almost all are grey.

[Eddie]

Why has grey become the main color used on seemingly most modern aircraft? Obviously there are some countries out there that have regional colors based camouflage painted on their aircraft but for the US military almost all are grey.

 

Because it's the hardest colour to spot in the air against the sky. Different countries may use slightly different shades of grey, but they all serve the same purpose.

 

Camouflage patterns in the traditional sense (greens & browns etc) are rather easy to spot unless you're looking at them from above against similar colour terrain.

[Griffin]

Us rotorheads only have lift, while fixed-wings have lift and thrust. We have to spend a reasonable amount of lift to generate thrust and since lift becomes more expensive (in terms of fuel efficiency) the higher we fly, fuel efficiency decreases as air density decreases.

 

At least that's what I understood from (e.g.) the Rotorcraft Flying Handbook (FAA-H-8083-21, 8-1 ff.).

 

My problem was looking at the details instead of the big picture I guess.

Planks use thrust to create lift but choppers use lift to create thrust. Simple :)

Thanks for bending it for my brains guys!

[sticks]

Helicopter fuel economic

 

Hi all

 

 

First post wee

 

 

Good questions you are asking griffin.

 

 

 

 

Stupid question, DEMANDS stupid answers!

Here is a stupid one (if you ever doubted)...

 

 

The question about helicopter fuel economics, I have a few thoughts on that:

 

 

As pointed out bay sobek the rpm is “fixed” but whey is rpm fixed?

A good reason for that I can think of, is the speed of the rotor blade at its ends, (there may be several).

 

 

The speed of the blade tips dependents on the rpm of the disk and the length of the rotor blades.

Lets say our speed at end of blade tip Is 900 kph and you are flying at a speed of 300kph,

 

 

The blade tips will at some point rotate “white the flight direction” and the relative speed trough the air will be 1200 kph ? At this speed you are roughly at speed of sound.

 

 

Anybody seen an helicopter with a delta shaped rotor disk?

 

So you cant step on the “pedal” to increase the rpm to gain more lift, what to do?

You Increase the rotor blades AoA as solbek pointed out.

As higher you go the air gets thinner and the rotor blades at an given AoA produce less lift.

 

 

You then compensate by increasing the AoA, but can you do that forever?

A show stopper can maybe be engine power. But What if you had unlimited power?

 

 

The Higher the AoA more drag you produce, but thinner air makes opp for some off the efficiency loss? Is the blades(airfoils) efficiency at producing lift at different AoA equal?

 

 

http://en.wikipedia.org/wiki/Lift_coefficient

 

 

Maybe others answers pointed down this lines, or maybe its all gibberish.

Please comment/correct.

[Melcar]

But What if you had unlimited power?

 

Then you could possibly exceed the speed of light.

[effte]

Rotor blade tip speeds and loads on the blades and the rotor hub would be reasons for limiting your RPM.

 

No, the blades are most efficient at a given AoA. As you approach the stall angle, drag increases drastically while lift remains the same (or even decreases). As you approach the zero-lift angle of attack, lift (and I'm probably being Captain Obvious here) decreases towards zero while the drag of course does not. Somewhere inbetween is the best lift/drag ratio.

 

As for flying higher, keep in mind that the speed of sound decreases with increasing altitude. Also a factor when considering the tip speed of the advancing blade.

[Yurgon]

Got a stupid question as well.

 

Why do fighter jets (almost) always take off with afterburner?

 

I think most runways are long enough for takeoff with full mil, saving quite an amount of fuel. Taking off with afterburner seems to be non-economic both in terms of the individual mission and in terms of an air force's overall fuel consumption.

[mvsgas]

Got a stupid question as well.

 

Why do fighter jets (almost) always take off with afterburner?

 

I think most runways are long enough for takeoff with full mil, saving quite an amount of fuel. Taking off with afterburner seems to be non-economic both in terms of the individual mission and in terms of an air force's overall fuel consumption.

Safety, depending on weight, it is safer to get to take off in AB.

[Yurgon]

Safety, depending on weight, it is safer to get to take off in AB.

 

How so?

 

The only explanation I can come up with is to have enough runway left to safely abort the takeoff at pretty much any point prior to the wheels actually lifting off the ground.

 

That safety margin would depend on lots of factors like aircraft weight, altitude, temperature, runway length etc. However, in most videos I've seen, jet fighters always take off with afterburner, even if they don't carry external payload.

