[GSchuld]

Paul Lipps (Elippse) made a mention of using carb heat to reduce engine rpm during cruise speed while keeping the throttle at WOT. The source is the thread in the propeller section below:

http://www.vansairforce.com/community/showthread.php?t=66424&page=2


Since many who may be interested in Paul's thoughts on the subject may not have noticed it in the propeller section, I'm starting this thread in hopes to continue the discussion in this section under it's own subject heading. Hopefully Paul will not mind my copy/paste of several of his posts from that thread.



Post 15:

Typically, a FP prop can be more efficient in converting horsepower into thrust, because it can have a more efficient tip and, especially, root planform. But as you say, by using the prop control to keep rpm low with high MAP can give more efficient flight. The trick with an FP is, instead of reducing power with the throttle, which gives less efficient engine operation due to the pressure drop across the throttle plate, is the use of carb heat, if available, to reduce engine power while at the same time increasing the engine efficiency due to the higher input temperature.
The heated induction also allows better fuel atomization and better leaning. On my plane I designed my latest prop to give 3000 rpm at WOT at 14,500 dalt. This gives me much more rpm for takeoff and climb. My climb rpm at best rate is now 2750, 50 rpm below rated. So by designing for more rpm at high altitude, I get all of the benefits of the CS for takeoff and climb, and by use of carb heat I can still keep the rpm down in cruise. On two recent flights, I was getting 4.9 gph at 180 mph TAS at 14,000 dalt! I know that this FF and TAS was real because my GPS said that it took a total of two hours for 341 miles and I used 10 gallons.


Post 17:

The "ram" effect, that is the conversion of dynamic pressure to increased manifold pressure, is necessary when you want to get more power and decrease induction losses. My curved divergent submerged NACA duct is giving me total pressure recovery when it's open, even with a K&N filter. When I close it carb heat enters the engine to reduce power but increase effciency.
To keep the rpm at or below rated requires more pitch, which means less static rpm and thrust, a longer take-off, and reduced climb rate. So I chose the route of lower pitch for better take-off and climb, and more speed if I want it, but when I want more economy I use the carb heat. On both of those two way trips wind wasn't a factor. I'm just trying to get the best all- around performance from the plane and prop.
I haven't tested for this yet, but according to my estimates, I will get 220 mph TAS at 3250 rpm at 1000' dalt if I want. My ROC is over 1700 fpm at 2750 rpm, 1000' dalt, 1350 lb, 105 mph IAS. Not too shabby for 125 HP O-235! With my previous 3-blade I was getting 1550 fpm at 2410 rpm at the same conditions, and a measured 213 mph at 2950 rpm, 1000' dalt. This prop design lets me trade off speed and performance or economy. Using the 10% above rated rpm rule, I should be able to turn 3080 rpm all day with no problem.


Post 20:

Hi, George! Yes, it looks exactly like Tony Higa's except three-blades rather than two. I don't get as much heating with my set-up since I get heated air from the inner fins of the front two cylinders rather than an exhaust muff and I think my valve in the inlet is leaking somewhat. I've seen 18C carb temp when OAT is 4C, so I'm getting about a 25F rise which would decrease power 2.5%.


Post 22:

Quote:
Originally Posted by Flybuddy2
Colder, denser air increases engine power. The reduced fuel burn via carb ht is solely a result of (another method) of reducing power. A prop spinning at 2750 in cruise is inefficient and not properly loaded for normal cruise power.

Yes, the density goes up 1%/5.2F decrease, but the engine efficiency goes down 1/2%/5.2F decrease, so that the overall effect is that the engine power increases 1%/10.4F decrease. If you'd like, I can cite this effect in C.F. Taylor. I'm not sure what rpm factor that you used to make the statement about a propeller not being efficient at 2750 rpm in cruise; can you cite your source?
So far I've designed props for 3750 rpm for biplane racing, 4500 rpm for IF1 racing, 7250 rpm for one UAV, and two different UAV designs at 6500 rpm. The one thing that has always stood out with my designs is that they have almost no noise, so that power isn't being converted into noise. This has been demostrated in an anechoic chamber test of one of my UAV designs which has demonstrated remarkably low noise!. This is very important with UAVs as it helps them to avoid detection by the bad guys!
In my propeller design equations the only thing that has anything to do with propeller efficiency is lift distribution, parasite drag, and mass flow. If you know of something that I am missing, I request that you let me know as I am always trying to improve my product.


