Hello,
I have a question. But first let me tell you a little story. Last week I was going over some mountains and bad weather and found my self at 9700 ft. This was my first in a 170. Well I was a little surprised to see the indicated airspeed dropping a good 10 MPH at this altitude. The indicated airspeed was 90 MPH and the ground speed per GPS was 95 MPH (don’t know how were the winds). The pressure altitude was probably even higher due to the hot weather here in Brazil.
Considering that the manual recommends a maximum RPM of 2450 during cruise, at what density altitude would you cruise for best ground speed? Would you maintain 2450 or would you go higher (considering that at altitude 2450 is less than 65% of power). And another scenario: Say you are going for the best range what altitude and RPM would you use? The manual says 2200 for best range but what would YOU use and what altitude?
My airplane has a 53 prop and there is a good change that the IAS is not indicating properly. Also I have not checked the tachometer, but I get static 2200-2300 RPM.
I always read the topics here and have learned a lot about this plane. Thanks!
Altitude/RPM for best ground speed and best range?
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
There are a couple of assumptions which must be made to address your question. The first is that you are operating an A or B model 170, with the standard engine. The second is that your airspeed indicator, altimeter, and tachometer are accurate.
The operational performance data available from the Owner's Manuals of these airplanes indicates that the best/longest range is obtained between 38 and 43 percent power at altitudes between 5000 and 12500 feet. The charts are not specific with regard to climb/descent performance. Many early/small Cessna performance charts specifically state that cruising performance data does not include fuel/time/distance to climb to or descend from altitude. For that reason there is some ambiguity with regard to the Cessna 170A/B performance charts.
But generally, in my own experience, it is better to cruise at or as near to 7500 feet pressure altitude as possible. This altitude allows most of the benefits of True Airspeed gains VS Fuel Consumption VS Attainable % Power, ...while avoiding time to climb/descent computations which can become excessive to reach higher altitudes. Of course, weather, terrain, and winds all greatly affect the decision. A strong wind is especially worth considering, and it is generally better to remain low and use high power against a headwind, while going high and use low power with a tailwind. (The extreme example is the 100 mph airplane flying into the 100 mph headwind. ANY increase in speed REGARDLESS of fuel consumption penalty is an IMPROVEMENT in both groundspeed and miles-per-gallon.)
Therefore, assuming the wind is constant at all altitudes (very uncommon) then with a headwind I'll choose to fly lower in order to achieve higher horsepower availablilty,...generally at/below 5000',... While with a tailwind or calm wind, I prefer to fly at 7500' and use 2450 RPM.
As regards power settings, ...your comment that 2450 is recommended as a maximum cruising rpm is not correct. That rpm is a recommendation for all considerations of efficiency, fuel, speed, and engine longevity. It is not a "maximum" limit of recommendation. The 145 Continental may be operated at 2700 continuously without limitation, (provided your prop will actually allow that much rpm. Most props will not.)
At/Below 2500' I usually operate at 2450 rpm because at that pressure altitude 2450 rpm equals 65% power. And generally up to 7500' I still use 2450-2500 rpm and accept the one or two percent power loss, but that also happens to coincide with wide-open-throttle (WOT). (I do not recall ever seeing my rpm increase above that achievable with WOT at 7500'. For that reason, it's my choice of favorite cruising altitude.)
Best range is another matter. (Notice that 2100/2200/2300 rpm will achieve practically the same range at 7500'/10000'/12500' respectively. But careful/aggressive leaning is necessary to actually achieve any fuel/range benefits. The main consideration at those altitudes will still be weather, wind and terrain. Also, light operating weights are almost a necessity to utilize the higher altitudes effectively.
Hope that helps.
The operational performance data available from the Owner's Manuals of these airplanes indicates that the best/longest range is obtained between 38 and 43 percent power at altitudes between 5000 and 12500 feet. The charts are not specific with regard to climb/descent performance. Many early/small Cessna performance charts specifically state that cruising performance data does not include fuel/time/distance to climb to or descend from altitude. For that reason there is some ambiguity with regard to the Cessna 170A/B performance charts.
