More power always better?

[Stevey]

Hey everyone,
Sorry its been so long since Ive been here, Ive been VERY busy! Ive had a question bothering me for some time now. Is there ever too much power in an airplane? I mean is there ever a point when a propeller simply cant transfer any more power into thrust? . I realise certain propellers have certain limitations, but say you had a race propeller made for a 3500hp plane. Is it possible to transfer that power into thrust efficiently? I also realise big planes like C-130's have lots of power they transfer but Im talking for speed, race wise.

If this isnt possible what is the most feasible power output? One more question, what is the most efficient rpm for an airplane to run? I realise again this depends on the prop but Im looking for an average. I was once told the prop tips attempt to go supersonic at 7000rpm. So would a good speed rpm for a race plane be... 4500-5500?

Thanks Steve

[Peashooter]

Re: More power always better?

Who boy, where to start?

[Skyracer]

Re: More power always better?

Who boy, where to start?

Maybe with high school geometry and physics?

[Mluvara]

Re: More power always better?

Propellers are a very interesting field and certainly are very dynamic. The biggest issue facing racers is running the tips up to or near mach. With high forward velocity and high rpms, the tips can be near mach, and the prop will become less efficient. The mach all relates to diameter and forward speed, so there are two velocity factors involved.

The answer for RPM comes down to both the motor and the propeller diameter, and relates to mach. A propeller can basically be designed for whatever RPM you choose. At Reno (with the exception of smaller aircraft like bipes, F1's, and sport class), there are very few who choose to design their own props. Some have tried in the past on the unlimiteds and they haven't been successful. This isn't to say that it can't be done, but the expense of such an experiment is outside of many budgets.

Michael

[AAFO_WSagar]

Re: More power always better?

Propellers are a very interesting field and certainly are very dynamic. The biggest issue facing racers is running the tips up to or near mach. With high forward velocity and high rpms, the tips can be near mach, and the prop will become less efficient.

Mike, how much does "forward velocity" play into tip speed? I know I've heard many transonic tips on both float planes going pretty slowly (forward velocity)and T-6's without a "great" amount of "forward velocity"..

I'sn't tip speed independent of forward velocity?

[Mluvara]

Re: More power always better?

Propeller tip velocity (rotational) and airspeed (axial) velocity are two components of propeller mach number. You can calculate the rotational velocity of the tip and then get a number that appears to be well below mach and wonder why you are making so much noise. Well, then add the forward (axial) velocity and you might just be in a huge inefficiency area.
I haven't run the numbers on a wide variety of props, but with the larger diameter you have the easier it is to get up in mach number.

In geek terms:

Vtip=(Vaxial^2+Vrotational^2)^1/2
or
Tip velocity equals the square root of the sum of the squares of axial and rotational velocities.

Here's a reference for anyone wanting to dig further.

http://www.mh-aerotools.de/airfoils/pylonprops_1.htm

Michael

[Stevey]

Re: More power always better?

Maybe with high school geometry and physics?

I have high school physics. I dont remember the section on calculating the efficiency of propellers and tip velocity though. Must have skipped that class. There are some things you cant learn well enough sitting in a classroom you must go out into the "realworld" and search for or experiment with "realworld" answers.

Thanks for the help Michael! So basically if you sift through all the designed props out there or even if you had one custom made there is a prop to handle basically any amount of power. One more question, if the diameter needs to be smaller so the tips do not go above mach wouldnt that mean in having a smaller propeller you wouldnt be pushing as much air? Thus providing less thrust.

Sorry to all of you who think my question is dumb, some of us havent had the pleasure of being around airplanes alot or learning about them. This is one area I have never even started learning until the other day.

Thanks Steve

[G. van Dyk]

Re: More power always better?

if the diameter needs to be smaller so the tips do not go above mach wouldnt that mean in having a smaller propeller you wouldnt be pushing as much air? Thus providing less thrust.
Thanks Steve

I'm sure others will add or correct my comments as required.

The engine RPM you want to run at is determined purely by the horsepower curve of the engine. You need to run the RPM where the curve is at it's highest, any higher or lower RPM reduces the horsepower an engine can produce. However doing engine mods can and will change the height and location of the curve on the graph.

The constant-speed propellor is the secret. The prop will change pitch to keep the engine at it's optimum RPM. When the pilot increases power with the throttle, the prop pitch increases and he goes faster, till the airplane's drag matches the horsepower being made. To answer your last question, you're mostly correct, the smaller diameter will not push as much "volume" of air (read mass or weight) but it affects acceleration and not speed. Think of the early jet airplanes where the engine would push a small volume of air (think of the intake size on an F-80) at high speed. The airplane would take forever to get off the ground but the top speed was much higher then the prop driven fighters. If the small diameter prop can run at a course enough pitch you can keep accelerating to the limit of the airframe. Here you're talking about efficiency and that's where the aeronautical engineers earn their money.