Post 24:

Any time the throttle plate is not wide open there will be a pressure drop that the engine must use power to overcome known as pumping loss. That's why if you reduce power by the use of carb heat, which also makes the engine more efficient, you can operate at WOT. The advantage of a CS is that you can reduce power by decreasing rpm through use of a higher pitch with WOT. These engines have a very flat torque curve, almost constant torque around the rated rpm, which is why the engine power is related linearly with rpm. More rpm, more power, and more friction loss; less rpm, less power, and less friction loss.
Rubbing friction is known as coulomb loss, and the friction FORCE is almost constant, so the friction POWER is force X rpm; the higher the rpm, the greater the friction power. Friction power goes up with rpm as does engine power; they are locked together. It's all a trade off! That's why Reno racers run their IO-360s at 3200 rpm-3750 rpm and O-200s at 4100rpm-4500rpm.
But the propeller efficency can still be high at high rpm if properly designed!


<END>

Paul,

Have you recorded any data that shows how much of a fuel burn improvement you are experiencing when reducing rpm with carb heat at WOT at higher cruise altutude (say above 10,000ft.) compared to keeping your submerged ram air inlet supplying cool pressurized air while reducing rpm with throttle setting only. Just curious...

George

 

[David Paule]

The engine efficiency while using carb heat might be degraded by the carb heat installation and the pressure recovery across the inlet, heater muff, and plumbing back to the carb. It's not necessarily better than the loss across the throttle. It might be for a particular installation or it might not be. It's not something to make a blanket statement about.

And most of what carb head does, aside from using a different air flow path, is to raise the density altitude of the engine operation without actually climbing. There's a bit of heat recovery, too, which might be worth trying to account for - bet that's too obscure to isolate, though.

The increased density altitude means that the engine develops less power and uses less fuel, of course. The prop is still operating in the real density altitude that you're flying. You can design a fixed pitch prop to perform at its best at that condition or any other condition, but you need to be clear what it's design point is with the buyer. For example, if his hope was to win the short take-off contest at Valdez, he might prefer a different design point.

Dave

 

[hevansrv7a]

A point of confusion (mine)

If I understand Paul's various statements, this summary is right:

1. Higher rpm's are mostly less efficient although you can design some of that out of it. This is about the prop.
2. Reducing power with intake heat (versus throttle plate) helps by reducing pumping loss and perhaps by increasing engine temp.

OK, here's my problem:

• If you reduce power (HP) with fully open throttle via intake heat and yet want to maintain speed, you must use higher RPM to get the same HP or speed because you must have a finer pitch to do it. Again, this must be designed into a FP prop. With a CS prop, this applies directly.
  
• Reason: for a given airplane on a given day, the same speed at the same altitude requires the same thrust power, right? Equally, for a given engine at a given altitude, if you reduce Mean Effective Pressure, you must increase RPM's to have equal power (ignoring friction delta). If you are already WOT and then reduce power with intake heat, you reduce MEP; you must either slow down or adjust pitch.
  
• This seems like adding efficiency to the engine while subtracting it from the prop. Paul may have the numbers to show the beneficial inequality here, but I don't have them for either way. I am ignoring mixture since it should be adjusted for each case.
  
• The increased efficiency from a hotter engine is about the amount of energy lost to cooling. I doubt that intake temps affect that very much all by themselves. Paul pointed out that engine efficiency has a net gain of about 0.5% per 10 degrees F ambient as a result of thinner air (good for equal MP) versus cooler air (bad for thermal efficiency).
  

Seeking clarification...

 

[elippse]

If Paul pointed out that engine efficiency has a net gain of about 0.5% per 10 degrees F ambient as a result of thinner air (good for equal MP) versus cooler air (bad for thermal efficiency).
[/LIST]Seeking clarification...

First, friction is a constant force regardless of sliding speed. That means that friction horsepower is linearly proportional to rpm, as is horsepower at WOT, since torque is pretty constant over the range of rpm we operate these engines at. So it turns out that basically friction horsepower is a constant percentage of torque horsepower at WOT, ignoring small second-order effects.
As to clarification of the relationship between induction temperature and power, I will point out again the little formula in the upper-left of a Lycoming power chart that shows that the power correction for temperature is the square-root of the ratio of the standard absolute temperature divided by the actual absolute temperature. Now everyone knows that according to Boyle's law, the density of a gas increases inversely in proportion to the ratio of the absolute standard temperature divided by the absolute actual gas temperature, and as you lower the induction air temperature, the charge density goes up, which it does. In other words, if the induction temperature drops 5.2F the density increases 1%. But the Lycoming correction doesn't show that, does it? It shows that the power goes up 1% with a 10.4F drop.
So if you get density and power going up 1% with a Ts/Ta drop of 1%, but engine efficiency and power dropping off 1/2% with a Ts/Ta drop of 1%, you end up with Lycomings correction factor, which is what CF Taylor writes about. 'Doesn't seem right, does it?

