The International Cessna^® 170 Association

It's a nice idea, and a fine piece of work, Bruce. Thanks for the contribution.

When will the takeoff/climb graph be ready?

By the way... I think it's important to realize something with regard to a graph such as this one:

If I am running at 2450 with FULL THROTTLE at 7500', and it results in a particular airspeed...... the fact that I change to a prop with a lesser pitch, say from 55 to 53,.... does not mean I will actually lose airspeed if I remain at full throttle.

Instead, what it means is, that I'll INCREASE rpm at full throttle. The NEW rpm will likely result in similar or possibly even GREATER airspeed. However.....this chart likely indicates that if I change props and keep the SAME rpm, my airspeed will result in a reduction as indicated in that chart. OR.... that a change in prop AND a change in RPM to that specified in the same ROW of the chart, will likely result.

Such a small change may not affect cruise speed as much as it will increase RPM realized during Take-Off.... and therefore will reduce distance req'd and increase ROC. It will also result in a greater fuel burn per mile travelled, and more piston travel within the cylinder, and likely increase engine operating temps.

(You can't spend money without it costing you.)

The factory's orignal choice of a 7653 prop for this airplane is probably the best compromise of all choices available. If you want to reduce trip fuel and reduce engine RPM at cruse, then a 7655 cruise prop will give such result, at the cost of takeoff/climb performance. If you want better takeoff/climb performance, then a 7651 climb prop will give you that, at the cost of additional fuel burn and increased engine RPM during cruise. The airspeeds achievable with such prop changes will be limited by the engine's ability to produce horsepower (thrust) within the achievable RPM limits. Again, think of a car with a 5-speed transmission. The standard 7653 prop is like cruising around in 4th gear. The 7651 is like doing so in 3rd, and the 7655 is like being in 5th.
A similar correlation will exist during drag-racing (as in takeoff/climb). The finer pitch 7651 prop will accelerate better than the 7655 coarse pitch prop.

Atmospheric conditions being the same, it will take a certain brake horsepower to give a certain speed in any given airplane as long as the arcraft configuraton remains unchanged. At a given mixture, fuel burn is directly proportional to induced horsepower developed. Induced HP less friction HP equals brake HP on the output shaft. The limited RPM variation usable in cruise will limit much variation in friction HP loss. Fuel consumption should be very little different with variation in prop pitch when flying at equal air speeds.
After installing the Cont. IO-360 engine in my '52 C-170B using a FP prop, my fuel burn at stock engine speeds was the same as with the stock engine.

gahorn wrote: If I am running at 2450 with FULL THROTTLE at 7500', and it results in a particular airspeed...... the fact that I change to a prop with a lesser pitch, say from 55 to 53,.... does not mean I will actually lose airspeed if I remain at full throttle.

Instead, what it means is, that I'll INCREASE rpm at full throttle. The NEW rpm will likely result in similar or possibly even GREATER airspeed. However.....this chart likely indicates that if I change props and keep the SAME rpm, my airspeed will result in a reduction as indicated in that chart. OR.... that a change in prop AND a change in RPM to that specified in the same ROW of the chart, will likely result.

You are right George you won't change the throttle but the RPM will change resulting in perhaps the same speed. When i was trying to figure out how much pitch I wanted to add to my Cub prop I talked to an engineer at Sensenich. He told me a good rule of thumb that they have developed over years of twisting props of all sorts. Wish I could remember exactly but I believe it was about 50 RPM loss or gain for every inch of pitch added or removed. In the case of the Cub the A65 I had could turn the prop at 2500 RPM in cruise, 200 RPM over red line. I wanted to add just enough pitch so that I could just red line the engine at cruise. I had then add 3 inches of pitch to what ever they found the prop to be and it worked out perfectly. The A65 could turn the prop at 2350 or just over red line at full throttle.

So George in your stated scenario and using that rule of thumb by changing to a 53 from a 55 you would gain about 100 RPM or 2550RPM. Remembering the rules that the first chart seems to indicate by changing to the 53 but adjusting the throttle to the same RPM, you would lose about 5 mph. But since you are not adjusting the throttle Sensenich rule says you would gain 100 RPM and using the second rule from the chart you would gain about 5 mph. Guess what, you going the same speed. This also works out to what BL is saying. At the lover RPM and 55 pitch the engine is working as hard as it is with the flatter 53 inch at 100 rpm higher.

So why not run a flatter pitch and run the engine at a higher RPM and go just as fast as with a cruise prop, you can to the limit of the red line. Why not then run a high pitch so we can go faster without exceeding red line, you can. But as George points out your takeoff and climb will suffer with the 55 inch prop because the 53 inch will excellerate you airplane to flying speed faster.