But generally, in my own experience, it is better to cruise at or as near to 7500 feet pressure altitude as possible. This altitude allows most of the benefits of True Airspeed gains VS Fuel Consumption VS Attainable % Power, ...while avoiding time to climb/descent computations which can become excessive to reach higher altitudes. Of course, weather, terrain, and winds all greatly affect the decision. A strong wind is especially worth considering, and it is generally better to remain low and use high power against a headwind, while going high and use low power with a tailwind. (The extreme example is the 100 mph airplane flying into the 100 mph headwind. ANY increase in speed REGARDLESS of fuel consumption penalty is an IMPROVEMENT in both groundspeed and miles-per-gallon.)
Therefore, assuming the wind is constant at all altitudes (very uncommon) then with a headwind I'll choose to fly lower in order to achieve higher horsepower availablilty,...generally at/below 5000',... While with a tailwind or calm wind, I prefer to fly at 7500' and use 2450 RPM.
As regards power settings, ...your comment that 2450 is recommended as a maximum cruising rpm is not correct. That rpm is a recommendation for all considerations of efficiency, fuel, speed, and engine longevity. It is not a "maximum" limit of recommendation. The 145 Continental may be operated at 2700 continuously without limitation, (provided your prop will actually allow that much rpm. Most props will not.)
At/Below 2500' I usually operate at 2450 rpm because at that pressure altitude 2450 rpm equals 65% power. And generally up to 7500' I still use 2450-2500 rpm and accept the one or two percent power loss, but that also happens to coincide with wide-open-throttle (WOT). (I do not recall ever seeing my rpm increase above that achievable with WOT at 7500'. For that reason, it's my choice of favorite cruising altitude.)
Best range is another matter. (Notice that 2100/2200/2300 rpm will achieve practically the same range at 7500'/10000'/12500' respectively. But careful/aggressive leaning is necessary to actually achieve any fuel/range benefits. The main consideration at those altitudes will still be weather, wind and terrain. Also, light operating weights are almost a necessity to utilize the higher altitudes effectively.
Hope that helps.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
Thanks Gahorn,
My 170 is an A model with a standard engine, but with a very non-standard tail (it has a squared tail!) I am not sure how this tail got in the airplane but it flies pretty well, nevertheless I am waiting for the right time to get it back to the original round tail.
Thanks again,
Donovan
My 170 is an A model with a standard engine, but with a very non-standard tail (it has a squared tail!) I am not sure how this tail got in the airplane but it flies pretty well, nevertheless I am waiting for the right time to get it back to the original round tail.
Thanks again,
Donovan
Donovan,...is your airplane serial number available? What is it? (I suspect you actually have a converted 172 with 170A wings.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
The factory once planned to make a 170 C model with the squared tail. I've seen an old photo of it before they changed their mind and converted it back to round tail feathers, and used the design instead on the early 172. (This is the basis for Frank's username, 170C. Frank actually has a 172 with the Bolen tailwheel modification.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
I don’t have the serial number here, but could look for it. The airplane is an A model from 1951, but something weird happed sometime ago. This is what I have figured it out: The airplane was towing banners in Rio de Janeiro, the guy that owned that plane said he modified the plane to have more control authority, increasing the area of the horizontal stabilizer, vertical stabilizer, rudder and elevator. This is what I was told, but if you look at the others 170 flying banners no other one have this king of tail. What I believe it happed is that for some reason he put a 172 elevator, then to make things look more natural (perhaps more like a 180) where the horizontal stabilizer is round he made squared (simply added a plug there) (the original stabilizer is there), he did the same with the vertical stabilizer, but the rudder I believe is the original attachments but with a new skin over longer ribs. About 5 years ago, when I was starting to think about buying part of this aircraft, I posted on the old 170 forrum a picture of the aircraft tail and asked if anyone had seen something similar, I had a hope that this was perhaps a known modification.
At one time I had a 170 rudder and elevator ready to install, but my partner said: ‘Its flying great why mess with it’, and I agreed. One thing we have to be careful is that the CG is a little back, so with 4 adults its possible to have the CG very out of envelop.
Regarding the RPM stuff, over the weekend I am probably going on a cross country so I will do some testing at various altitudes and RPM to see what is best. Next week I will post here what I found out.
Thanks
Donovan
At one time I had a 170 rudder and elevator ready to install, but my partner said: ‘Its flying great why mess with it’, and I agreed. One thing we have to be careful is that the CG is a little back, so with 4 adults its possible to have the CG very out of envelop.
Regarding the RPM stuff, over the weekend I am probably going on a cross country so I will do some testing at various altitudes and RPM to see what is best. Next week I will post here what I found out.
Thanks
Donovan