 

[hevansrv7a]

Not clear yet.

First, friction is a constant force regardless of sliding speed. That means that friction horsepower is linearly proportional to rpm, as is horsepower at WOT, since torque is pretty constant over the range of rpm we operate these engines at. So it turns out that basically friction horsepower is a constant percentage of torque horsepower at WOT, ignoring small second-order effects. OK, but torque is torque. What is the definition of "torque horsepower?"This means there is more friction loss at higher RPM for the same HP, assuming you can increase BMEP to that HP can be equal which in turn requires a CS prop.Since it is agreed that the faster rev's are less efficient for the prop and less efficient for the engine, then slower rev's if you can to it, for the same HP are better.

As to clarification of the relationship between induction temperature and power,
This is where I fall off the wagon because I think that Lycoming is talking about ambient temp, not induction temp, except by assuming that ambient controls induction and we are talking about altering that relationship on purpose.
I will point out again the little formula in the upper-left of a Lycoming power chart that shows that the power correction for temperature is the square-root of the ratio of the standard absolute temperature divided by the actual absolute temperature. Now everyone knows that according to Boyle's law, the density of a gas increases inversely in proportion to the ratio of the absolute standard temperature divided by the absolute actual gas temperature, and as you lower the induction air temperature, the charge density goes up, which it does. In other words, if the induction temperature drops 5.2F the density increases 1%. But the Lycoming correction doesn't show that, does it? It shows that the power goes up 1% with a 10.4F drop.
You have to remember that the correction is applied to equal MP, too.
So if you get density and power going up 1% with a Ts/Ta drop of 1%, but engine efficiency and power dropping off 1/2% with a Ts/Ta drop of 1%, you end up with Lycomings correction factor, which is what CF Taylor writes about. 'Doesn't seem right, does it?

First, let's all chip in to send someone to California to tutor Paul on using paragraphs, bullets, etc.

OK, the real questions are inserted into his text, above.

 

[elippse]

First, let's all chip in to send someone to California to tutor Paul on using paragraphs, bullets, etc.

OK, the real questions are inserted into his text, above.

What is torque HP? Well, torque X 2 X pi X rpm / 33,000 = horsepower.

It is absolutely NOT agreed "that the faster rev's are less efficient for the prop". I'm not sure where this bit of nonsense came from, but it is definitely not based on reality!
I just designed a model airplane prop that turns 29,000 rpm at 200 mph, and I've designed props for UAVs that turn 6500 rpm and 7250 rpm,, plus my designs for IF1 race planes turn 4500 rpm, and for biplanes turn in excess of 3500 rpm. One of my UAV designs was recently tested in an anechoic chamber and gave 1/4 the noise of the prop it replaced, and even that amount of noise was due to the prop not being-in and sealed to a spinner.

How about you send someone to teach me about paragraphs and bullets and I'll send someone to teach you about power and propellers. Agreed?

 

[hevansrv7a]

First teach me to read..

Paul said: "How about you send someone to teach me about paragraphs and bullets and I'll send someone to teach you about power and propellers. Agreed? "

I must have mis-read what he had said, earlier. My bad. First, seek to understand.

Just to test the idea, I used Andy Bauer's program to design three props.
Each prop is 72" with a standard Van's spinner, uses 135 BHP at 8000' to go 200 mph. All were 2-blade props.

Prop #1 runs at 2500 RPM. The program predicts 89.6561% efficiency.
Prop #2 runs at 2700 RPM. The program predicts 89.9444% efficiency.
Prop #3 runs at 3500 RPM. The program predicts 90.5567% efficiency.

There are some folks out there who believe the opposite, but I'm convinced that Paul and Andy are in agreement on this point and so, I'm a believer.

Some interesting observations:
On the one hand, with higher tip speeds you might expect higher tip losses from induced drag. On the other hand, as the RPM's increase and the length remains constant and the HP remains constant, the prop area must decrease and that means, by definition, smaller chord and greater aspect ratio. Aspect ratio is a factor in induced drag. Hmm. Also, the air being accelerated by the prop is accelerated 39.6% more at 2500 than at 3500 and the mass being accelerated is thus greater for the higher RPM. This is all for equal BHP and equal TAS, remember.

So, don't you wonder (I now do) why the auto-engine guys are gearing down their prop speeds?

Some days it gets really interesting.

Thanks, Paul.

