Hot or cold blades.

[birdy]

Question to anyone,
Would the tempreture of the blade compared to the air have a bearing on their performance or efficiancy???

If so in what way??

[Victor Duarte]

hi david, i cant tell you this as i m not a gyro flyer, but just considering the inner properties of the materials, composites are given to have a good behaviour in temperature VS alu
but i dont know about alu blades sensibility to heat , do they loose stiffness ? .. you may ask a chopper pilot, in hot days thay are very concerned with the perfs of their machines.

one thing is shure, in hot days the perfs are lesser than in a cold day because air density, but do you know what are cold days ;)

just throwing this tread further, hey no REAL specialist for birdy ??

cheers

[birdy]

Do I know cold??
Is - 11c cold enough,in an open machine for 6 hours.[wunder wot the wind chill factor was at 40 mph at -11c.]

The hot thin air affects choppers more than gyros coz the rotor is driven.
My prop,and to a lesser extent the engine suffers in the heat,but the rotors seem to be the same.
They obviosly spin faster but require no more power than usual to do the same job.

Haveing a temp range here of over 60c from max to min,[over 12 mounths] means I git to see a big difference in thrust,but theres little difference in rotor effect.

On a 48c day ,if left in the sun for a while,the alu will get close to 100c,[boiling point].
What affect would this have on 48c air??

[Ted Eggleston]

Birdy,
I imagine that the temp value for 'active' blades would be slightly less than the surrounding air temp in which they were working due to the continual airflow over the material.

In your post you suggested that "The hot thin air affects choppers more than gyros coz the rotor is driven. My prop,and to a lesser extent the engine suffers in the heat,but the rotors seem to be the same. They obviosly spin faster but require no more power than usual to do the same job."

Can you develop your comments further given that in FW training we were taught that temperature effects performance dramaticily. If your engine and prop suffers from high temps (thinner air), so too would your rotors, would they not? The loss of lift derived from the rotors in less dense air needs to be overcome by an increase in rotor speed given that they are a constant angle of attack.

Sincerely
Ted

[birdy]

I'm only go'n onwhat I'v experianced Ted.I don;t know the reasons why,I only have theories.
The reason I reckon that "autorotating' blades arn't affected as much is coz they gain their energy from the air itself,so the reaction to thinner or thicker air equals itself out ,same as the lift/drive of the blade dose under different loads.
The loss of lift in thin air is automaticaly compensated with more autorotating rpm.This higher rpm would cause greater friction with the air,but thinner air is applying less friction,so it balances out.

[darrellwittke]

The rotor system in a gyro is powered by the pneumatic drive coupled to the engine which I remember is about 40%? efficient. (Which is why gyro's are gas guzzlers) Helicopters are planetary gear driven with 90%? efficiency.

Both have the same amount of power available at low temps or high temps, no power increases with higher ambient temperatures.

So I would hazard a guess the helicopter suffers more than the gyro at higher air temps because of the difference in rotor RPM's?

Doesn't a helicopter spin at higher rotor RPM's and isn't the top speed of the rotor fixed? The gyro isn't so I would surmise that the rotor can overspeed in high temperature air while the helicopter can't, it must increase blade pitch.

Therefore there is something that isn't as efficient in increasing blade pitch versus increasing RPM's? Perhaps it goes back to moving a lot of air a little (higher rotor tip speed at 2 degrees) than moving a little air a lot (increasing blade pitch to 8 degrees)?

Where is Chuck B.? Did he get tired of straightening us country bumpkins out?

[birdy]

The way I see it is ,a heli's blades are mechanicaly driven,so you are limited to how much 'power' you'v got to pull Gs.But when you pull Gs in a gyro ,theoreticaly you'v unlimited power.The trouble with helis is when you need more power[thiner air]you'v got less coz of the decreased power in thiner air.

But the question was how a blade's efficiancy is affected,gyro or heli,when there is a difference in temp between the air and the blade.

[darrellwittke]

Do you mean the question is if the rotor being at 32 degrees (0 celsius) flies different that if it was at 212 degrees (100 celsius)?

Also, Birdy, what do you mean by the theoretically having unlimited power when pulling G's? The gyro rotor is mechanically powered up to the pneumatic drive, and the only excess power coming into the system is from momentum or inertia (ie. potential energy being converted to kinetic energy) so I would say the amount of power is limited.

I should go back and read the whole thread possibly again, do you need density altitude versus performance charts such as helicopters have?

Thanks for your replies. darrellwittke

[StanFoster]

Birdy: I had posted a few months ago how I noticed after a flight that the outer few feet of my blades are definately warmer than the inboard section of the blades. I just happened to debug it right after a flight..and had never noticed the tips warming up like they do. Now I catch myself after a flight checking the warmed up blades.

Stan

[birdy]

Yeh Darrell,Wot would the performance difference be,if any,between a blade at ambient temp[say,35c] and a blade heated to 100c in the same temp air??

