Innefficient Descent (going fast)

[Tommy]

8 hours ago, 201er said:

Maybe if you saved a couple bucks a flight you could afford some proper tires.

If you need to save a couple bucks a flight you shouldn't be flying - certainly not with your parents' money...

[Alan Fox]

Mike , I am cancelling any savings you may have accrued , Next time I work on your plane , you are getting surcharged 20 dollars , for the time I waste reading your "STUPID FUCKING POSTS "    If you are that worried about fuel , take the car !!!

[Tommy]

54 minutes ago, Browncbr1 said:

What a fantastic waste of time

Agree. 

The differences can be measured in seconds and millilitres. Not to mention the conclusion was drawn by someone with no engineering / flight testing background using data extracted from one plane, couple of descents, and imprecise measurements.

What's next Mike @201er ? The best power setting and FF for taxiing? 

[Bob_Belville]

6 hours ago, carusoam said:

 

8) My favorite method of descent was captured by a few people... Lower the nose, increase, then maintain, MP, while descending, use an IAS below Vne.  Lower IAS (yellow arc limit) in bumpy air.  Maximizes fun, speed, and selfies that include the GPS....   clearly not the efficiency topic 201er is describing...

 

+1

[steingar]

14 hours ago, teejayevans said:

Some of us enjoy flying and want to prolong it.

Buy a Skyhawk.  You'll fly plenty.

[bradp]

I would like to protest the lack of appropriate use of significant figures in this thread. 

Thats what they call a double entendre in France.  

[Browncbr1]

It is more efficient to descend with +7 degrees angle of attack..   

[bradp]

5 minutes ago, Browncbr1 said:

It is more efficient to descend with +7 degrees angle of attack..   

Apparently the angles of attack against Mike for posting this are varied.  

Sorry I didn’t get any sleep last night. 

[Hank]

8 hours ago, carusoam said:

8) My favorite method of descent was captured by a few people... Lower the nose, increase, then maintain, MP, while descending, use an IAS below Vne.  Lower IAS (yellow arc limit) in bumpy air.  Maximizes fun, speed, and selfies that include the GPS....   

Unless I'm on a short trip, which will never happen with a climb to 10,000 msl, there's not much MP to increase; at 10,000 there is zero additional MP available. Must be something to do with Anthony's extra 130 hp . . .

So in my simple little plane, I leave power alone and push on the yoke for 500 fpm. As I come down, MP increases all by itself and the mixture leans out, so I occasionally retard throttle and advance mixture to maintain the cruise values for MP and EGT. Once I level off and bleed off the extra airspeed that I converted from altitude, I start adjusting power to slow to pattern speed. As long as I'm able to drop flaps by midfield downwind, that's good enough and I can land.

Cost and time as calculated above are cheaper and faster than Mike's super slow descent, but not quite as much as Byron's. But the math is so much simpler, I just count thousands of feet from TPA to where I am using my fingers, double that number and start down that many minutes plus one or two out from my destination.

So far, engine performance has been good, the costs manageable and I'm rarely sprinting for the FBO. 

[Hank]

11 hours ago, 201er said:

Let me try this on a level you'll understand, you're going to have to spend more time at the self serve pumping the extra gas you burned so you really just spent extra money and barely saved any time.

Because I never travel at Carson's speed, I will burn more gas than those who do.

Because my carbureted C won't go very LOP, I will burn more gas than my injected brethren who do.

Because my C has fewer aerodynamic improvements than your J, I will burn more gas tha  you.

But my descent profile, even allowing for a few more minutes at cruise power after you slow down for descent, uses less fuel than your slow speed, slow rate descent, because my profile more efficiently converts potential energy (altitude) into kinetic energy (forward speed).

I also have half of your ground run for takeoff, or less. The first couple videos I saw of yours, I thought were high speed taxi runs . . . .   

[201er]

3 hours ago, Browncbr1 said:

What a fantastic waste of time

Ok, go back to talking about Timmy, Trump, and divorce.

[mooniac15u]

Nobody has mentioned the most fuel efficient descent profile; mixture to idle cutoff and pitch for best glide speed.  Descend at 0 gph and restart the engine at pattern altitude.  Or better yet, descend all the way to the runway with the engine off.