Hope this helps
Cheers
Gerry

[Skyracer]

Re: More power always better?

I have high school physics. I dont remember the section on calculating the efficiency of propellers and tip velocity though. Must have skipped that class. There are some things you cant learn well enough sitting in a classroom you must go out into the "realworld" and search for or experiment with "realworld" answers.

Sorry to all of you who think my question is dumb, some of us havent had the pleasure of being around airplanes alot or learning about them. This is one area I have never even started learning until the other day.

Thanks Steve

No such thing as a dumb question; just dumb answers. My response was more in response to Peashooter's question, "Who boy, where to start?", than a response directed at your questions/assumptions. Having been through high school physics myself, you are right (I don't remember anything about calculating prop efficiency either). But I do remember a little bit about fluid dynamics, speed of sound, etc. With a little practical application of what I remember from geometry class, it is pretty easy to see that the tip speed of a prop of a given diameter turning at a given RPM will be less than the tip speed of a prop of a larger diameter turning at the same RPM...and, throw in what we remember about fluid dynamics, speed of sound, etc., from physics class; now, we have a starting point!

Best thing to remember about props (and a whole lot of other things about air racing in general) is that very little can be carved in stone. What is, will get you "in the ballpark"; beyond that it takes a lot of tweaking, tinkering, and testing.

[Peashooter]

Re: More power always better?

In theory, a prop can always be developed to handle the power of the engine that's intended for the airframe. Sometimes that challenge is making sure that the airframe can handle the prop. An historic example is the bent wing of the Corsair that was designed to accomodated the 13' diameter of the prop. Another is the larger tail volumes required on the contra-prop-equipped Mustangs Red Baron, Precious Metal and the hybrid Miss Ashley II to maintain directional stability.

A great example of propeller efficiency and the losses due to transonic tips speeds at Reno is found with the Wright R-3350 as applied to most Sea Furies and the Rare Bear. Listen to the videos (e.g., Valley of Speed -- search Reno Air Races on YouTube) of say Spirit of Texas or Riff Raff versus the sound of September Fury and Rare Bear. The latter two don't have the transonic "howl" of the former because they have the "slow nose case" gearing that slows the prop RPM and allows a more efficient use of the available power. Also note that September Fury and Rare Bear are also about 10% faster than the non-slow nose case-powered airplanes.

My personal favorite, as sounds go, is that of the 3-bladed P-3 prop (actually a hybrid of parts) on Rare Bear. Instead of a raspy howl at speed, it produced a low frequency "whump-whump-whump" that you could hear and FEEL as it flew past at race power. You can sample this as well by watching a C-130 or P-3 Orion takeoff. The power in each case is about 4,000 hp plus or minus -- plus, actually -- but in all cases they produce that bass response that you can feel in your chest when you're close by.

Those of us who fly fixed pitch props on small aircraft live a life of compromise. Prop selection is like riding a one-speed bike -- do you want accelleration or top-end speed? I currently have a great solution on my Rutan Long EZ; a Hertzler "Silver Bullet" prop. It accellerates and climbs at about 80-85% of what a dedicated "climb prop" (i.e., flat pitch) would provide, but it's also the fastest prop I've had on the airplane.

As I've stated to Gary Hertzler more than once, that prop is magic!

[Stevey]

Re: More power always better?

Ok, thank you all for your responses. I do believe I understand a lot better now. Ive also been reading about props and how they perform etc. The reason I ask all of this is I may have been given the chance to build a plane. A racer to be specific.

I am debating over wether I should do it or not and the last few months {on top of years before that} have been dedicated soly to research {and working but you know what I mean}. Im quite sure I can do it skill wise, I used to work with composites {fiberglass, kevlar, carbon fiber}, I have experience welding and doing sheet metal work, I have experience doing machineing {lathe and mill} and I also build engines {mainly V8's for muscle cars/drag cars}.

I realise I will need to learn alot as I go along specific to airplanes and I know that it would most likely take at least a couple {more I bet} years to do. I have done some designing and had it approved by a aeronautical engineer, I have thought over what I would use for a powerplant. Ive even built a small wind tunnel and built a model of the plane and did some testing. To be honest Im scared of the time commitment {between work and girlfriend} and Im scared I may fail, which would mean my friend/advisor would lose his money. I rebuild cars and build boats.... not planes . What do you guys think?