Everyone I talk to says R22s blades are underpowered here in summer.IOW,they are limited in the avalable power.
A gyro isn't coz the temp seems to have little affect on their performance,only the engine/prop is affected..

[Aussie_Paul]

I don't think you are correct there Birdy. Gyro blades suffer in the heat as does any airfoil. When I was training at Bond springs some years ago the rrpm gained 5 from ground level to 1000' agl.

Aussie Paul. [:)]

[birdy]

Yeh Paul,I know they spin faster in hoter or thiner air,the hot/cold rev range on the extru blades on the RAF will go from 325 in winter to 365 in summer at S/L.
What I'm saying is they don't nessaserily rob more power from the engine,more like the engine revs harder coz of the prop push'n against thinner air.

In the heat,a R22 can't manouver as fast coz it runs out of power.A gyro,while it may lose thrust[prop] it dosn't lack lift.Blades take longer to get to flying rpm when taking off coz flying rpm is higher in the heat.The extra run needed is caused by the need to gain more rpm,or inertia.It dosn't take longer coz of less eficiancy in the heat,more coz they need more rpm.The loss of lift is automaticaly compenssated with more rpm,and I reckon coz the air is thinner,the extra rpm don't cause anymore drag ,so you haven't lost any lift efficiancy. IOW,you can still pull the same Gs in heat as in cold.

When you gained 5 rrpm Paul,how many prop rpm did you gain??Was that prop rpm increase caused by thinner air on the prop or rotor inefficiancy at alt??

None of this has anything to do with my origional question tho;

Wot would the performance difference be,if any,between a blade at ambient temp[say,35c] and a blade heated to 100c in the same temp air??

[stuart]

Victor, What are perfs? thanks stuart

[darrellwittke]

I was hoping a true blue engineer would give a proper answer... but in their stead I will offer my uneducated opinion.

If I remember right, a steel beam of 1000 feet (315 meters) when heated to 1000 degrees will grow 4-6 inches in length. Not very much when down to the span of a rotor airfoil.

Aluminum would expand slightly more than steel, considering aluminum engines go from below 0 degrees (celsius or fahrenheit) to operating temperature at 190 to 200 degrees fahrenheit without enough expansion to knock off the close tolerances an engine requires to operate, I would hazard a guess that aluminum expansion with temperature is not that much in the dimensions were talking.

So in summary, I would say that the common sense answer (I hope!) is the performance difference between a heated and expanded and unheated contracted blade is not enough to effect the performance.

It makes sense to me the R22's are limited in summer because their rotor's can't overspeed to compensate for the thinner air as a gyro's can.

Did Chuck B. get mad at the Goldsberry thread and leave us? Hope not. darrell wittke

[birdy]

Darrell,The R22s blades can't overspeed,but they can add pitch to counter the thinner air.More pitch=more drag..........when you need it least.

I hadn't ever thought of the slight expansion of the blades material when heated,and agree that it'd be so minimal you would'nt notice.
The way I look at it is the effect the heated blade has on the air in contact with it.

What I'v noticed is if the machine is parked in the sun for a while and the HOLE blade is quite hot[ecseading 100c] theres not a noticable difference in TO distance.[to cooler air]
Whenever I do a quick fuel stop,say 60 seconds,the TO distance is considerably shorter.In mid summer the TOP of the blade will heat faster than the bottom,and in round 60 secs ONLY the top will be heated, so only the air passing over the blade contacts the hot material.

Hot blade....heated air....thinner air on top,=more lift................maybe??

I don't know,I'm only a SCG but theres a noticable difference between the to blade temp situations.

[Victor Duarte]

Victor, What are perfs? thanks stuart

sorry stuart, for the delay i didnt see you were answering me somthing, i just shut up and let exp people answer..

but ok, what i mean by perfs in a chopper : the ability to lift a load whith a given engine power , in cold air, the air being more dense, the lift is better, no need a high pitch so no need to open gases...

if hot air, the air is thinner, the blades dont lift the same way, the pilot needs to increase pitch, so (on piston engines) to compensate with more gases, more fuel, higher pitch, more drag, lower rate of climb.. BTW the chopper will "vomit" (dont laugh please i dont know the english expression, but it means that after hover and rise up, on transition to fw flight, the chopper will skid on the hover to the ground) and with poor air, this "vomit" can be dangerous..

i just must say, i observed it during 3 years passed with a crop sprayer on Bell 47 only.. but in summer, with chemical tanks filled, he had to keep the gas tanks under the half to be able to take off...

i dont think gyro rotors "create" magic lift by hot days, perhaps you dont feel it because you just can add some gases..untill you get satisfying lift.

Birdy, interesting question about hot metal contact, i must say i didnt think about that... why not ? pushing to the limits, if the top of the blade was at red heated, there are chances the lift would be bad or null. dont know, but can be useful to avoid black paint... and i didnt know you have cold days in OZ ;)

thank you

[darrellwittke]

It is an interesting phenomenon you've pointed out to us Birdy, I congratulate you for your keen observations, curiosity (especially after all the hours you've flown) and willingness to post what you observe.