On my 170 which had what was suppose to be a 53" pitch prop, I was not happy with the final speed of the aircraft. I looked at everything involved including rigging which is when I discovered the wing adjustment I mentioned earlier. After exhausting all other possibilities I came to the conclusion that either the prop was not as it was marked, which is a distinct possibility for any prop, or at the very least the prop was not performing like most other propellers of the same make and model. I was happy with the takeoff and climb (for a c-145) and the static rpm seemed on the high side. So I had my prop checked by a prop shop and told them to add an 1" pitch to what ever they found. The shop said the prop was within limits for a DM7653 and so they twisted it to 54. And that is how I ended up with a non-standard DM7654 which I'm happy with.

BTW Another limit we have that restricts what we do with our props is the static RPM limits. When ever you are looking at all these numbers and thinking about a change make sure what ever change you make will not put your engine/prop combination outside those limits.

N9149A wrote:... by changing to a 53 from a 55 you would gain about 100 RPM or 2550RPM. Remembering the rules that the first chart seems to indicate by changing to the 53 but adjusting the throttle to the same RPM, you would lose about 5 mph. But since you are not adjusting the throttle Sensenich rule says you would gain 100 RPM and using the second rule from the chart you would gain about 5 mph. Guess what, you going the same speed. This also works out to what BL is saying. At the lover RPM and 55 pitch the engine is working as hard as it is with the flatter 53 inch at 100 rpm higher.

Not exactly, but we're discussing micrometer calibration when we're marking it with a grease-pencil and cutting it with a hatchet.
In the scenario I described, by reducing pitch I will realize an increase in RPM which is an increase in horsepower. (See the TCM power charts.) Increased horsepower will result in higher airspeed... not same airspeed. (Of course it will also result in higher fuel consumption and increased engine wear.)

N9149A wrote:... So why not run a flatter pitch and run the engine at a higher RPM and go just as fast as with a cruise prop, you can to the limit of the red line. ....

Because, as already noted, you will burn more fuel and increase engine wear. But as I've mentioned many times, I sometimes wish I had the better takeoff/climb of a flatter prop. With my 7655 I just avoid short strips unless I'm light.

Dam it I just didn't type so you understand what I was saying. If you reduce your pitch from 55 to 53 AND reduce the throttle to maintain RPM at the RPM you had with the 55 then you won't be increasing horsepower. But if you DO NOT adjust the RPM and the result of the reduction to 53 would increase RPM then you would increase horsepower (and prop speed) and you may actually go the same speed.

I think we're talking the same differently

I'll never forget what one of my mechanics told me when I was Service Manager at Gulf-Coast Honda in Houston (back in the early '70's) when he and I kept re-hashing the same conversation...

He said, "I'm not sure I understand all I know about this."

While the new administrator of the FAA is dodging all the rolling heads from the "Sling seats in the back of C-170S" fiasco, he is going to have apoplexy when he finds out that Bruce is running a 54 inch prop pitch when the TDCS calls for a 51, 53.or 55.

blueldr wrote:While the new administrator of the FAA is dodging all the rolling heads from the "Sling seats in the back of C-170S" fiasco, he is going to have apoplexy when he finds out that Bruce is running a 54 inch prop pitch when the TDCS calls for a 51, 53.or 55.

That's news to me also, I'm sure glad my stock McCauley 7654 was twisted up to 55" 36 years ago!

With my 51" pitch I get 90Kts at 2200rpm. I get 105Kts at 2500rpm (maybe 110).
This not at SL, but at 1000'.
Cheers Eric

ginbug92b wrote:51 pitch is a climb prop. In full throttle level flight what is the maximum RPM that you get?

What I get at full power is only 2,500rpm.. I gess that answer the question. Probably I have a climb prop and that is also why I am not getting more power from my engine.

Juasiel, climb props result in HIGHER/MORE rpm than cruise or standard props. A standard prop, at sea level, full power, brakes locked.... should produce only 2230-2330 rpm.

I'll say what George has said a bit different. A static RPM check is the RPM your engine can achieve on the ground with the brakes locked at full throttle no carb head. The static RPM range allowed for our 170s with the McCauley 1A170 is 2230 to 2330 RPM. You have said that your prop is stamped DM7651 which we know to be a climb pitch and so if your prop is a DM7651 you should be able to get close to if not 2330. If you can't then I would suspect your tach to be inaccurate BUT we also can't rule out poor engine performance do to any number of things such as mag timing, throttle linkage adjustment and on and on.

Juasiel you also say you can't get more that 2500 RPM. You didn't say when you are trying to do this but if it is in level flight at sea level or close to it then I suspect your tach is inaccurate BUT we also can't rule out poor engine performance do to any number of things such as mag timing, throttle linkage adjustment and on and on. You should easily be able to get 2700 rpm at or near sea level with a DM7651 My engine can turn my 1A1170 DM7654 to 2650-2700 RPM.