Thanks Steve
PS: This man said he wouldnt give all the money but he will be giving me a very good jumpstart and putting me in contact with sponsers.

[TOR_DP]

Re: More power always better?

Slow nose cases are really to get the engine RPMs up to the peak HP/TQ of the engine.

Most props don't vary their RPM much (if at all)...

For example: In your (hypothetical) race Mustang, you enter at the guide pylon at full throttle (and plan to leave it there...) and control the engines' power via the prop control (pitch angle from the prop govenor) and monitor the power via the manifold pressure gauge. As you increase pitch the power required from the engine to maintain the desired RPM goes up.

So given a desired prop RPM vs. anticipated airspeed = Diameter to prevent trans-sonic blades, how then can we apply more of the engines' energy to the air?

Alter the airfoil and chord of the prop blades...

By example:

Compare (on your own) the length vs width of the DC-3 prop on Steadfast and the length vs width of the prop on Race 57... and a standard Mustang... and Sept. Pops... and so on.

Most unlimited racers I've talked to just home the throttle and jockey the prop control/ADI/Nitrous/Secret Juice #267...

[Peashooter]

Re: More power always better?

Ok, thank you all for your responses. I do believe I understand a lot better now. Ive also been reading about props and how they perform etc. The reason I ask all of this is I may have been given the chance to build a plane. A racer to be specific.

I am debating over wether I should do it or not and the last few months {on top of years before that} have been dedicated soly to research {and working but you know what I mean}. Im quite sure I can do it skill wise, I used to work with composites {fiberglass, kevlar, carbon fiber}, I have experience welding and doing sheet metal work, I have experience doing machineing {lathe and mill} and I also build engines {mainly V8's for muscle cars/drag cars}.

I realise I will need to learn alot as I go along specific to airplanes and I know that it would most likely take at least a couple {more I bet} years to do. I have done some designing and had it approved by a aeronautical engineer, I have thought over what I would use for a powerplant. Ive even built a small wind tunnel and built a model of the plane and did some testing. To be honest Im scared of the time commitment {between work and girlfriend} and Im scared I may fail, which would mean my friend/advisor would lose his money. I rebuild cars and build boats.... not planes . What do you guys think?

Thanks Steve
PS: This man said he wouldnt give all the money but he will be giving me a very good jumpstart and putting me in contact with sponsers.

I've said it before, "Dreamers dream, doers do" -- go for it! If you've got the resources, the talent can be found to shepard ANY design/development issue -- likely right from this board. Tell the GF that you're both departing on an incredible adventure. (Which ain't no lie!)Peas

[Warren_C]

Re: More power always better?

I'd recommend that you join EAA, then find someone who is building an experimental a/c near where you live, then volunteer your services to that individual. Most folks that build kit a/c take years to complete them.

[BillRo]

Re: More power always better?

Although this discussion has been about Unlimited aircraft, Formula Ones regularly run their fixed pitch props to over 4,200 rpm and, at the speeds the fast planes fly (250 m/h), prop tip mach number is an issue. For most prop airfoil sections, drag rises significantly above about M=0.92 and it will limit the attainable rpm especially at cold temperatures.

As a crewchief I developed a table of prop diameter vs rpm at a fixed speed (240 m/h) to determine the prop tip Mach number. This table can be seen in the Formula One Design Guide at if1airracing.com under Library/Rules/Design Guide and it is in Figure 7. As was explained before the tip speed is the vector sum of the tip rpm and the forward speed, but the speed of sound varies significantly with temperature.

Formula One engine power is limited by the induction system size requirements (carbureter and valve seat sizes) to about 140 HP at about 4,200 rpm when it drops off. Sometimes people overspeed a little past at the end of the straight so that as the rpms drop off in the turns the engine falls back into the power band. If your airplane will only pull 3,900 rpm you can use a 56" prop and avoid tip mach issues. But if you want to run it out to 4,300 you had better cut 2" off each tip and thin the section.

More power always better? Bruce Bohannon used a Miller Pusher "Pushy Galore" to make time-to-climb records. There are no engine rules so Bruce added a nitrous system to provide about 200 hp? (a figure that should be possible with a 200 cu in engine). He was unable to make Pushy go much faster than 235 m/h which it did under IF1 rules. He concluded that the prop efficiency was degraded by the wing/tail wake that the prop blades hit three times every revolution. Lesson: if you want to use a prop driven pusher, make sure the prop disc is well away from the wing or tail wake.

BillRo