I don't have any real answers, just gut instinct (nothing wrong with discussing with that though) but I wonder if your performance increase in take offs after the quick fuel stop don't come from your engine being warmed up to close tolerances and providing more horsepower?

It seems to me that the blade would cool off to ambient air temperature in less than a 100 rpm's, given it has that much air (100 to 300 mph) passing over it when spinning.

Any chance of it being the warmed up engine causing better thrust?

[birdy]

Good point Darrell,[another this SCG didn't think of],but I reckon that different temps on a 2000 hour old donk wouldn't have as much affect so as to feel it .
The observation I'v made is more rotor related coz not only do they wind up faster but after a "horsed" liftoff,if I bank hard at 10' to turn tail[turn down wind] they seem to do it easly.Normaly,if I try to do that the blades feel sluggish in responce for a few seconds,as if they arn't yet up to fly'n rpm.When the blades are in the "hot top" condition they can be pulled round as soon as you break ground.

Also ,the area of the blade critical to any change is the leading top edge,witch is solid in the extruded type,and would hold heat longer.

BTW,my gut tells me near all I need to know,I'm always listen'n to it intently,especialy when it's empty.

[darrellwittke]

If a "hot top" condition can give you the lift to maneuver when normal flying conditions, with an already warmed engine, doesn't, then I guess we can discount the horsepower theory.

How about rotor rpm's? I'm thinking maybe the "hot top" blades increase the turbulence of the boundary layer and thereby allow faster rotations and more lift? Just guessing but I would theorize that it is like vortex generators but on a small laminar scale. Sounds as good as any I suppose right now, until a qualified answer comes along.

Don't know what else, what could cause an increase in lift in such conditions? The atmosphere should be less dense with bright sunlight, thereby negating horsepower increase due to more dense air. Blade performance should also suffer in such conditions, not increase.

An interesting conundrum you've mentioned Birdy. I don't discount anecdotal evidence as much as Chuck B. or other heavily instrumented engineers so I believe you've got a little known rotor trait (or not discussed much) here. Maybe us SCG's can talk and figure it out or maybe some pertinent knowledge regarding this will be cast our way?

So what is your conjecture as to performance increase Birdy? Sincerely darrellwittke

[birdy]

I just reckon that the air contacting the hotter top side of the blade is thinner coz it's being heated,improving lift.
I probably should fit a rotor tac,but that would be a wast of time coz I'm never look'n at the instruments anyway.

[darrellwittke]

Seems to me that the air contacting the top of the blade will be hotter and thinner which should decrease performance (or lift)? Wouldn't cold dense air on top cause more of a vacuum and more lift?

More lift, from the conditions you described, must come from faster rotor RPM. That's my theory at least (for now) and I'm sticking to it. Any chance you might put on a rotor tach. and answer this conundrum? (Which you started by pointing out I might add :) )

[birdy]

Taco,slim chance.
I thought there'd be someone here who'd put us streight,maybe they is sick of my dribbl'n.

[darrellwittke]

What the heck happened to Chuck B.? And Doug R. never even commented.
By God, it's finally happened, everyone that actually knows something has pulled all their hair out over us and are now receiving hair transplants at Ken's place in San Diego!

[birdy]

Mmmmmm..........,they reckon good help is hard to find.

[Chopper Reid]

What the heck happened to Chuck B.? And Doug R. never even commented.
By God, it's finally happened, everyone that actually knows something has pulled all their hair out over us and are now receiving hair transplants at Ken's place in San Diego!

Thats good Darrell !! :)

[Doug Riley]

There's a lot going on in Birdy's little puzzle. Here's one way to go at it:

1. Static effects (balloon theory): When you heat up a little chunk of air, the pressure tends to go UP, not down. The pressure increase if the chunk of air stays the same volume is proportional to the temp increase (measured from absolute zero). E.g., heating a chunk of air from 50 deg. F to 200 deg F increases its pressure (if volume doesn't change) by about a third. Clearly a pressure INCREASE isn't helpful if it happens on top of a wing.

As soon as the chunk of air heats up, however, it will expand (if it can) in response to the pressure rise. If it's not constrained by anything other than surrounding cooler air, it'll keep expanding until the pressures inside and outside the chunk are the same. By then, its density will be lower than the surrounding air and so the chunk will float and then rise. The chunk has become a little thermal. Inside this thermal, the static pressure is the SAME as outside, but the air density is less. So far, no extra lift (you could get static lift if you put a bag around the rising thermal and made a little hot-air balloon).

2. Dynamic effects: The air next to the top of the blade is less dense than the air next to the bottom, but the STATIC pressures are the same. In aeroDYNAMICS, however, we're looking at what happens when we get the molecules moving. AeroDYNAMIC lift comes from slinging molecules about. We get a little reaction force each time we sling a molecule out of its pre-existing path. The more molecules slung per second, the more sharply they're slung and the faster they end travelling up post-sling, the more aerodynamic reaction force we get.