[EtherealN]

Well, think of it like this: the most dangerous moments of flight is takeoff and landing. Using the AB at takeoff does use more fuel, but increases your safety - and it's not exactly a horribly long time that you'll be using the AB anyhow, so the fuel cost isn't too bad. (If 10-20 seconds of AB constitutes a fuel problem your plane shouldn't have an AB in the first place. :P )

 

Another thing to remember is that if, on each takeoff, you get to bird-safe(ish) altitudes in 30 seconds instead of 60 seconds, over the lifetime of that aircraft that actually adds up to a LOT less time spent at risk of eating a bird. Not sure how big this one really is, but it's another thing to consider beyond just abort distances on the runway.

[Yurgon]

Another thing to remember is that if, on each takeoff, you get to bird-safe(ish) altitudes in 30 seconds instead of 60 seconds, over the lifetime of that aircraft that actually adds up to a LOT less time spent at risk of eating a bird. Not sure how big this one really is, but it's another thing to consider beyond just abort distances on the runway.

 

Sounds like a good point. :-)

[Yurgon]

A little thread resurrection for no apparent reason. But I've been wondering for a while: how do left-handed pilots cope with aviation? Aren't (almost) all cockpits created for right handed pilots?

 

And vice versa: how do right handed pilots cope with Airbus's side stick when they're flying the plane from the left seat?

[cichlidfan]

It's been discussed before but in my mind it is not any different than some American postal workers who must drive a right hand drive vehichle at work and then drive home in a left hand drive. They just get used to it.

[Griffin]

I've wondered about it too. They start getting used to it early in Cessnas. When you adjust the pull-push throttle, you have to control the yoke with left hand. Many ultralight aircraft also have a stick between your legs and a throttle in the center console. So if you sit on the left where all gauges are, you have to control with left hand. It does sound very weird to me too and I can't really imagine flying nearly as good with left hand. :huh:

[LawnDart]

A little thread resurrection for no apparent reason. But I've been wondering for a while: how do left-handed pilots cope with aviation? Aren't (almost) all cockpits created for right handed pilots?

 

And vice versa: how do right handed pilots cope with Airbus's side stick when they're flying the plane from the left seat?

 

It isn't as hard as it may seem! You usually start out in the left seat as a student pilot (unless you're learning in a tandem seat a/c), then many move on to fly as instructor from the right seat. Later you'll be right seat as a First Officer and eventually move back to left seat as Captain if you pursue an airline job. Which hand you fly with isn't really a big deal; you improvise, adapt and overcome. I remember thinking it would be awkward flying from the left seat again after years of instructing and flying as co-pilot when I went for my ATP checkride, but I immediately reverted back to prior training and didn't even think about it until after I had already finished the ride.

 

I think it's actually way harder to get used to brushing your teeth with your left hand if you're right handed and vice versa than flying an a/c with the stick/throttle in your "opposite" hand. (However, I have mastered brushing my teeth with my weak hand as well). ;)

 

 

EDIT: Something that actually takes a little more getting used to than any muscle memory or motorskills is the angle at which you look at the instruments when flying from the right seat in a Cessna for example. The distance between the needle(s) and the face of the altimeter/heading indicator creates a slight gap that when viewed from the angle where you're sitting at, you'd have to purposely fly ~20' too low/~5 deg left of course to be right on target altitude/heading (which you'd see if you were viewing the instrument head-on). Any general aviation flight instructor knows what I'm talking about.

Edited July 17, 2012 by LawnDart

[NoJoe]

I agree completely with LawnDart's points. I also though it would be awkward to switch back and forth between right and left seats when instructing, but it turns out it's pretty seamless. I now consider myself to be interchangable. ;)

 

I think the biggest thing to get used to, in addition to the instrument parallax, was the sight picture out the front of the plane. You get used to where the engine cowling sits on the horizon, which is different in a left or right turn. But when you switch seats, the sight picture is backwards. I kept wanting to climb in left turns and descend in right turns (usually it's the other way round).

 

--NoJoe

[Alicatt]

I agree completely with LawnDart's points. I also though it would be awkward to switch back and forth between right and left seats when instructing, but it turns out it's pretty seamless. I now consider myself to be interchangable. ;)

--NoJoe

Same as in a LHD car a RHD car and a motorbike I have one of each and swap between them without any problem.

