Cruise Speed Loss By Weight

[201er]

Supposedly you gain speed on a trip as you burn off fuel or cruise faster light than heavy. How much of a difference in speed does it actually make? Anyone have real world numbers/experience on this?

[carusoam]

Mike,

Theoretically speaking, Expect one or two knots by improving balance.

Adding a few pounds in the right place (within the legal envelope) will keep your trim from hanging out in the wind.

A tool box in the baggage compartment to minimize up-trim during solo flights comes to mind.

Always calculate your W&B accurately before this type of experiment. Don't rush or exceed any limits.....

Changing the balance alters the aircraft's resistance to stalls.

Best regards,

-a-

[gsengle]

Quote: carusoam

Mike,

Theoretically speaking, Expect one or two knots by improving balance.

Adding a few pounds in the right place (within the legal envelope) will keep your trim from hanging out in the wind.

A tool box in the baggage compartment to minimize up-trim during solo flights comes to mind.

Always calculate your W&B accurately before this type of experiment. Don't rush or exceed any limits.....

Changing the balance alters the aircraft's resistance to stalls.

Best regards,

-a-

[PTK]

Mike, you are stating the obvious but I think the opposite is true. A heavier airplane must, by definition, have a higher AOA than a lighter one. It must fly faster in order to overcome the higher drag and maintain altitude. As it progressively sheds weight the AOA decreases. Less drag and thus lower speed required to maintain altitude. Wing loading and trim play major roles as Carusoam noted.

What I've said assumes non-accelerated flight.

[201er]

Does anyone have any actual numbers? The POH suggests a gain of as much as 10ktas over a drop of 440 pounds. I'm asking about actual real world figures. Anyone know?

[Jerry 5TJ]

I see about 1 knot per hour increase in TAS when in cruise at constant power and altitude, which suggests about 1 knot increase for every 50 pounds of weight reduction in my "C" model.  (9 gph x 6 pounds per gallon) operating at 2000 to 2200 pounds typical weight and 65% power.  

 

 

[jetdriven]

It can't be that much.  Our 201 at 95% power ran 186 mph with one person, and after a couple mods went 187 with 3 people.  It then ran the same speed with two pilots.  Then 189 with 3.    I would swag 1 knot per 300 lb wih no cg change.  Load the airplane aft it cancels the higher weight.  

[Hank]

Quote: allsmiles

Mike, you are stating the obvious but I think the opposite is true. A heavier airplane must, by definition, have a higher AOA than a lighter one. It must fly faster in order to overcome the higher drag and maintain altitude. As it progressively sheds weight the AOA decreases. Less drag and thus lower speed required to maintain altitude. Wing loading and trim play major roles as Carusoam noted.

What I've said assumes non-accelerated flight.

[Hank]

Forward CG will require more trim, which increases drag and slows you some at any weight. Moving CG aft, within reason, will reduce the out-of-trim drag and let you go faster. I've never paid close enough attention to IAS on long flights to remember a difference, I usually spot-check it when making power/altitude changes and rarely record settings, temp, altitude and IAS for reference but not more than once per flight. I'll have to start watching for this.

[fantom]

Quote: Hank

Please, leave the physics to those of us who studied it, and I'll not advise you on how to clean/whiten your teeth. Although I did get Dentistry Merit Badge in the Boy Scouts! 

[PTK]

Thank you Hank!  

So...Hank, in level unaccelerated flight what determines the amount of lift generated by the wing?

[Hank]

Quote: allsmiles

Thank you Hank!  

So...Hank, in level unaccelerated flight what determines the amount of lift generated by the wing?

[PTK]

Hank, I said "less drag and thus lower speed required to maintain altitude." I'm not referring to acceleration. I specifically said non-accelerated flight! Would you agree that in a given configuration the speed of the airflow over the wing and AOA determine how much lift generated? And that an increase in either of these parameters will increase both drag and lift? 

I'm sending you a pm; I don't want Fantom to get too excited!

[Hank]

Quote: allsmiles

Hank, I said "less drag and thus lower speed required to maintain altitude." I'm not referring to acceleration. I specifically said non-accelerated flight! Would you agree that in a given configuration the speed of the airflow over the wing and AOA determine how much lift generated? And that an increase in either of these parameters will increase both drag and lift?  

Take two identical Mooneys. Load one heavy, leave one light. Fly beside each other at any reasonable altitude.

The heavier aircraft will be at a higher AOA if they fly the same speed [increased AOA for increased lift].

If they both fly the same power settings, the heavier aircraft will be slower [higher AOA for increased lift creates more induced drag, slowing the heavy aircraft].

In any given configuration, altitude and power setting, the airflow over the wing and AOA determine the amount of lift generated, which will equal the aircraft weight in level, non-accelerated flight.