As the air loses density, we can't sling as many molecules per second -- because there aren't as many available in a given volume of air. The amount we displace each one and the speed of displacement are the same in heavy or light air, since these things are functions of the wing's shape, speed and AOA. Note that the standard equations for lift and drag make these items a linear function of air density. I.e., less density, less aerodynamic lift.

(A dramatic example is the difference in lift between a wing operating in air and the same wing at the same speed in water. Water is denser and you get a whole lot more lift at the same speed and AOA.)

So my conclusion is that, IF you're succeeding in producing a temp difference between the air above the blade and the air below it, it won't add to lift and might even decrease it.

P.S.: You have to wonder if the temp difference really happens, though. With a 7" chord blade at 350 mph, that chunk of air is in contact with the blade for about .001 (a thousandth) of a second. How much heat transfer can happen in that time? Besides, given the rapid change-over of the air near the blade, isn't the blade going to cool down in a hurry?

I'd be looking at other things that might affect performance after only a brief stop -- say, the engine staying closer to its warmed-up temp.

[birdy]

thanx for shoot'n my theory down Doug.

The weather is warming up here now and a couple of days ago I was do'n a mate a favour,on a hot day.
I left here at first light and landed at his camp bout a 1/2 hour later to make a plan.
I woz a bit dissapointed with wot he called a "strip".It ran cross wind,was plenty wide enough,but only bout 150m long,with 60' trees at ether end.[he dosn't fly ether]
Such a strip dosn't usualy bother me coz the ferel can handle it.I toped up with fuel and took off,cross wind,25/30c* and JUST cleared the trees at the west end.
Still ,I wasn't faised,done that plenty of times[you tend to expect it from non flyers]
2 hours later I was back for more fuel.As I flared I felt the normal thin air at ground level and was starting to wunder if I'll ever get off again.
A quick refill and taxy as far as I could to the east end of this strip,the temp now was just past 40c.Held the prerotor on till it would go no more and opened full tap.Except for the temp increase,the conditions were the same as 2 hours earler,but I lifted off at least 50m shorter than when it was cooler.????????????????????

Same weight[less a little fluid from the piss I had]
Roughly the same time between landing and T/O
Wind was same ,only gusting instead of constant.
I refiiled 2 more times with the same effect.
All 4 T/Os were the same pattern,only the first one was longer.

Just out of interest,where's the peak touque rpm on a 912???

[darrellwittke]

Well, now that Doug R. has put us straight with an engineers cool eye, I will relinquish my last foolish theory and jump to a new one.

Your description of the weather conditions caught my eye, birdy. The gusting winds are probaly indicative of an unstable atmosphere, and with Doug R. mentioning air parcel heating, it has jogged my memory cells.

As you may or may not know, I have read (in Dennis Pagan's Understanding the Sky) that small scale thermals grow on the ground in a bubble shape (much like the bubbles on the bottom of a kettle before it starts to boil.) Once they have enough growth, energy and/or a disturbance they release, turn over and radiate their energy upward assuming a homogenous shape as a rising parcel of air.

So, although the above knowledge is not completely descriptive, I believe the performance increases you are seeing may be coming from your landing strips heating up and producing a little bit of rising air. Any words on if the strip varies from the surrounding landscape greatly? I assume it is much like our scab-rock pinyon-juniper desert country, and if so then the open areas can heat up a fair amount more than the shaded (although not very much) surrounding areas. Especially with the sun towards the higher angles. Any correllation on sun position/time of day to performance increases?

Any credence to this theory?

PS. Thanks again for the input Doug R, any chance you could, if you would, answer my questions re twisted rotors in the rotor blade section? Oops, theory of aerodynamics section, I mean.

Regards, darrellwittke

[bones]

Darrell,
You could be onto something there as from when i was hang gliding the thermals need a trigger to set them off the ground, ie rotor ground effect thus making them lift off the ground and in doing takes the rotors with it..
Birdy how long after TO does this extra lift last for ? if only 50-100' after say 50' you waould start to out fly any thermal that lifted off at the same time...
going back to my corner now,was thinking this for awhile and was hoping someone else would say it first that way if it is crap, i can say it wasnt me, Thanks Darrell :)

[birdy]

Interesting points bout the thermals,but theres one very reliable thermal trigger in this country,gum trees,the bigger the mass of trees the stronger the thermal.The strip I was using this time was running longside a creek full of gum saplings,with a big thermal over it.If the air is go'n up there,then it'd be cum'n down at or near the strip.Now that I think bout it,it's worse,the strip was on the down wind side of the creek,and that is where the big sinks usualy are.I do remember that it was a stagger[big sink] till I was past the edge of the saplings.So if anything,the conditions,and the location of the strip were acting against the poor little gyro.

Bones,the "extra lift" is only enough to break ground,if theres trees in front,you gota fly between till you git AS.

[Doug Riley]

Birdy, does your prerotator perhaps work better when it gets warm? Getting the most possible RRPM is the best way to shorten your takeoff in a gyro.

