The International Cessna^® 170 Association

I am a firm beleiver that there is no such thing as a stupid question. The ony way we learn things is to ask when we don't know. With that being said, here is my stupid question

On December 17, 2003 I took a flight in my plane to celebrate the Wright Brothers first flight. I climbed my lowly Skyhawk up to 10,500 feet for no other reason that to say I was that high in my plane. I tooled around at the altitude for a short time and noticed something. My airspeed was only indicating about 95mph instead of the 115mph I see at, lets say, 5000. I didn't think much about it at the time.

Last night I took a 100 mile cross country to attend an AOPA safety seminar. I flew out at 5500' and cruised at 115mph. On the way home I flew at 8500' and only indicated 95 mph.

I watched the GPS and took into account the near zero wind at that altitude and have concluded that thats all the faster I was going. I surmised that as I got higher, with the change in airpressure, that maybe the airspeed indicator would not give me an accurate reading. But as I compared that to the GPS I don't know that I beleieve that now.

So the stupid part of the question is this, does a plane such as mine (non-turbo charged) slow down like that with altitude due to the airpressure being less and the engine therefor not producing as much power?

Below 5000 I stay in the green arc of the tach at 2450 RPM. Above 5000 I run it at 2600 rpm which is in the yellow arc.

I would love to hear you take on this....

Dave

I have another part of this question now. With this being said, what do you normally cruise at on a cross country flight? Let's assume a normally aspirated engine, something like my Skyhawk or a Cherokee.

Dave

Hi, Dave!
I'm going to move this to The Pilot Lounge, since it's a general flying type question, OK?
I believe what you experienced relates to atmospheric density/True Airspeed/winds aloft.
As we climb, the atmosphere loses density. (The air molecules are farther apart.) This has several effects. 1- The performance of our normally aspirated engines begins to taper off (also the props.) 2- The aerodynamic "drag" on our airframes also tapers off resulting in a higher True Airspeed for any given Indicated Airspeed.
With no wind, groundspeed will increase with altitude up to the "critical" altitude of the airframe/engine combination, which is where the engine can no longer make rated power and performance begins to decrease again. In other words, if you cruise at 65% power (which is about 95 h.p.), at some point of altitude the engine will no longer meet that power setting due to atmospheric losses (loss of manifold pressure in a normally aspirated engine). The result of no longer producing selected power output will be a loss of true airspeed.
In the 170, that altitude is about 7500'. Notice that the Crusise performance charts in the Owner's Manual shows that 2450 rpm at 2500' is approximately 65% power. However 2450 rpm at 5000' is only 60% pwr, and at 7500', 2450 rpm produces only 55% pwr. Therefore, a constant pwr setting of 2450 rpm will result in slight decrease in TAS as altitude is increased, because 2450 rpm is less pwr at increasing altitudes.
BUT...if you maintain 65% pwr by allowing your rpm to increase as your altitude increases, then your TAS will increase. 65% pwr at S.L. (2350 rpm) only produces 118 mph TAS, while 65% pwr at 2500' (2450 rpm) produces 120 mph, and 65% pwr at 5000' (2525 rpm) produces 123 mph, and 65% pwr at 7500' (2600 rpm) will result in a TAS of 125 mph. In other words, constant power output results in an increase in TAS, up to critical altitude, which is about 7500' in the Cessna 170. Notice your miles per gallon is also improving.
Altitudes above 7500' will begin to reduce performance again because you cannot maintain the 65% power. Since you are reducing your power output above that critical altitude, your TAS will begin to reduce as well, even though aerodynamic drag is continuing to reduce.
In the 170, you'll notice that with standard engine/prop combinations, wide open throttle/2450 rpm/and optimum speed all happen to coincide at about 7500'. The chief advantage of cruising at higher altitudes is endurance/range improvements due to lower fuel consumption. It may also allow you to take advantage of cooler temps, better winds aloft (especially/usually if north/eastbound in the U.S.) and smoother rides.
Hope this explanation helps. More detailed info is usually available in flying instructional handbooks and references regarding True Airspeed, Density Altitude, and such topics.

Thanks George. What is your opinion on running the O300 engine in the yellow arc?

At 5500' to maintain 74% power I need to run at 2600 rpm. This is in the yellow arc.

Dave

You can run the O-300 at red line all day long, if you like. It's not limited in that regard. Just make sure the prop you have doesn't have an rpm range that is limited. (Some Sensenich props have rpm ranges which continuous operation should be avoided.)

Thanks you George. Where can I find information such as that pertaining to the prop? My prop is indeed a sensenich.

Dave

See your Type Certificate Data Sheet for your particular aircraft, engine, and prop combination for limitations on the prop. (if any).