The heavier aircraft generates more lift to offset its heavier weight by increasing its AOA. This also increases drag. At typical WOT power setting, the heavier aircraft is slower than the lighter weight aircraft at the same altitude. If the two aircraft maintain the same speed, then power settings and AOA are different, with the heavier aircraft having higher power settings and higher AOA. I don't think I can notice a ½º difference in AOA, but my plane will descend if I trim it level and lean forward in my seat.

When flying at reduced power settings, as in the pattern, flaps enter the picture. Higher airspeed is maintained on final when heavy due to:  1) stall speed increases with weight due to increased lift required; 2) the heavy plane is at a higher AOA, closer to the stall AOA [typically ~18º for light GA], and higher speed lowers AOA. Flaps lower stall speed, modify AOA and increase drag so that you can descend without increasing speed and floating across the airport.

[xftrplt]

Hank,

I suspect, alas, your pedagogical efforts are futile.

[jetdriven]

Never confuse someone with facts or physics.  It confuses people.

[Hank]

For the OP, I've never noticed the difference in IAS because I've never looked for it; what matters to me is groundspeed, because that is fuel burned. IAS is only important in the pattern [always the same except on final where I adjust for weight], in turbulence and up/down drafts and mountain waves. I do admire it during descent, and am ready to level out and slow down if it's rough.

Even WOT, I still manage to cruise in the green, and only get near the yellow in descent; to reach yellow, I need to be >500 fpm. She accelerates nicely with gravity!

I have some upcoming trips whose weights will vary significantly. Now I'm going to have to remember to check airspeed on them!

On my recent 4.4 hr. trip, I was watching groundspeed and counting time on my fingers vs. fuel in the tank. Still landed with >1 hour left. My legs are usually 3 hours or less, sometimes 3.5 into the wind, so I don't often see a significant weight change from fuel burn. [ 3 hr x 9 gph x 6 ppg = 162 lb, not much]

[N601RX]

It's very noticable in a 150.  The diference in one pilot and 10 gallons and being at gross or slightly overweight is almost 10mph.  I've noticed this on several ocassions. 

Ask any of the RV builders or racers.  They go to great lengths to keep them light.

[rob]

Hank: I have a few issues with what you're saying. 

1. IAS is important always, as a metric of how the plane is performing. You should be familiar with what your plane indicates in particular configurations- any deviation could be an indication of a problem. I'm not advocating keeping track down to individual knots, but you should have a general idea of where the needle rests under normal operating conditions.

2. You state that 162lbs is not much. At what point is the weight enough? That last .5 when you're flying into the wind starts 162 lbs ligher than your first .5. That's akin to one standard FAA person missing. It's over 6 percent of your gross weight. I'm no engineer, but I think we can agree that 6% is significant, can't we? If not, at what point is it significant? 

[Hank]

Quote: rob

Hank: I have a few issues with what you're saying. 

1. IAS is important always, as a metric of how the plane is performing. You should be familiar with what your plane indicates in particular configurations- any deviation could be an indication of a problem. I'm not advocating keeping track down to individual knots, but you should have a general idea of where the needle rests under normal operating conditions.

2. You state that 162lbs is not much. At what point is the weight enough? That last .5 when you're flying into the wind starts 162 lbs ligher than your first .5. That's akin to one standard FAA person missing. It's over 6 percent of your gross weight. I'm no engineer, but I think we can agree that 6% is significant, can't we? If not, at what point is it significant? 

[jetdriven]

Always calculate density altitude, then look at your IAS with the equal power settings.

[rob]

I flew a 660nm flight earlier today in my M20C and was blessed with smooth air and relatively consistent winds at 9000'. Temperatures were well above standard for the duration but didnt vary much from 14-16.  I made a mental note to check the IAS and TAS as reported by my Aspen at about 5gal intervals on my fuel totalizer. What I concluded is very unscientific, so be kind. 

The plane was loaded with full fuel, me(210) and about 65lbs in the baggage compartment. I had my seat fully aft and the autopilot with altitude hold was doing the flying. Airspeeds were pretty stable and the air was mostly clear. 

At the top of climb the plane settled into a TAS of approximately 167mph. This didn't change initially, though after 5 gal burn I noticed that it began to fluctuate more toward 168 than 166. At 10 gal burned, it was pretty steady at 168mph. I didn't notice another change until I had burned 30 gallons, at which point I was flying 169-170mph TAS. Unfortunately, I was given a descent at this time, so I don't think Any data beyond that had meaning, though at 7000' I was indicating 172/173mph. 

 

FWIW - At the initial 9000, power was WOT (about 21") and the prop was set to 2500RPM. I was leaned to approx. 8.4gph. Winds were quartering tailwinds at approx 30 mph and 30degrees. 

[Steve Dawson]

How does your angle of attach change if your CG changes very little with the adding or subtracting of weight in the aircraft and/or you trim to keep a similar AofA?

I think it has more to do with F=MxE .

Where are the aeronautical engineers? 

Quote: Hank