I'm thinking of items like a rubber drive wheel getting slightly sticky and not slipping once it gets warm, grease in a gearbox getting softer -- that sort of thing.

[Doug Riley]

Darrell Wittke: Where's that post re twisted blades? I couldn't find it.

[bones]

Doug, Birdys feral has an electric starter on it for the prerotator

[Doug Riley]

Does the starter rev higher when it gets warm?

Here in the north country where it gets very cold in winter, you do have to warm up an electric tool to get full rev's at colder temperatures. Batteries make more juice when they're warm, too.

[Chopper Reid]

Birdy, for once I have to disagree with you. My experience is that the hotter it gets, the performance of my gyro drops off noticeably,not the engine but the gyro just needs more power to perform the same as it did when cool.My gyro is fairly heavy, not like your "feral" so I know that makes a big diference [weight wise ]and I need another foot or so in rotor length and that would make a big diference to performance.
Your observations had me puzzled at first but I think its just that your machine has a incredible power to weight ratio with big rotors . I think this is backed up by some of the manourves you do, I know I couldnt even try to come close. Your gyro set up works very sweetly.

[birdy]

Yeh Doug,electrical stuff CAN work better when its at an ideal temp[not sure wot ideal is tho],but it can also get too hot.Some of the modern vehicals have all sorts of electrical problems here when it gets hot.I don't think that the temp differences we're talk'n bout here would have effect to this extent ether.
I couldn't tell bout the starter rev'n harder coz I havn't got a rotor tac,and I have the prerotor on till just befor I break ground,so it'd be hard to tell wot the rpms are get'n up to.

Chopper,mate, your definatly not the first person to dissagree with me.
I know the thinner air makes the prop less eficiant,needing more rpm to have same effect,and when its hot[45c+] I'm usuly bout2/300 rpm up on the rotax,but I'm refering here to the rotor only.

Also,The ferel's setup isn't anything startling.
The 912 would only be put'n out,I reckon 60 horses at best,the machine would weigh close to 180/200 kg. The blades are probably longer than normal at 26',but they are only adequate.

[MichaelBurton]

Not having done any work in this area I amgoing to give my "uninformed" opinion. I know that opinions are like belly buttons " everybodys got one, there not polite to show in public and everybody doese it anyway. So hear goes.

What if the air passing over the rotors expands as it comes into contact with the heated portion of the blades. I would then need to accelerate. This accelation would lower the airpressure as per Bernuli.

Just a thought

[birdy]

Wots Bernuli???

[MichaelBurton]

In the 1700s, Daniel Bernoulli investigated the forces present in a moving fluid. The equation states that the static pressure ps in the flow plus one half of the density r times the velocity V squared is equal to a constant throughout the flow, which we call the total pressure pt of the flow. There are two ways to look at a fluid; from the large, macro scale properties of the fluid that we can measure, and from the small, micro scale of the molecular motion and interaction.

Thermodynamics is the branch of science which describes the macro scale properties of a fluid. One of the principle results of the study of thermodynamics is the conservation of energy; within a system, energy is neither created nor destroyed but may be converted from one form to another. Assuming a steady, inviscid flow we have a simplified conservation of energy equation in terms of the enthalpy of the fluid:

ht2 - ht1 = q - wsh

where ht is the total enthalpy of the fluid, q is the heat transfer into the fluid, and wsh is the useful work done by the fluid.

Assuming no heat transfer into the fluid, and no work done by the fluid, we have:

ht2 = ht1

From the definition of total enthalpy:

e2 + (p * v)2 + (.5 * V^2)2 = e1 + (p * v)1 + (.5 * V^2)1

where e is the internal energy, p is the pressure, v is the specific volume, and V is the velocity of the fluid. From the first law of thermodynamics if there is no work and no heat transfer, the internal energy remains the same:

(p * v)2 + (.5 * V^2)2 = (p * v)1 + (.5 * V^2)1

The specific volume is the inverse of the fluid density r:

(p / r)2 + (.5 * V^2)2 = (p / r)1 + (.5 * V^2)1

Assuming that the flow is incompressible, the density is a constant. Multiplying the energy equation by the constant density:

(ps)2 + (.5 * r * V^2)2 = (ps)1 + (.5 * r * V^2)1 = a constant = pt

This is Bernoulli's equation. If we make different assumptions in the derivation, we can derive other forms of the equation.

It is important when applying any equation that you are aware of the restrictions on its use; the restrictions usually arise in the derivation of the equation when certain simplifying assumptions about the nature of the problem are made. If you ignore the restrictions, you may often get an incorrect "answer" from the equation. For instance, this form of the equation was derived while assuming that the flow was incompressible, which means that the speed of the flow is much less than the speed of sound. If you use this form for a supersonic flow, the answer will be wrong.

