The International Cessna^® 170 Association

Interesting! My experience is 170's get a little faster with more load, especially to the rear. I understand this is because the downforce the elevator needs to generate is less. What are others experiences?

Bruce
I think it is more a factor of where the load is than how much it is. While hauling one of my plus size friends ( I'm no lightweight either) he decided to move his seat back to take advantage of the shade. Next time I looked at the airspeed it was 3-4 mph faster than we had been doing all day. Intrigued by this I had him move forward again and the airspeed drifted back down. When he moved back into the shade the airspeed went back up again. I'm sure if I'm wrong it will quickly be pointed out, but my conclusion was that moving from a forward CG to a more neutral
position caused the speed to increase.

When discussing cruise speed, keep in mind that any speed is a function of horse power. The airplane doesen't care if you develop that horsepower at 2200, 2400,or 2600 rpm.
The engine has a specific fuel consumption. That is, a certain amount of horse power per pound of fuel per hour.
For example, when you are developing 100 horse power the fuel burn difference between 2400, 2500, and 2600 rpm will only be the difference in the friction loss of the higher rpm.
The advantage in the thinner (lower pitched prop) is that you are able to generate the rpm necessary for a given horse power at a lower air speed.
The advantage of a longer, thinner prop is the prop efficiency of a large disc and high rpm at a slow air speed, such as during take off and climb.
There is not going to be a lot of difference in your fuel burn at cruise air speeds whether you have a 51, 53, or 55 inch pitch. There will probably be a noticeable difference in the noise level when operating at equal air speeds.

4583C wrote:Bruce
I think it is more a factor of where the load is than how much it is. While hauling one of my plus size friends ( I'm no lightweight either) he decided to move his seat back to take advantage of the shade. Next time I looked at the airspeed it was 3-4 mph faster than we had been doing all day. Intrigued by this I had him move forward again and the airspeed drifted back down. When he moved back into the shade the airspeed went back up again. I'm sure if I'm wrong it will quickly be pointed out, but my conclusion was that moving from a forward CG to a more neutral
position caused the speed to increase.

You're not wrong. When a conventional aircraft like the 170 is properly loaded, the CG is forward of the wing's center of pressure, where the wing's lift appears to be concentrated. The horizontal stabilizer and elevators develop a "down-force" in cruise that balances the C.G.

Since overall lift has to equal weight in order to achieve level flight, the lift generated by the wing has to equal the aircraft weight plus the tail's down force (this is oversimplified, but don't let facts get in the way of the truth!). If you move the C.G. closer to the aft limit, less down-force is required, so the wing doesn't have to generate as much lift. Now you can lower the nose and fly level at a slightly faster airspeed. This characteristic is normal for an airframe with a horizontal stabilizer in the tail.

Best Regards,

John

Blu Elder....I'm really swimming upstream here, (sorry to argue with my elders....), .....but.... that's not exactly correct.

"For example, when you are developing 100 horse power the fuel burn difference between 2400, 2500, and 2600 rpm will only be the difference in the friction loss of the higher rpm." You aren't forgetting about propeller efficiencies are you? Fixed pitch props develop highest efficiency only at specific rpm at specific air density. Changing the rpm changes hp losses. Perhaps you meant that SFC varies with throttle setting and density altitude?

All other parameters being equal,...more weight means more induced drag. For the same hp that means slower speed. If that wt is distributed anywhere but at the center of lift...more drag will be produced in order to balance the aircraft...and a loss of optimum speed will occur.

All other things being equal, there IS a difference in fuel burn versus prop pitch....dependant upon how you define "cruise speed". (Did you mean "resultant speed" (whatever speed results)....or did you mean "same speeds"?)

"When discussing cruise speed, keep in mind that any speed is a function of horse power." .... yes, and drag. Which is affected by weight and CG (which I believe is Paul's observation) and a few other things.

Pauls observation refers to the fact that the tailplane ordinarily produces downward "lift". This creates drag. Moving the CG such that minimum "downward lift" is necessary....results in less induced drag and therefore increase speed with no other changes.

Are we measuring with micrometers again?