That being said, there is the "occasional" moment when you have to think carefully about which side of the road you have to drive on it can be embarrassing when going from Europe to the UK or vice versa.:smilewink:

Esp. the motorbike as it has no reference as to which side of the road it should be on.

[LawnDart]

I agree completely with LawnDart's points. I also though it would be awkward to switch back and forth between right and left seats when instructing, but it turns out it's pretty seamless. I now consider myself to be interchangable. ;)

 

I think the biggest thing to get used to, in addition to the instrument parallax, was the sight picture out the front of the plane. You get used to where the engine cowling sits on the horizon, which is different in a left or right turn. But when you switch seats, the sight picture is backwards. I kept wanting to climb in left turns and descend in right turns (usually it's the other way round).

 

--NoJoe

 

Good follow-up! Steep turns from the L vs. R definitely feel/look different.

 

After a while it does turn out pretty seamless though. I have a harder time not reaching for a knob or button that's usually on my left whenever I'm sitting in the other seat, only to find myself slamming my hand into a panel or something. :doh:

 

The sight picture and muscle memory isn't really an issue when switching from flying with your right hand to flying with your left hand. Running checklists and instictively using the correct hand to reach for knobs is another story! :lol:

[Yurgon]

Cool, thanks for the explanations, guys!

[Bushmanni]

Ok but that doesn't make complete sense to me. Engine governors try to keep the rotor RPM constant withing a certain range. If the RPM is constant then the blade pitch is what determines the amount of lift. Also shouldn't it be easier to spin the blades at the same RPM in thinner air?

That's why it makes sense to me that the laws of physics should make it more economic.

 

Some other things on the other hand also determine the fuel efficiency:

 

- OAT; if we use ISA and it's +15 C at the sea level and ~ -3 C at 3000 m, would the temperature drop be enough to call the engine efficiency increase worthwile?

 

- Max IAS with the increase of altitude. Your maximum speed will decrease with altitude and according to my last sortie, the max IAS is lower than what the EPR gauge cruise power setting would be. Ie, I can't reach the cruise power setting because my maximum speed warning will go off. Also the maximum speed at an altitude is lower than the cruise speed at sea level that is equivalent to cruise power setting. I hope that made sense. :)

 

Additional question came to mind;

 

Would increasing the governor RPM setting allow you to fly higher (because increasing blade pitch would make them stall without additional speed) as long as the rotor RPM is within flight manual limits?

 

While blade drag for given AoA is less at higher altitude so is lift. By increasing AoA you get further away from the AoA sweet spot ie. to make up the lost lift you need to increase AoA which increases the drag even more due to decreased efficiency.

 

Helicopter fuel efficiency depends on two systems, rotor and engine. Turboshaft engine efficiency increases as air temperature decreases ie. it gets better at high altitude. Rotor efficiency decreases with altitude due to increased profile drag which results from increased blade AoA. Which one has more "authority" depends on rotor system performance characteristics, engine performance characteristics and gross weight. Rotor system and engines are what you have so the only variable is gross weight.

 

By determining specific fuel consumption (fuel/distance) as a function of gross weight for the helicopter in question at different altitudes gives a set of curves that usually coincide at some point. That crossover point is the gross weight that gives the same fuel efficiency at all altitudes. With lower gross weights you get better fuel efficiency at altitude and with higher gross weights you get better fuel efficiency at sea level.

 

This is because at low gross weight the blade AoA is lower and the increase required when going higher is also less so the loss of efficiency at high altitude is smaller than with higher gross weight.

 

Rotor system design operating RPM is usually the highest structurally possible minus some safe margin so increasing the RPM isn't usually possible. As the aircraft weight decreases during mission so does the required AoA. Rotor system properties are designed so that the blades operate as close to the optimum AoA as possible during typical mission. This means that the AoA is more than optimal at high gross weight and less than optimal at low gross weight. Due to this at low gross weight reducing RPM would allow using higher AoA that provides better efficiency. RPM reduction has one nasty side effect as the tail rotor RPM is also reduced which leads to reduced tail rotor authority and increases chances of tail rotor related loss of control.

[Yurgon]

Got another one. :-)

 

With the TM Warthog, the two throttles can be linked together mechanically. Is this also the case with the real aircraft?

 

To broaden the question a bit: Is it common for multi-engine aircraft to have their throttles linked/not linked?

 

With unlinked throttles, wouldn't there be too much of a risk of asymmetrical thrust because of pilot error?

[Eddie]

Put simply, no.