We can make another interpretation of the equation by considering the motion of the gas molecules. The molecules within a fluid are in constant random motion and collide with each other and with the walls of an object in the fluid. The motion of the molecules gives the molecules a linear momentum and the fluid pressure is a measure of this momentum. If a gas is at rest, all of the motion of the molecules is random and the pressure that we detect is the total pressure of the gas. If the gas is set in motion or flows, some of the random components of velocity are changed in favor of the directed motion. We call the directed motion "ordered," as opposed to the disordered random motion.

We can associate a "pressure" with the momentum of the ordered motion of the gas. We call this pressure the dynamic pressure. The remaining random motion of the molecules still produces a pressure called the static pressure. At the molecular level, there is no distinction between random and ordered motion. Each molecule has a velocity in some direction until it collides with another molecule and the velocity is changed. But when you sum up all the velocities of all the molecules you will detect the ordered motion. From a conservation of energy and momentum, the static pressure plus the dynamic pressure is equal to the original total pressure in a flow (assuming we do not add or subtract energy in the flow). The form of the dynamic pressure is the density times the square of the velocity divided by two.

I was thinking that the heat could add energy to the system by expanding the air and increasing the velocity on the upper / heated part of the blade.

Bernoulli's equation is also used on aircraft to provide a speedometer called a pitot tube. A pressure is quite easy to measure with a mechanical device. In a pitot tube, we measure the static and total pressure and can then use Bernoulli's equation to compute the velocity.

[birdy]

No one will belive this,but i see wot your saying,and it makes sence to a SCG. ;) [Mind you,them numbers scared the crap outa me when I saw them,so I skiped and just read the writing. :p ]

"I was thinking that the heat could add energy to the system by expanding the air and increasing the velocity on the upper / heated part of the blade."

That is how I understood the increase in performance,only couldn't explain it.The hot part of the blade is only exciting the air passing over it.If the blade cools back to ambiant temp after a short while,then that heat energy has gon somewere,and the effect of that energy transfer is the better performance of the blade in the way you explained. :)

Not only are you a smart man Mic,you can also explaine it to a SCG,and thats no mean feat. :D

[MichaelBurton]

Sorry about all the math just thought someone might want to se why it could work.

[Vance]

Michael, I would think given the time the blades are in touch with any one part of air and the volume of air that the rotorblade is in contact with that there would not be enough heating of the air to measure a change in velocity. It is very impresive support of the therory though. Thank you, Vance

[MichaelBurton]

Stranger things have been known to happen.

[birdy]

And it's the best explanation so far to understand why my blades perform better in the "hot top" situation.

[Doug Riley]

I keep falling back on the simple gas equations from high school chemistry class. The ones that relate P (pressure), V (volume) and T (temperature).

IF there's enough time for a significant heat transfer (we're talking a thousandth of a SECOND for the air to travel from leading edge to trailing edge over the tips of 7" blades at 375 mph), then, yes, the molecules will pick up more kinetic energy. The immediate effect of this is to INCREASE the pressure on the upper side, however. That is, none of the added molecular motion is of the "ordered" type; it's all of the random variety. Random molecular motion doesn't create lift, just pressure in all directions at once.

After the newly-heated air has a chance to bulge up above the blade surface as a budding micro-thermal, the pressure goes back to the ambient air pressure (but not below it). I suspect the blade will be long gone by the time this pressure equalization happens, however.

I still think that there's a mechanical explanation for Birdy's experience. Without a rotor tach, though, it's all guesswork.

[Udi]

Interesting discussion. You guys really make me feel at home (actually at work) with the ideal gas equation (PV=nRT) and Bernoulli's equation.

I am not sure I am clear about something. Is the premise of this discussion that the blades are warmer than the air before the flight because they were sitting in the sun? So the question is -- would hot blades have different lift than cold blades?

In my opinion, hot blades would have better lift than colder blades, for a reason that was not mentioned in this thread yet. Here goes:

There is a very narrow layer of air that is "stuck" to the blade and is not moving as fast at the air outside of this layer. It is called the “boundary layer”. The thickness of the boundary layer depends on the viscosity of the air and the speed of the blades.

The viscosity of air is a function of temperature. Warmer air has a lower viscosity than cooler air. It's like warm honey compared to cold honey, although the differences are not as dramatic.

If the blades were hot, the air in the boundary layer would be hotter. This air is moving over the blade relatively slow (because it is stuck to it) and so it has a good contact with the surface of the blade. It heats up quickly. A warmer, less viscous, boundary layer means there is less friction between the outside air and the blade. Less friction - less parasite drag.

If the parasite drag is lower, the blades would spin-up to a higher RPM with the same torque from the prerotator. Faster prerotation - faster takeoff.

This effect would not last for long, however, because the airflow would cool the blades down to the ambient air temperature pretty quickly.

That's my theory, and I am sticking to it.

Udi

[Vance]

Udi, that is really cleaver. I still find it hard to believe that the difference could be measured by the seat of David's pants. What percentage of difference do you think it would make? Thank You, Vance

[Udi]

...What percentage of difference do you think it would make?
Well, I am very sorry you had to ask that, Vance, because I went to the books to look for an answer for you, and what I found blew my theory away. The viscosity of gasses, unlike liquids, goes up with temperature. The viscosity of air is about 10% higher at 100 deg c than at 30 deg c.