I can't believe anyone burns fuel on a descent when you can let gravity do all the work.

[Browncbr1]

9 minutes ago, 201er said:

Ok, go back to talking about Timmy, Trump, and divorce.

didn't you read my previous post?  I want to talk about AOA...   

[Hank]

22 minutes ago, 201er said:

Ok, go back to talking about Timmy, Trump, and divorce.

No, no--Timmy, Tommy, Trump and the latest sexual misconductor . . . . Seems like there's a new one every hour.

[Bob - S50]

Whether you think this was a waste of time or not, thought exercises are a good thing.  They make us think about alternative ways of doing things.  I do stuff like this all the time so you are not alone Mike.

Makes us think about stuff like:

How much wind change does it take to make it worthwhile to change altitudes? (very little)

What's the most efficient altitude for flight's under 300 miles? (lower is better but it makes very little difference)

If I'm running a bit short on fuel due to a headwind, but still want to try and make it to my destination without a fuel stop, should I speed up or slow down? (if you've been cruising at our normal 65 - 75% power then slow down)

[Raptor05121]

Guys, give it a rest. We're already in Mooneys. We're 4 steps ahead of anyone in an Arrow or Comanche, just be happy with what we have and fly your plane how you want

[jetdriven]

remember the Vz climb profile seems to use more fuel when it’s actially the exact same due to climb as Vy and several minutes faster. And don’t forget the LOP discussions. And the myth of oversquare.  They all started like this, someone challenging what we assumed as fact, and hadn’t been assuming that it was wrong all along. 

[201er]

11 hours ago, jetdriven said:

AT mikes behest, same climb and cruise, only difference is the slower 2200 rpm descent profile. 

the "A" 140NM trip only costs $3 less and is 7 minutes faster.

the "B" 300 NM trip is 1$ cheaper and 6 minutes faster.  

 

So, descend at high speed, cruise power, save money, and get there sooner.

Both trips are identical up till the top of the descent of the economy profile so I will ignore everything prior to that point. We can bring that back in later for total trip comparison once we can agree on the difference strictly between descent profiles.

One parameter I disagree with and modify is that the descent will be done at 6gph rather than 8gph and i will suggest that at 5000ft will be 135ktas so changing it to 70nm 135ktas@6gph for economy descent profile.

 

Economy descent:

0 mins cruise + 30 mins descent 135ktas@6gph uses 3g total.

 

Byron descent:

12 mins cruise 155ktas@9.5g uses 1.9g + 13mins descent 184ktas@10g uses 2.16g. 12mins@155ktas + 13mins@184ktas = 70nm. Total time 25mins and 4g. That's one extra gallon for 5 Hobbs minutes saved.

 

Using your $35/hour tach time we can compare 

30 mins of tach time at 2200RPM = 25.7 mins of tach time

25 mins of tach time at 250RPM = 24.4 mins of tach time

Difference in tach time of 1.7 mins yields $0.99 more tach cost for economy descent while saving 1g of gas which is $5 average. Net savings of $4 for 5 extra minutes still.

Edited December 1, 2017 by 201er

[jetdriven]

add in the cost of the trip from level off, and then add the descent profile to it.  See what that looks like.

[201er]

39 minutes ago, jetdriven said:

add in the cost of the trip from level off, and then add the descent profile to it.  See what that looks like.

I don't think it will matter except for changing the proportion of the role the descent savings or speed up plays on the overall trip.

For 140nm trip, 99nm from top of climb.

Economy descent profile:

99 - 70nm = 29nm of cruise, 11.2mins Hobbs, 10.9mins tach, 1.8 gallons

+ 30 mins Hobbs descent 135ktas@6gph, 25.7mins tach, 3 gallons

total 41.2mins Hobbs, 36.6 mins tach, 4.8 gallons

$35 x 36.6mins tach + 4.8g x $5 = $45.35

----

Byron descent profile:

99 - 40nm = 59nm of cruise, 22.8mins Hobbs, 22.2mins tach, 3.6 gallons

+ 13 mins Hobbs descent 184ktas, 12.7mins tach, 2.2 gallons

total 35.8mins Hobbs, 34.9mins tach, 5.8gallons

$35 x 34.9mins tach + 5.8g x $5 = $49.36

----

Results in same numbers I offered before that ignore the cruise prior to first descent profile. 1 gallon saved for 5 extra minutes for 1.7 mins of extra tach. Still $4 saved.

