L/D IAS/TAS?

[201er]

Is L/D or Carson speed based on IAS or TAS? I'd imagine they (and the concept of aerodynamic efficiency) are based on IAS. In this case it would seem that upon reaching a certain altitude, going any higher would actually greatly reduce efficiency (particularly in NA airplanes but turbos as well).

 

Cruising at 11,000ft the other day, my IAS was around 110 KIAS (even though TAS was over 130KTAS and GS was 170). Since best L/D is around 100 KIAS, it wouldn't take going much higher/slower that I would actually be losing efficiency once I dip below 100 KIAS?

 

It looks to me that in neutral winds, 11,000ft is about the efficiency service ceiling of the M20J because even though KTAS will increase by going higher, by dipping below L/D it won't outweigh the decline of speed. Is this right or not?

 

Before you start criticizing me for nit picking about efficiency, it's more about understanding the subtleties of it to be able to apply in a broader way.

[201er]

Yeah, I figured. It seems like anything over 9k ends up becoming quite challenging and inefficient again. Below 10,000ft, it seems like I can do a good job of running LOP and staying cool. Over 10k, I find myself no longer LOP, struggling with temperatures, trailing cowl flaps, etc... that unless I have to be that high or have a tailwind, it doesn't seem like a mechanism for extending cruise range or nmpg.

[John Pleisse]

I agree. At 11,000, I notice a significant nose-up moment and the last and best hopes for the highest TAS. Is this the best LD/max? No.

[jetdriven]

I have read that maximum range is not related to altitude.   I need to do more reading, but it seems the only reason to fly higher than a palm tree is to 1.) take advantage of increasing tailwinds, or 2.) to avoid "pumping losses" from a partially closed throttle below optimal altitude in cruise.

 

It seems that for piston planes, 123 KTAS is 123 KTAS regardless of altitude. I am so jet driven that altitude is everything, i had to go back and read more on it.

[201er]

Byron, I think you're wrong. But there may be truth in what you say. It seems that LOP on 10GPH, I get ~150KTAS whether I'm at sea level or 4000ft! Down low, to get 10GPH and 150KTAS, I'm turning the prop 2400RPM, up higher it's 2600RPM. Yet the TAS is the same. How do you explain that the GPH and TAS is the same at varying altitudes? This only seems to correlate this way when LOP though?

 

At around 8000ft, I seem to be able to do 5-6GPH cruise by bringing things way back. As I get higher, I'm moving toward peak and battling excessive CHTs that my fuel flow is higher! Flying above 8k ends up seeming less efficient. Still, when ROP I don't think you can beat being at 6-8k when it comes to fuel flow to TAS!

[garytex]

Testwest, where are you in our hour of need?

Gary

[Skywarrior]

"It seems that for piston planes, 123 KTAS is 123 KTAS regardless of altitude."

 

 

123 KTAS is 123 KTAS no matter what vehicle.

 

(I minored in Aero. Eng. and have slept at a Holiday Inn Express a few times.)

[Dave Marten]

Gents, reference your speed, power, vs altitude graph in your J model POH Section 5. For a constant % power you can generate more TAS as altitude increases. As you increase alt you squeeze out more speed for a given %HP UNTIL, as the chart depicts, you just can generate the HP (WOT and no more manifold pressure to give). The dreaded "% power not available" line which really cramps your style. Neglecting winds you can work the chart right to left entering with a target speed and %power that you'd like to cruise at and then optimize your altitude based on OAT (at altitude). 201ers intuition that 6-8k is the sweet spot is reflected right there in the chart (depending on OAT). You'll also note that much above 10K it'll be tough to generate power on warmer than standard OAT days. If it is a warm summer day then yes, that lower altitude will get you the speed. Another interesting tidbit for you Carson guys is if Carson (and I'm no carson expert) is about 132 IAS in a J and you're at 8K STD day then you'll about 160 KTAS. Conicidence or design point for the airplane? I like to think the design worked out as intended. Be careful out there and don't hit any palm trees!

[jetdriven]

L/Dmax and Carson speed are two different things. L/Dmax gives best NMPG and still air range, while Carson's speed takes into account the fixed costs of the airplane and the cost of time. 

 

It depends on what you are trying to do. My recent trip from FL to TX was all about making the trip at a certain arrival time and with minimum trip fuel. 128 KTAS was more expensive, in dollars for the trip, to fly than 100 KTAS, but if you add the cost of the fuel stop, it was a wash. Faster would not have sufficient reserves, slower was later and more expensive.  We did arrive nonstop in 6 hours and 44 gallons. Landing to take on fuel would have burned more fuel for the trip, and arrived at the same time, or the same fuel and arrived later.  It was a unique situation.