Now we have to find another spin to make my theory work... (this is what we call "engineering")

Ok. I think I found it. At 100 deg c the air is 22% less dense than at 30 deg c. So, if the boundary layer is 10% more viscous but 22% less dense then maybe the parasite drag is lower.

Please don't ask me any more questions like that, Vance - I hate proving myself wrong...

Udi

[Al_Hammer]

The air flow immediately adjacent to the surface of an airfoil acts
differently than the general airflow. This area is referred to as the
boundary layer. By energizing this thin layer of air, the air flow can
be controlled and forced to act in a modified manner.

In the F-4 there was Boundary Layer Control when the flaps were
extended. BLC was achieved by routing hot bleed air from the engines into
the wing trailing edge and then through small holes in the wing
surface. This caused delay in airflow separation at low speed (high
AOA) thereby delaying onset of buffet and allowing for slower landing
speeds.

OK, not exactly a hot wing, but hot air over the wing. I thought it was interesting.

I don't agree with the earlier suggestion that a gyro rotor doesn't have less performance in hot air.
The rotor may spin faster, but that just means you have to fly faster to maintain that rpm, just as a plane has to fly faster on a hot day to maintain lift at a given angle of attack. It is always more efficient to produce thrust with less velocity imparted to the air. A rotor spinning faster is moving a thinner, less dense parcel of air at a higher speed than rotor in cooler air. It takes more power from the engine to fly the rotor in less dense air, plus the engine has less power available, as was mentioned.

[Vance]

I am really sorry Udi. I still think that you are brilliant and creative.

Lets back up, The air is thinner and the rotor spins up better and the giro takes off faster. The blades being hot are not the sorce of the better take off performance.
If the starter is happier at the higher rpm things could get better. If the wire is a little on the small side or the battery is small, the increased rpm might make more difference than you would expect. Thank You, Vance

[Udi]

Thanks, Vance. I hope you know I was just kidding. My theories are being challenged every day in my line of work. The only way to advance science is to come up with hypotheses and then prove or disprove these theories.

To tell you the truth my first thought as I was reading this thread was that Birdy's observations were not scientific enough (i.e. n=1) to draw any conclusions, but I figured I would play along and come up with a spicy theory... This forum is a lot of fun.

Udi

[Chopper Reid]

Birdy, I do have a rotor tacho and on a day of 40c plus, the revs increase from approx 350 to 360, thats with 27 ft alloys. I see you have 27's and I would really need probably 29 ft rotors for the extra weight I'm carting about to be on an equal power to weight ratio with you despite the fact I use a 2.2 Suby producing 120 horses. I generally only use 3800 revs to 4000 on a really hot day so I'm not using a lot of those 120 horses but a lot of torque and the 2.2 is probably twice the wieight of a 912.I must weight my gyro and just see what it does actually weigh in at then we can get a bit more scientific. Do you use a 60 litre seat tank ?

[birdy]

Nothing I do could be classed as scientific Udie,but the constant better blade performance in the "hot top " situation don't take a scientest to notice,but it mite need one to figure.
I'd heard bout the 'boundry layer 'thing too,but woz hope'n someone of higher inteligence would think of it first.

That quote from Al is interesting.

The ferel has only 26'ers Chop,and I can tell by look'n that they sipn faster in the heat.The RAF 28' AKs spin alot faster in the heat,25/35 rpm more than in wnter.

[Udi]

Birdy - what do you mean by "hot top". Can you say again when exactly the performance is better?

Udi

[darrellwittke]

Of Udi's request, Birdy. I've been away and glad to see the qualified people are shedding some light on this (anecdotal but believable) situation. (BTW, Doug R. the twisted questions are in the theory of aerodynamics section about 100 days old. The core question is do tapered blades equal twisted blades in efficiency? Thanks for looking.)
To recap what I understood Birdy was noting is that when the blades warm up on the top skin only, they have a noticeable performance increase. This is over both when the blades are warm all the way through, top and bottom and/or when they are completely cold. I understood Birdy has noticed this (with all those hours, I believe in his capacity to tell the performance differences as well as instruments) even when the engine has been warmed up to operating temperatures, but the blades have differed as to top only or whole blade heated.

You also mention, Birdy, the ability to turn much more responsively with "hot top" conditions, which to me means you have more rotor RPM. And although I think we concluded the gyro does not suffer as much performance loss as a helicopter does in the hotter temperatures, it still does suffer a performance loss. However, in "hot top" conditions your take-off roll is noticeably shorter and your maneuverability is higher and your climb rate is better (at least for awhile.) Is this accurate?

Mentioning the gusting winds at the Gum tree strip still catches my eye, however. Having gusting winds versus the steady winds you mentioned previously is not indicative of having the same conditions or atmosphere.
I would say, especially with the feral (being light and big rotors) set up for the low end of the speed scale, you caught a gust at the right time (as you know, a 2-4 mph wind can have a dramatic effect) to provide the performance increase noted.

