What is the normal cruise speed at sea level?

[juasiel123]

I know this is an old topic and concern for all of us, but really would like your opinion on this.

Up to what I have been reading from other members, they mentioned that they get on their 170's up to 114mph at cruise speed or 65%. That seems great, even on the 170 manual say that Cruise speed: 105 knots (121 mph, 195 km/h), which is also great.

My problem is that I only get on my C-170a, 85mph with 2200rpms or 90mph with 2350rpms. Not even close to a 100mph, and I fly most of the time at sea level. Engine is a C-145. Have also double check with gps and the Airspeed Indicator is accurate as well. I think the prop I am using is a Mcauley standard prop, not sure the dimensions or anything, but will check it out and give more info.

The thing is that I am always flying on the white arc of the Airspeed, I only get 100 or 110 on descend and that's about it.

I was thinking that maybe needed some wheel pants or something, but accordingly of what I have read, that is not significant improvement in airspeed gain.

Any thoughts? would really appreciate

Juasiel

[davevramp]

Check the Tach. Mine was reading low by 225 rpm a new MITCHELL TACH is hard to beat for the price. What is your fuel burn? Do you lean?
I get 105 to 107mph at 2000ft at 2200rpm and burn 7.5 to 8 gph. If I don’t lean it goes to 9gph.

[GAHorn]

Your tach accuracy, your prop "pitch" (and model should be verified to be correct) will affect your cruise speeds as will your aircraft's trim, rigging, and drag from accessories, antennas, oversized tires, etc. (Not to mention those huge external speakers you fly around with.)

The FIRST thing I noticed about your message was...... you aren't using the same power setting as others are using. You are using a lower power setting because you are using a slower rpm, therefore you cannot expect to achieve the same speeds as others unless you can determine if your engine and prop are also standard installations and are operating correctly.

[juasiel123]

Your tach accuracy, your prop "pitch" (and model should be verified to be correct) will affect your cruise speeds as will your aircraft's trim, rigging, and drag from accessories, antennas, oversized tires, etc. (Not to mention those huge external speakers you fly around with.)

The FIRST thing I noticed about your message was...... you aren't using the same power setting as others are using. You are using a lower power setting because you are using a slower rpm, therefore you cannot expect to achieve the same speeds as others unless you can determine if your engine and prop are also standard installations and are operating correctly.

Hi gahorn... About the tach, it has been recently overhaul,, so its accuracy don't think that could be questioned,, (unless wasn't overhaul right ). I do lean while cruising, but wanted to take as reference sea level, so we could avoid such problem to be taken under consideration.

To the situation that I am refering is when I fly in a normal way, the four speakers that I fly with, really make a lot of drag, so I take them off the wing strut as soon I finish the job. The only accesory That I have is the com antena and is only one and tires are the standart 6.00 x 6.

Plane haven't been with me for a long time, couldn't tell what is the pitch on the prop. The only info. on the prop is the Model which is 1A-170-DM7651 S.N. 54256 McCauley Prop. How can I find out the pitch on my prop?

What is the normal power setting? Think I read at the POH that 2200rpm couuld be use for economic cruise and 2350rpms for fast or cruise speed, but even flying with 2400rpms, haven't achieve the 100mph. Really began to think that the 170 was slower that the C-150 or very close, until I read some post.

Juasiel

[ginbug92b]

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

[Bruce Fenstermacher]

The DM7651 means that when new the prop had a 76" diameter and 51" pitch. Assuming your prop has not been re pitched you have answered one question. And that is that your prop is a climb prop and so you will be slower than the book RPM and speed you are trying to match.

The fact that you tach was recently overhauled does not mean it is correct. My tach is recently overhauled and is within 50 rpm at 2700. So even overhauled tachs can be off. The only way to know for sure is to check it against an electronic tach.

Even though you are using a GPS to calibrate your airspeed unless you measure under very critical criteria there is room for error.

You would be surprised how planes can look the same and have different drag. My plane once had each wing eccentric adjusted opposite each other. You couldn't tell it from any other plane yet the wings had much more drag than others because of the adjustment. I picked up several miles an hour by adjusting both eccentric bushings neutral even with the resulting aileron adjustment needed to now keep the plane level.

There are lots of little things that cause drag and it is sometimes difficult to realize the drag is being produced. People who try to make their planes faster by cleaning them up make many little improvements here and there and though small they all add up.

I think the major reason you are seeing slower airspeed results is probably your climb propeller coupled with tach and airspeed inaccuracies. Find out what they are and compensate for them and you will most likely find your plane to be within 5 miles an hour of the book values.

[bsdunek]

Well explained, Bruce.

My 170A has the original DM7651 prop. At 2450 by my tach(?), at 2000 feet, I show about 118 mph. It's been that way for 58 years.

I've found the biggest difference between similar planes is the accuracy of the instruments. If you can get an optical tachometer to check against yours, then fly both directions between two known landmarks on a quiet morning or evening, you will bet a more accurate number.

As George says, rigging is important too - that should be the next thing to check if your speeds are still slow.

