Max Speed vs Max Range altitudes

Leonard_Smith_nz · #1 10-31-2006, 05:40 PM

All,

As a relatively new RV6 pilot (+- 30 hrs and loving every minute of it) I'm getting to the curious stage of ..."OK, I can fly this thing now, but how do I fly it well"...

I'm wondering what density altitudes pilots fly at for firstly, max TAS and secondly, max range. I appreciate that many things in aviation are a compromise and an interplay between competing interest e.g. at higher altitudes we have less dense air to traverse, but our engines aren't able to make rated hp.

For the purposes of the question assume normally aspirated and smooth air from sea level up.

Any thoughts appreciated.

Bryan Wood · #2 10-31-2006, 06:43 PM

Here's a link to the CAFE foundation tests. There should be no better information than this.

http://cafefoundation.org/v2/research_aprs.php

Best,

fodrv7 · #3 10-31-2006, 07:39 PM

Leonard,
The maximum cruise speed for a given percentage power will be at the full throttle height. Full throttle height is the height at which you have full throttle and are just attaining, say 75% power. Any higher and power will decrease (and hence TAS) due to decreasing manifold pressure. Any lower and TAS will decrease for the same IAS
Full throttle is easy to establish if you have a variable pitch prop, but more difficult with your fixed pitch prop.
Other RV drivers with FP props will hopefully chime in here to give you some guidance on settings- which I am unable to do, as I have a CSU.
But, to answer your questions, Max Cruise TAS and Max Range to attached graphs should clarify where to operate the aircraft to satisfy these requirements. The Graphs are not for an RV, but are excellent as the layout graphically demonstrates the effect of Altitude on TAS and Range.
In the first graph you can see that at given % power, TAS increases until the Full throttle height and then decrease above that height. Also, the full throttle height is higher for a lower power setting. In this graph 75% is available to 6,200? and 45% to 14,000?.
So max cruise TAS would be at 75% power at 6,200? giving 175KT.
The second graph tells a different story. It shows that as you climb above Full Throttle Height, whilst TAS decreases slowly, range increase dramatically, as the decrease in Fuel Flow is more significant than the decrease in TAS. Compared with the Max Cruise 75%, 6,200? 175KT which gives a range of 715nm, 65% at 13,000? (which will comfortably clear Mt Cook) gives a range of 860nm.
More importantly, fuel burn in the second case is 83% of the first. ie. 715/860= 0.83.
So, what you need to establish is where the glorious looking ZK-VRV approximates Full Throttle Height without over revving.
That will give you max TAS.
The higher you go above that the higher the range. Well, to a point, but I am sure that would apply up to 13,000?
If you are lucky some FP boys will chime in now with power settings and if you are really lucky Kevin H.
Pete.

n5lp · #4 10-31-2006, 07:45 PM

Quote:

Originally Posted by Leonard_Smith_nz

...I'm wondering what density altitudes pilots fly at for firstly, max TAS and secondly, max range....

Max TAS is easy for normally aspirated airplanes, full throttle and RPM as low as you can go. You will burn some fuel.

Now when you talk tradeoffs, it gets more complicated.

fodrv7 · #5 10-31-2006, 07:55 PM

Absolutely Larry.
That is why the Schneider trophy was fought out at sea level.
And is fun to flog an RV around at 500', Full throttle and Max RPM..... for a couple of minutes.
But I suspect Leonard is enquiring about Max CRUISE TAS.
Pete.

n5lp · #6 10-31-2006, 08:46 PM

Quote:

Originally Posted by fodrv7

Absolutely Larry.
That is why the Schneider trophy was fought out at sea level.
And is fun to flog an RV around at 500', Full throttle and Max RPM..... for a couple of minutes.
But I suspect Leonard is enquiring about Max CRUISE TAS.
Pete.

Yes maximum cruise power is generally considered to be 75% power but Lycoming approves continuous 100% power with the O-320 and O-360. So there you go!

Leonard_Smith_nz · #7 11-01-2006, 01:07 AM

Peter,

Thanks for your excellent reply.

The graphs provide a clear explanation of what the trade-offs are. I would imagine that 'Full Throttle Height' for 100% power is sea level and then TAS will decrease with altitude. This is consistent with your explanation of a different curve for a given power setting.

I'd like to come back to the 75% curve and see if my understanding is correct. My RV6 is fitted with a FP Sensenich, so I'd have to increase RPM to maintain 75% power with increasing altitude. If we say that 'full throttle height' is 8000 ft, then above that altitude I could gain the benefit of decreased fuel flow through appropriate mixture control without sacrificing power or TAS.

Does this sound correct?

Thanks

fodrv7 · #8 11-01-2006, 01:23 AM

Leonard,
The difficulty is knowing what percentage power you have set, as you have a fixed pitch prop and no Mainfold Pressure gauge.
If at 1000' you set 2500RPM for the climb at 100KT and that happens to be 75% power, and you then accelerate and climb out at 130KT you will have to throttle back to keep 2500rpm. What power do you have now? I don't know.
But what I have found, in the Lycoming manual, is that your engine, the O-360, is rated at 75% power at 2450RPM.
Now I assume that is Full Throttle at 2450RPM.
It would seem to follow then, that if you therefore climbed to the altitude where with your FIXED pitch prop and full throttle you were pulling 2450RPM the engine would be at 75% at FULL THROTTLE Altitude.
Sorry, I can't offer more. I have spent to many years driving Auto-throttle controlled kero burners and my only current Fixed Pitch time is in the Tiger Moth, which does everything at the same RPM (1800) and IAS (70KT). Climb, cruise and descent.
Come on you FP Blokes, jump in here with a bit of specific guidance for Leonard.
Pete.

fodrv7 · #9 11-01-2006, 01:26 AM

Quote:

Originally Posted by Leonard_Smith_nz

Peter,
I'd like to come back to the 75% curve and see if my understanding is correct. My RV6 is fitted with a FP Sensenich, so I'd have to increase RPM to maintain 75% power with increasing altitude. If we say that 'full throttle height' is 8000 ft, then above that altitude I could gain the benefit of decreased fuel flow through appropriate mixture control without sacrificing power or TAS.

Does this sound correct?

Thanks

Not quite.
AT 8000' you would get your highest possible cruise speed for 75% power.
Above that you would loose a little TRUE airspeed for a much bigger reduction in fuel burn and so a better range.
Pete.