So, getting back to Udi's request, do you Sir Birdy have another anecdote or observation you can describe to us whereby you are relatively positive the weather conditions stayed the same (preferably with no wind.) I suspect you have seen this condition many times and have such anecdotal evidence to offer. Please be as descriptive as you can. (Especially regarding "hot top" conditions ie. temperature, cloud cover and type, surrounding vegetation, wind speed, runway color, time of day, anything else you remember.)

As always, thank you for your time reading these meanderings.

[birdy]

Darrell said,
"You also mention, Birdy, the ability to turn much more responsively with "hot top" conditions, which to me means you have more rotor RPM. And although I think we concluded the gyro does not suffer as much performance loss as a helicopter does in the hotter temperatures, it still does suffer a performance loss. However, in "hot top" conditions your take-off roll is noticeably shorter and your maneuverability is higher and your climb rate is better (at least for awhile.) Is this accurate?"

I couldn't av summerised it better me self Darrell. ;)
And wot's this SIR ****?? :rolleyes:

I know its only anecdotal,but when you go to the extent of pacing the avalable runway,just too see how long it isnt,then to find the machine bounce off much earler than you expect,and you got to get it right coz your ass is on the line,I reckon me recolections are pretty accurate.IOW,its not idle observation.

Yes,I'v been noticing it for years,I'v only described the one bout the short strip run'n cross wind ,on a hot day with the big gums coz it was fresh in my mind.

I understand wot your say'n bout the occasional fluke'n the right gust Darral,but as an indicater of how lucky I couldn't possably be :p ,I'v NEVER had to abort a TO coz I'v run out of room :D .[wunder how many muster'n blokes could say that.? :eek: ?]

I'v been ponder'n this for years,I didn't just notice it yesterday.

Please,I'm not git'n stroppy,just try'n to give the "evedance" some weight. :)

[birdy]

I know Steve,but its shorter,and thats the puzzelment.

[Doug Riley]

Udi: By "static" I really did mean "static." As in "not moving." No movement, no Bernoulli. The point was to separate the possible "hot air balloon" (aerostatic) effects from the air-density effects on the aerodynamic process. It's easier to consider them one at a time.

I don't know enough about fluid dynamics to fill a thimble. However, Marks Engineers Handbook states that the viscosity of gases INCREASES with temperature -- pointing out that this is just the opposite of honey and other liquids. Do you have more info than Marks's cryptic statement? How does it play into boundary layer thickness? If you could thin out the boudary layer, you could increase performance.

Some old-time experiments in this area involved things as crude as punching holes in the upper surface of the wing and attaching a vacuum source to suck the boundary layer away. I wonder in this, case, though, if heat doesn't produce an adverse effect on boundary layer... to the extent it does anything at all.

[Udi]

Doug - yes, the viscosity of air is directly proportional to temperature, which is the opposite of what I said in my first post. I should have also been more careful in the conclusions I made in that post - which admittedly were not well thought out.

The thickness of the boundary layer depends on a few parameters. First, you have to know whether the boundary layer is laminar or turbulent. In principle, the boundary layer is laminar at, and close to, the leading edge. It may become turbulent further back, close to the trailing edge.

The thickness of the laminar boundary layer is a function of the distance from the leading edge, and a reverse function of a square root of the Reynolds number. As you know, the Reynolds number is directly proportional to free air velocity and to air density, and reverse proportional to the air viscosity.

The drag for a laminar boundary layer is directly proportional to the air density and to the square of the air velocity, and is reverse proportional to the square root of the Reynolds number.

Do the math - assuming a laminar boundary layer, the drag is directly proportional to the square root of the viscosity, the square root of the density, and to the velocity to the power of 1.5.

So, assuming the rate of increase in viscosity is about half the rate of decrease in density of air due to temperature, the net effect of heating the boundary layer should be a reduction in drag (which is supported by some of the methods mentioned above for reducing boundary layer drag).

Let’s play with some numbers - if the viscosity of the air increases by 10% due to temperature and the density is reduced by 20%, the net effect on boundary layer drag would be a reduction of about 4%. Can this kind of short-term reduction in drag explain the faster takeoffs Birdy is experiencing?

Udi

[Cobra Doc]

Think we can figure out how to get Alex "Twistair" some hot blades? Looks like they would really improve his performance! (Took a while and my brother's help, but we got the video to work, once.)

[Victor Duarte]

hi all, udi, doug,
the initial post was about the effet of heated blades in ambiant air if i remember.
Udi, interesting your opinion on boundary layer, i try to have info on this to reduce noise on a nozzle and unbind the layer by heat or micro-porous material is one of the solutions i investigate.

but the initial question is till interesting, imagine a heated surface, how behaves the air ?

a lot of new surfaces are investigated on swim suits, non adhesive surfaces, autocleaning surfaces... not to hijack this, just to say "please go on"

thanks