[GAHorn]

The DM7651 means that when new the prop had a 76" diameter and 51" pitch. Assuming your prop has not been re pitched you have answered one question. And that is that your prop is a climb prop and so you will be slower than the book RPM and speed you are trying to match.
...

I believe what Bruce meant to say is, that the Indicated airspeed will be slower. Your RPM at similar throttle settings and indicated airspeeds will be faster/higher rpm. (It's as if you were driving your truck around in second gear continuously.... Your top speed will be less, even though your engine speed will be more.

[Bruce Fenstermacher]

Yes George you are right I didn't say that very well. The book RPM is predicated on a DM7653 prop. So if you match the RPM in the book with a DM7651 prop you will be going slower. Or if you go as fast as the book speed then you will have to be using a higher RPM setting.

Here is a quick chart which shows the relationship or pitch, rpm and speed. This chart assumes the prop to be 100% efficient which means among other things there would be no airframe drag. We know our props are not 100% efficient and that there is airframe drag so your airframe with a given prop and rpm will not be able to achieve the chart speed.

PropRPMSpeedChart.jpg

What we can take away from the chart is this. There is about a 4 to 5 mph difference between a 51 and 53 pitch prop at the same rpm and another 4 to 5 mph. We can also see from the chart that a difference of 100 RPM makes about 4-5 mph difference. OK I know you won't see 10mph switching from a 51 to a 55 pitch prop because drag increases with speed so it will actually take more pitch than 55 at the same rpm to get a 10mph difference. And just increasing RPM 100 rpm will not see the 4-5 mph difference again because of the increase in drag. I'd say maybe 3 mph for every 2" of pitch and or 100 rpm

[GAHorn]

Bruce, I'm curious as to the source of that graph. I'd like to know because I'd like to make other comparisions.

[Robert Eilers]

Great graph Bruce - thanks - it answered a number of questions for me. I have been contemplating having my 7653 twisted to 7651 in an attempt to obtain improved take off and climb performance. I have a much better understanding of pitch affects on air speed, but what I would like to see is a chart/graph reflecting takeoff and FPM climb comparions based on prop pitch. So far, I have not been able to locate a chart that compares pitch and T/O, Climb performance.

[GAHorn]

Everyone should understand that whatever the source of that graph, there are a lot of variables involved. A change of pitch cannot guarantee the exact results predicted in any graph.

An example is my airplane which has few external drag devices. At 7,500 ASL it trues out 104 kts (119.6 mph) with a 55 pitch McCauley at 2450 RPM, and my tach is accurate when compared to a digital tach-checker. That doesn't mean that a reduction in pitch to 53 is going to be exactly as predicted in the chart, due to the fact that we're not measuring in steel.... we're measuring in very slippery air, on different days. (And we haven't yet determined if my airspeed indicator is accurate, although I believe it is because I've flown formation with others and it seems to agree.)

Comparing my airplane to the chart, it appears that my airplane correlates to the chart's 55 pitch prop at 2300 RPM. I'd guess that changing my pitch to 53 might result in similar airspeeds as the 2300 RPM line in that graph, i.e. I'd lose about 5 mph.

What's more helpful to me is seeing the differences between what my prop achieves versus that graph's prediction. I'm curious at what altitude and other atmospherics that chart represents, and with what airplane.

[lowNslow]

The chart is just showing the theoretical math with no slippage. It is showing the pitch x rpm and then converted to miles per hour. No prop is 100% efficient.

[Bruce Fenstermacher]

Oh boy I may have created a hornets nest of expectation with that chart.

I wanted to show a simple comparison of the effect of changing pitch and rpm. I did say that the chart DOES NOT take drag into consideration and it assumes 100% efficiency of the prop, which never happens. I tried to explain as George has pointed out there are many many variables involved and so the chart can not be used to say that your airplane or any airplane for that matter should be able to achieve a speed for the rpm.

I created the chart in Excel. It's attached at the end of this post for anyone who wants it. It represents, as I've said, a fictitious airplane that has no drag and has a 100% efficient prop. It's pretty easy to create. On this airplane a 51" prop would move forward 51" for every revolution of the prop. So at 2700 rpm the prop (and airplane) would move forward 137700 inches or 11475 feet in one minute. In 60 minutes the prop (and airplane) would move forward 688500 feet or 130.39 miles an hour. (688500/5280). The chart is just duplicated out showing the results for all the pitch and rpm ranges.

I hope I didn't get anyone excited that I may have discovered a secret chart somewhere.

I've used a similar chart before to see what I wanted to re-pitch both my Cub and 170 props to. I flew my plane at different RPM and recorded the speed achieved. I then compared that to a 100% chart like this one and figured out what the inefficiency was and used it to correct the chart. Then by changing the pitch I could see how the rest of the chart was effected.

PitchRPMSpeedChart.xls

[jrenwick]

Bruce, I understood what you were doing. Everyone, this chart is only used to get a theoretical estimate of what a small change in pitch will do to airspeed vs. RPM. It gives you a delta only -- not an absolute value.

John