The important thing to realize is a comparison of what $4 buys you in cruise vs in descent. 1 extra gallon spread out across 3 hours is a +0.3gph fuel flow. That's enough to buy a few knots and spread across that span of time may actually get you to the destination faster than a slower cruise with faster descent.

[201er]

What the question really ought to be is that with a budget of X gallons of fuel and the purpose to travel Y distance, what climb, descent, and cruise profiles will give the best blend of speed or the greatest savings. Which strategies give you better bang for the buck? It gets even more complicated with wind. But first you have to understand the basics in a no wind case before adjusting for wind.

Is your gas money better spent to climb fast, cruise slow, and descend fast? Or will it serve better to climb slow, cruise a bit faster, and then descend slow? 

My understanding is that climbing, cruising, and descending at the fuel flow that gives you Carson's speed will give you the best speed for the buck in all phases of flight (give or take a little). However, if you desire to drop an extra $4 into your total flight cost, my suspicion is that you are better off spending it on the cruise rather than during the descent. Why? Because there is less parasitic drag at low IAS up high than at high speed descent down low. Working on scenarios that demonstrate this.

Edited December 1, 2017 by 201er

[Skates97]

If I was calculating ways to save $4 on a flight because I had a company making hundreds or thousands of flights a day and I was trying to put more money on the bottom line, maybe this would be more beneficial. However the reality is that the $4 savings is going to be used up at the drive through window because nobody wanted to cook dinner one night... 

[201er]

7 minutes ago, Skates97 said:

If I was calculating ways to save $4 on a flight because I had a company making hundreds or thousands of flights a day and I was trying to put more money on the bottom line, maybe this would be more beneficial. However the reality is that the $4 savings is going to be used up at the drive through window because nobody wanted to cook dinner one night... 

Look, it's not really about the $4. It's about learning how to get the most out of our airplanes for what we already spend on them. Even for those who choose to fly ROP to go as fast as possible, cruising at 150ROP rather than 100ROP will actually make you go slower while spilling fuel out the exhaust with no benefit. It's about identifying how to optimize our flying strategy to get the most out of it be it speed or savings.

[mooniac15u]

47 minutes ago, 201er said:

What the question really ought to be is that with a budget of X gallons of fuel and the purpose to travel Y distance, what climb, descent, and cruise profiles will give the best blend of speed or the greatest savings. Which strategies give you better bang for the buck? It gets even more complicated with wind. But first you have to understand the basics in a no wind case before adjusting for wind.

Is your gas money better spent to climb fast, cruise slow, and descend fast? Or will it serve better to climb slow, cruise a bit faster, and then descend slow? 

My understanding is that climbing, cruising, and descending at the fuel flow that gives you Carson's speed will give you the best speed for the buck in all phases of flight (give or take a little). However, if you desire to drop an extra $4 into your total flight cost, my suspicion is that you are better off spending it on the cruise rather than during the descent. Why? Because there is less parasitic drag at low IAS up high than at high speed descent down low. Working on scenarios that demonstrate this.

What your calculations have demonstrated is that the differences in descent profiles are so insignificant that wind will almost always be a far more significant factor in determining when you should descend.

[jlunseth]

I typically pull about an inch of MP off and tip the nose over to a 500 fpm descent.  The inch comes back quickly because of the ram air effect.  I don't know what Carson says about this, all I know is that if I don't start down at the South Dakota or North Dakota border on days when I have a good tailwind, I ain't gonna make it in time and I will either spend time spiraling down south of my home field (KFCM), which is pretty inefficient, of I will get some kind of cram down from ATC.  If I was doing a GS of, say, 200 kts, I am now probably doing 230 kts or a little more, and it is going to take me about 45 minutes to get down from altitude, to pattern altitude, and of course find a way to slow the plane down.  I don't know where Carson is at that point, but I think he is somewhere behind me, probably still out over Wheaton or Morris.  All I know is I have an old bladder and need to get to ground faster than him.