 

Maximum range in the 1977 POH is 1110 NM at 40% power, 2000 RPM, at 8000 feet, 135 KTAS.  It is also the same at sea level, 1110 NM, but 37% power, 103 KTAS.

 

Vy at 12,000 feet is listed as 80 KIAS at gross weight, so Carson speed at that altitude would be ~105 KIAS.  This number may be in error.

[Dave Marten]

L/D max is by definition best guide speed. So yes, use your own aircraft's number and away you go. Lower gross weight slower L/Dmax slower Carson. Ballpark of 120KIAS dependent on GW and model. Yes, if you pull way back to below 55% power then you can look at section 5 again and see you are now not gaining range w/ altitude (no surprise), but look at your 65%+ range charts...altitude matters there. The copy of the J POH I have does not list anything below 55% and 2400 RPM (Jan 96 w/ rev . Can't comment on 2000 RPM cruise range. Carson- "It thus turns out that the best rate of return on excess fuel expended, as measured in additional airspeed will occur when...a 32% increase in airspeed above the optimum will result in only a 16% increase in total fuel used...the flight time will be reduced by 24%....This is clearly the best return in airspeed increase (and hence reduction in flying time) on excess fuel, and as such, must be regarded as the "least wasteful way of wasting,i.e., the most productive use of excess fuel for cruising purposes." 16% more fuel than L/Dmax but 1/4 the less flight time. CAFE KIAS may be higher than carson after taking into account prop efficiency (designed for an optimum rpm range), etc. But, if that means I can complete 25% more flights than somebody flying at L/Dmax. Means I'm completing 4 of the same trips vs 3 and with only a 16% fuel penalty over L/Dmax. I'll defer to those who max CAFE scores, that is a more useful measure of efficiency. Norm wrote his a thesis on it (literally I believe).

[201er]

Jim, I'm not sure this applies to the climb the same way because the vectors are altered. We're really just talking about cruise here. Also, I don't think Carson speed relates to Vy as it does to best L/D. Best glide in my 201 is 98-105 KIAS depending on weight. Then Carson speed must be 129-138 KIAS depending on weight.

 

And Dave, your notes based on POH seem to hold true for ROP flight but not so much for LOP. Can someone explain why it is that it seems like there is no gain in TAS for higher LOP cruise at equal fuel flow? Is it because the increase in RPM to develop equal power higher is a wash with the gain in TAS over the wings?

[201er]

Ah yes, MPH. I hate how they keep mixing everything around like that.

 

85-91KIAS = Best Glide

 

112-120KIAS = Carson Speed

[Dave Marten]

201er, For your climb use the Vz profile (previuos thread on LOP climbs-ref testwest's work). WOT/2700 RPM, Target EGT, 115 KIAS until the airplane won't climb 500 fpm, then 500 fpm until speed drops to Vy.

[testwest]

This is one awesome discussion!

I think we got most of the points......Carson's Speed for the M20J is 115 KIAS, or 113 KCAS. This is for sea-level standard day conditions, and remember at these conditions you have about 2 knots of position error, so your ASI reads about 2 knots high, assuming no instrument error.

Vy (and by implication, Carson's Speed), decreases with altitude a little bit. However, the magnitude of the decrease for altitude, or changes for gross weight and temperature are not huge.

Trying to keep track of all these little differences is bread and butter for a piston airplane performance engineer, but unlikely to be rigorously applied by a "usual and ordinary" pilot. That's why the Vz profile uses the published sea level standard day Vy as the starting condition. Weight, temperature, engine not quite putting out 100%, all that stuff falls out to the 500 FPM performance limit in the profile.

So the technique becomes very effective, while being easy to apply every day and insensitive enough to the actual precision of a pilot's flying technique to remain viable. I ran sensitivity analyses for all of those conditions during the thesis work.

For cruise, the parameters of interest are velocity made good, and mpg. This means winds are an overriding factor in cruise efficiency as measured by the CAFE score. What I have done is put the Vz climb profile and my 65% LOP cruise speeds and fuel flow into the advanced performance profile on fltplan.com ...and I use the wind matrix to predict the altitude which yields the best CAFE score.

Your choice of altitude to maximize your 65% LOP cruise CAFE scores is how you get the balanced CAFE efficiency out of these airplanes day after day. Down low you may be at 2200 RPM WOT into a headwind, and up high you may be 2600 to make that 65%.

You may be slightly better off going 70% power LOP into a headwind, but doing a 2 variable condition wind optimization (altitude and LOP power) is beyond anyone's capability to model at this point.