 

[SHIPCHIEF]

I'm more interested in the efficiency gains from running carb heat. The prop that I have 'is what it is'. This is something I can do with the existing equipment.
In a previous post someone mistook power for efficency.
I'm thinking of reduced power output and reduced pounds of fuel per brake horsepower hour. That equates to more distance over the ground per dollar.
The variables:
1) Increase intake air temp using engine waste heat.
2) lean the mixture.
3) Advance the ignition @ low charge air densities.
How, in a practical way, is this done? Leaving the throttle wide open, apply 'some' carb heat, then lean some, then do it again until...what?
The airspeed slows down to the desired power delivery?
What are the limiting factors for the engine? Will excess intake temp cause any engine problems like detonation or ring scuffing?
Do you need ignition advance, like LightSpeed? Should you have and induction temp sensor? Exhaust O2 sensor? How to read the EGT & CHT?
Some one mentioned potential flow restrictions in the air heat system, this could be designed with the intent to reduce pressure loss, even include the pressure recovery system.
I guess I'm asking about operational guidelines and the best air preheat layout.

 

[elippse]

"On the one hand, with higher tip speeds you might expect higher tip losses from induced drag. On the other hand, as the RPM's increase and the length remains constant and the HP remains constant, the prop area must decrease and that means, by definition, smaller chord and greater aspect ratio. Aspect ratio is a factor in induced drag."

Actually, aspect ratio would never be used in a precision propeller design program, since the induced angle of attack would be arrived at through momentum and force equations. AR is just an attempt to modify the lift and drag of an airfoil for less than an infinite wing-span. The equation for induced drag is: W^2 / (Q x B^2 x pi x epsilon); no AR here! You can rewrite this equation to substitute AR instead of B^2 but then you have to add wing area, so AR is not necessary to arrive at induced drag.
I'm not really sure how you would even go about calculating AR on a propeller since implicit to using AR implies that the flow and dynamic pressure is everywhere the same which on a propeller it definitely isn't.
A famous phrase that is used with computer programs is GIGO, garbage in, garbage out! This can refer to both the input data and also the program assumptions. I can take a program that predicts a linear trend from a few data points and use it with a restricted set of data points to show that the earth will heat up 500F in the next twenty years or reach absolute zero in the same time span. Neither the computer program nor the data set is appropriate to the problem at hand. Computers can give you an estimated performance that can be carried out to umpteen decimal places and because it gives you an excellent goodness of fit for the data makes you think that you've got the world by the tail. But if the program is leaving out some important considerations or is making assumptions, such as you see in calculus where certain terms are left out, it will lead you astray with incredible precision. I know, I've worked with real-time programs since 1959!
Induced drag is not, repeat not, a loss! The so-called induced drag is the price of generating lift or thrust. If you didn't have it it would mean that you have perpetual motion.
The only real loss that you have in propellers is due to parasite drag and the whole propeller span is not operating at the same L/D. This comes about when the thrust is not properly distributed along the span, as is typical with most designs, in which almost all of the thrust is in the outer 25% of the blade, so that the inner portion is just along for the ride. Along with this is a crappy, non-aerodynamic shape in the prop root, and blades with way too much area near the tip where the parasite drag coefficient at high Mach is reaching for the heavens. Then you have the additional drag that comes from having big holes in the spinner surrounding the blade. So all of these effects take away from a theoretical propeller efficiency to one that is real world.

 

[hevansrv7a]

What I said vs what you thought I said

Yes, I did refer to the AR of the prop. I did not say it was used to compute the prop. It isn't. It's merely a result of the design. It was an observation about the results of the design program, nothing more.

You said "The only real loss that you have in propellers is due to parasite drag".

I am not the expert on prop design, but I just want to point out that you and Jack Norris disagree strongly on that point. He, interpreting BGT, believes that induced losses at the tip are critical.

Since your tips and his are both tapered and differ only in the precise shape and proportion of the taper, I suspect that your design ideas are not as far apart as your words, But heck, I don't understand either one of you!

 

[David Paule]

Back in post #7, the prop design program mentioned is apparently not one of the good ones. The 3,500 rpm prop has the blade tips going supersonic... That's going to reduce the efficiency a bit, don't you think?

Dave

 

[elippse]

Mach 1.0197 at 70F, and that's just the very tip speed, not to say anything of the critical Mach over the airfoil inboard of the tip!

 

[hevansrv7a]

You are correct, Dave

Back in post #7, the prop design program mentioned is apparently not one of the good ones. The 3,500 rpm prop has the blade tips going supersonic... That's going to reduce the efficiency a bit, don't you think?

