Larger diameter prop = more thrust ?
With the height of the front of the airframe that I have, looks like I could comfortably mount up to a 16" diameter prop. But with engine size compatibility for this plane I would only be allowed a 13"-14" prop.


Mando..........

That tells me that you can't put much faith is ANY of those specs.....
[X(]

If I had my .82 off of the plane, I'd just weigh it and post the results.

Prop efficiency is Thrust HP devided by Brake HP. Thrust HP is basically the thrust multiplied by the RPM, so you can basically divide thrust by the torque the engine is putting out. So airspeed is related only as far as it affects thrust. The low pitch prop will have little thrust at airspeed and will have a lower efficiency at speed than in climb and static. The high pitched prop is just the opposite. Since we are talking about increasing pitch and reducing diameter to keep the engine at the same RPM the high pitched prop will have more thrust and higher efficiency. Airframe and engine will not make a large differance here.

And if you are turning both props at the same RPM the higher pitched prop will have more thrust.

Muffler weight on the .72 is only .6 ounces? Pretty light, most fourstroke mufflers I have seen for that size are just under 2 ounces not less than one ounce. I suspect the .72 weighs 15 ounces without muffler and the muffler weighs 1.6 ounces.

Sorry, this is wrong. It has nothing to do with RPM.

http://www.answers.com/topic/thrust-power
http://www.auf.asn.au/groundschool/propeller.html

The actual airspeed achieved is the important figure. The theoretical thrust of a prop at a certain airspeed is insignificant if the aircraft never reaches that speed. You could have a model that is draggy enough that a high pitch prop will cause it to fly slower. Not only that, take off performance would be miserable.

It all depends on how much drag the model exhibits, desired airspeed, and available power. You won't make a DR-1 fly any faster or more efficiently by putting a square prop on it, unless you can reduce the RPM to allow a larger diameter at the same HP input. This is what BLW is eluding to.

You are right. I got axial speed mixed up with RPM

Its not the airspeed of the plane, its the increased airspeed of the air going through the prop

Nothing wrong here. Except the prop efficiency is the thrust time increased propeller airblast divided by horsepower. If the engine on that DR-1 is red lined and has more power left, then increasing the pitch will increase thrust, and prop speed, and will go faster, though probably by a very small amount. The prop will have a higher efficiency because it will have higher thrust. As for my model a larger prop makes it climb worse and fly slower, though this is because the engine cannot turn it as fast. A smaller prop with more pitch will make it fly faster but climb slightly worse using about the same engine power, however it should be more efficient though less effective.

Greg- thanks. Airspeed is important as it is calculated with the speed of the prop.

Sport pilot- the arguing is getting to be a drag here. I didn't want to dig out any books about it. You do factor in output power divided by shaft power input for thrust. But, that is only the beginning and you are fixated that this is the end of it. In short, it is thrust x velocity divided by thrust x velocity (prop) times velocity (airspeed). This is where the higher prop velocities of the smaller prop brings down the efficiency, as you already argued against.

OK, you are right and text books are wrong. If you didn't read or even browse the second link I posted, read the first paragraph under section 5.2.

This is getting old. Why do I bother trying to correct people's misconceptions on these stupid discussion forums? Especially when people don't care to listen. Why? So the others that think they are learning something, can learn the truth instead of repeating opinion, hearsay, false truths, half truths, etc......

If you are stating opinion please precede it with the usual internet method of, "I heard", "my friend told me", "I read online", "I think", etc.

If you are stating fact quote your source or experience.

There is a method to calculate power from thrust and air velocity, but this isn't thrust power as in the calculation of propulsive efficiency.

Prop efficiency is THP/BHP thats it. Another formula is (Thrust * Axial speed) / (Resistance torque * RPM). The factors you mention do affect the BHP but since we are talking about two props being turned by the same engine at the same speed then the BHP is the same. You are saying the highere velocity brings the efficiency down, but it doesn't it brings it up. You are confusing the tip loss with high RPM. This is not a factor here as both propellers are turning at the same speed, in fact the tip velocity of the smaller prop will be less so there will be less loss there also.

Edit xx Getting confused here. If a prop of the same diameter is used and turned at a constant speed then the thrust goes up as well as the velocity of the prop blast with increased pitch. Buy if the prop is made smaller to keep both speed and power equal then the thrust goes down and the speed goes up even more than in the first case.

Using the Thrust HP calculator if one is to turn a 12-6 at 10,000 RPM then the Speed is 56.83 MPH, HP is .889, and thrust is 6.22 Lb. Turning a 11-8.5 at 10,000 RPM results in; Speed = 80.49 MPH, HP is .889, and thrust is 4.39 Lb. Thrust x HP is almost the same with the smaller prop being slightly less, thus vey slightly less efficiency when static. However at an aircraft speed of say30 MPH then the speed (leaving velocity - incoming velocity) of the larger low pitch will only be about 27 MPH (likely better because the angle of attack would improve) and with the smaller pitched prop the velocity differance is 50 MPH. Thrust should come down about as much as the velocity; so at speed, the smaller higher pitched prop should be more efficient. I am sure there are times when the opposite is true, but most of the time I think the smaller higher pitched prop will be more efficient except when static.

I have been refering to a textbook as well as the internet. That second link doesn't work for me.

New, out of the box, Saito .72
Engine weight = 16.5 oz.
Muffler = 1.7 oz.

Yeah, I'd say the .72 and .82 Saito weigh very nearly the same. I don't want to take one of my .82's off a plane just to weigh it, but I'd say it is probably very close to your .72.

However, since I now have a brand-new 1.25 that followed me home, I thought I'd compare the actual weight to the listed specifications:


Looks like my 1.25 weighs pretty close to what the specs say!

Saito .82/stock muffler, Saito 1.25/TurboHeader and Saito 1.25/stock muffler, all nuts and washers in place plus home made velocity stack on the 1.25.