Get your tack checked against an electronic tach.

Well at 2450, I'm indicating between 105 and 110 MPH with a 53" pitch. If I'm close to gross wt., it's closer to 105 maybe a bit less. Even with adjusting the wings to a neutral setting at the last annual, and bringing the cable tensions into tolerence,the ball is slightly (1/4 - 1/3) out to the right. If I adjust the rudder trim it may come back and maybe I'll pick up some speed. Not that I'm counting or should I say calculating

Been off for a while, hope you are all doing well and enjoyed your summers...

Kyle T

N9149A wrote:I'll say what George has said a bit different. A static RPM check is the RPM your engine can achieve on the ground with the brakes locked at full throttle no carb head. The static RPM range allowed for our 170s with the McCauley 1A170 is 2230 to 2330 RPM. You have said that your prop is stamped DM7651 which we know to be a climb pitch and so if your prop is a DM7651 you should be able to get close to if not 2330. If you can't then I would suspect your tach to be inaccurate BUT we also can't rule out poor engine performance do to any number of things such as mag timing, throttle linkage adjustment and on and on.

Juasiel you also say you can't get more that 2500 RPM. You didn't say when you are trying to do this but if it is in level flight at sea level or close to it then I suspect your tach is inaccurate BUT we also can't rule out poor engine performance do to any number of things such as mag timing, throttle linkage adjustment and on and on. You should easily be able to get 2700 rpm at or near sea level with a DM7651 My engine can turn my 1A1170 DM7654 to 2650-2700 RPM.

Get your tack checked against an electronic tach.

I cheked this reading of RPM at level flight,, I say sea level, but actually flown at about 400 to 500'.You really think the tach could be giving bad readings?..

gahorn, I thought that a climb prop should have more pitch to perform as it is expected. I can maybe wrong but I thought that as more pitch a prop has, less RPM you get from an engine. or is teh oposite? ....

gahorn wrote:It's a nice idea, and a fine piece of work, Bruce. Thanks for the contribution.

When will the takeoff/climb graph be ready?

By the way... I think it's important to realize something with regard to a graph such as this one:

If I am running at 2450 with FULL THROTTLE at 7500', and it results in a particular airspeed...... the fact that I change to a prop with a lesser pitch, say from 55 to 53,.... does not mean I will actually lose airspeed if I remain at full throttle.

Instead, what it means is, that I'll INCREASE rpm at full throttle. The NEW rpm will likely result in similar or possibly even GREATER airspeed. However.....this chart likely indicates that if I change props and keep the SAME rpm, my airspeed will result in a reduction as indicated in that chart. OR.... that a change in prop AND a change in RPM to that specified in the same ROW of the chart, will likely result.

Such a small change may not affect cruise speed as much as it will increase RPM realized during Take-Off.... and therefore will reduce distance req'd and increase ROC. It will also result in a greater fuel burn per mile travelled, and more piston travel within the cylinder, and likely increase engine operating temps.

(You can't spend money without it costing you.)

The factory's orignal choice of a 7653 prop for this airplane is probably the best compromise of all choices available. If you want to reduce trip fuel and reduce engine RPM at cruse, then a 7655 cruise prop will give such result, at the cost of takeoff/climb performance. If you want better takeoff/climb performance, then a 7651 climb prop will give you that, at the cost of additional fuel burn and increased engine RPM during cruise. The airspeeds achievable with such prop changes will be limited by the engine's ability to produce horsepower (thrust) within the achievable RPM limits. Again, think of a car with a 5-speed transmission. The standard 7653 prop is like cruising around in 4th gear. The 7651 is like doing so in 3rd, and the 7655 is like being in 5th.
A similar correlation will exist during drag-racing (as in takeoff/climb). The finer pitch 7651 prop will accelerate better than the 7655 coarse pitch prop.

This is a very easy way to understand the pitch vs RPM issue and to make our minds clear about the real problem (well at least to me)
Now, just found and notice something on my prop that makes me to doubt even more. Taking a very close look at the numbers on my prop, found that, they seem to be remarked over some other numbers. So up to this point I am not sure if the numbers are real or not and wheter to tell the right pitch on my prop. It only says MD7651 with the number on top of the previous ones.

The facts are that at sea level (400 to 500') flying at level flight full power wich is 2,500 RPM (can't get more HP), I get 100 to 105 indicated mph and oil temp. of 200 to 205 f, OAT of 30 celcius.
I am really considering to remove the prop and take it to my nearest prop shop to have them inspect it and really specify what prop do I have.