Dave

You are right, Dave! The program does not look for mach#. I was looking for how RPM affects efficiency and ignored it (I should not have). I confirmed with Jack Norris that the program does not look at mach# because you are not supposed to design for that extreme. I could run the same program with a smaller diameter prop but the result would still be the same, proportionally. The program accepts as input both diameter and rpm so they have to be decided before it runs. It's up to the operator to avoid that error.

There are props on F-1 racers at Reno turning over 4000 rpm and staying sub-sonic. This finding is of value in looking at them, for example.

According to Jack's book, speed of sound is 1116.46 f/s at standard conditions and .9 of that would be about 1,000 f/s. for a 6' prop turning 3500 rpm, the circumference is pi*D = 18.85'. At 3500 rpm, that's:
3500 * 18.85 /60 = 1099.6. Therefore the example was flawed. It gets worse at higher elevations because it is colder.

 

[SHIPCHIEF]

So what about a little topical drift?
Isn't this thread about operating with carb heat at cruise power?

 

[David Paule]

In addition to the tip speed due to prop rotation, don't forget the forward speed component. Also the difference in the speed of sound at 8,000 feet per the example (it's about 31 feet per second slower there on a standard day.)

And in answer to the previous (no doubt hypothetical) question of why use a reduction gear, it's to allow a longer propeller at a lower rpm. What's the advantage there?

 

[zav6a]

Another operating mode

There was an interesting article in hot rod a few months ago about Smoky Yunick, I think, experimenting with salvaging waste heat to increase induction temp and efficiency. I think I recall them going for several hundred degrees induction temp. Our engines ought to be able to tolerate at least 250 inlet at low loads based on what turbo engines run.

 

[David Paule]

Our engines ought to be able to tolerate at least 250 inlet at low loads based on what turbo engines run.

Since that would drastically reduce the power available, even ignoring preignition, it might make more sense to either install a lower power engine in the first place, or a variable pitch prop, or both. Preferably both.

Since this discussion is nominally about the efficiency of running with carb heat on versus reducing power with the throttle, let's add in another option: just fly higher.

Dave

 

[L.Adamson]

I'm not buying all this...........

First off, I found that I prefer throttling back, rather than running WOP, and pulling the rpms back on my Hartzell C/S prop. A friend gets great efficiency fuel wise, by pulling the throttle back on his 0320 RV9A with C/S prop. I typically don't prefer flying that slow.

What I'm really not buying into.................is the throttle plate restricting the pumping action .......pressure or suction-- depending on how you want to define it. Sure, I've read it (about throttle restriction) in several articles, but I don't believe everything I read. I want real comparison proof,... as to gained efficiency.

I work with heating and air conditioning systems. If the required load is "smaller", then the cfm output of the blower will be less, and the duct work to handle it, is smaller in area, as well. When we run our Lyc's, etc. at a slower rpm, the cfm's through the throttle plate passage would be less. The way I figure it, a partially closed throttle plate would still allow enough cfm to keep static pressures in check.

This would be the same as a smaller duct, for a smaller air conditioning blower unit. And in the case of heating/ AC systems, the accumulated static pressure (resistance of duct, filter, coils) would be in check and everything just fine.

L.Adamson --- RV6A

 

[elippse]

Since that would drastically reduce the power available, even ignoring preignition, it might make more sense to either install a lower power engine in the first place, or a variable pitch prop, or both. Preferably both.

Since this discussion is nominally about the efficiency of running with carb heat on versus reducing power with the throttle, let's add in another option: just fly higher.

Dave

I agree; fly high! But that's because the major use of my plane is for cross-country flying, so I get up to 11,500 or 12,500 where the air is cool, clear, and smooth and has very little traffic. In my case I designed my prop to give me 3000 rpm at 14,500' dalt and I use the carb heat to reduce power and rpm.

The advantage of this high an rpm, for me, is that my static and climb rpm are much higher giving me enhanced takeoff and climb performance. 'Course a lot of people, especially those on this forum, just like to fly around at lower altitudes and have a lot of fun. For them this kind of thing is not what they want.

But the ones who race, such as the SARL crowd, where speed is king, might want to invest in a racing prop for use while racing, and go back to their cruise prop for the every-day kind of flying. Heaven knows that they spend enough time and money on little things to eke out that elusive 0.83 kts that they lost by last year that for them a very efficient race prop might be just the ticket. Every 3% increase in rpm gets you at least 1% more speed.

Then there is also the fuel-mileage competition at Copperstate, where again, ekeing out that extra mpg can make you a class winner! This is also where the use of induction heat and its efficiency benefit plus better lean operation will shine.