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dreel(at)cox.net
Guest

Posted: Tue May 30, 2006 6:47 am Post subject: How to add 15kt to cruise speed by adding weight

I've been exploring the CG range in flight testing of my RV8A and have found

an unanticipated cruise speed effect.  At 85.9 inches, I was getting 155kias

with 2300rpm, 23 in mp, 5,500 ft, and 42 degrees F outside.  At 79.6 inches,

I got 142kias with 2300rpm, 23 in mp, 4,500 ft, and 64 degrees F outside.

My whiz wheel gives true airspeeds of roughly 169kt and 154kt.  Quite an

improvement just by adding some weight in back.  Yes, the fast flight was

with a little bit higher weight!



There's lots of discussion of speed improvements in the archives, but

nothing related to CG location that I could find and certainly nothing so

easy to get and of this magnitude.  So, my question to everyone is have you

had similar experience?  Do you know anywhere this has been quantified or

further detailed?  After all, I'm just comparing two test flights here, but

theoretically, I'd guess the decrease in drag due to decreased pitch

stability requirements as the CG moves aft would explain the speed increase.



Excited!



Dave Reel - RV8A

recapen(at)earthlink.net
Guest

Posted: Tue May 30, 2006 7:14 am Post subject: How to add 15kt to cruise speed by adding weight

Have you tried simply altering the CG and not the weight?



I think I remember reading somewhere that the shifted CG changes stabilized AOA thereby changing drag characteristics.



Geek - not aerodynamicist...and I stayed at a Marriot this weekend instead of a Holiday Inn!



--

alexpeterson(at)earthlink
Guest

Posted: Tue May 30, 2006 7:25 am Post subject: How to add 15kt to cruise speed by adding weight

 	  Quote: 	
		   

 

 I've been exploring the CG range in flight testing of my RV8A 

 and have found an unanticipated cruise speed effect.  At 85.9 

 inches, I was getting 155kias with 2300rpm, 23 in mp, 5,500 

 ft, and 42 degrees F outside.  At 79.6 inches, I got 142kias 

 with 2300rpm, 23 in mp, 4,500 ft, and 64 degrees F outside.

 My whiz wheel gives true airspeeds of roughly 169kt and 

 154kt.  Quite an improvement just by adding some weight in 

 back.  Yes, the fast flight was with a little bit higher weight!

 

 There's lots of discussion of speed improvements in the 

 archives, but nothing related to CG location that I could 

 find and certainly nothing so easy to get and of this 

 magnitude.  So, my question to everyone is have you had 

 similar experience?  Do you know anywhere this has been 

 quantified or further detailed?  After all, I'm just 

 comparing two test flights here, but theoretically, I'd guess 

 the decrease in drag due to decreased pitch stability 

 requirements as the CG moves aft would explain the speed increase.

 

 Excited!

 

 Dave Reel - RV8A


	


Dave,



What were the fuel burns during those two runs?  One cannot state that the

power was the same, only that the MAP and RPM were the same.  You can vary

airspeed greatly with mixture.



A more aft cg is more efficient, but not that much.  (The tail doesn't have

to generate as much downward lift.)



Alex Peterson

RV6-A N66AP 755 hours

Maple Grove, MN

lsbrv7a(at)yahoo.com
Guest

Posted: Tue May 30, 2006 7:39 am Post subject: How to add 15kt to cruise speed by adding weight

David,

  That is true in my Bonanza as well, although I have never tried to quantify it.  Less drag from less elevator (ruddivator) down force.

  The Bonanza experts warn of CG change with fuel burn moving the CG aft and the possibility of elevator sensitivity during the landing flare resulting in over correction.

   

  Did you notice increased elevator sensitivity?



DAVID REEL <dreel(at)cox.net> wrote:

  



I've been exploring the CG range in flight testing of my RV8A and have found

an unanticipated cruise speed effect. At 85.9 inches, I was getting 155kias

with 2300rpm, 23 in mp, 5,500 ft, and 42 degrees F outside. At 79.6 inches,

I got 142kias with 2300rpm, 23 in mp, 4,500 ft, and 64 degrees F outside.

My whiz wheel gives true airspeeds of roughly 169kt and 154kt. Quite an

improvement just by adding some weight in back. Yes, the fast flight was

with a little bit higher weight!



There's lots of discussion of speed improvements in the archives, but

nothing related to CG location that I could find and certainly nothing so

easy to get and of this magnitude. So, my question to everyone is have you

had similar experience? Do you know anywhere this has been quantified or

further detailed? After all, I'm just comparing two test flights here, but

theoretically, I'd guess the decrease in drag due to decreased pitch

stability requirements as the CG moves aft would explain the speed increase.



Excited!



Dave Reel - RV8A



Sherman Butler

RV-7a Wings

Idaho Falls

		

---------------------------------

Be a chatter box. Enjoy free PC-to-PC calls  with Yahoo! Messenger with Voice.

LloydDR(at)wernerco.com
Guest

Posted: Tue May 30, 2006 8:10 am Post subject: How to add 15kt to cruise speed by adding weight

This has been discussed before on the 10 list. The heavy lift planes of

the military will shift loads in flight to do this same thing. How much

and to what degree, you would have to talk with a loadmaster, and they

tell real interesting stories!

So, yes it is possible, but to do it reliable in a small aircraft,

without getting your CG stuck in an aft position, is the big risk, and

not worth it in my book.

YMMV

Dan 

RV10 40269

N289DT

 

Do not archive

--

dan.beadle(at)inclinesoft
Guest

Posted: Tue May 30, 2006 8:39 am Post subject: How to add 15kt to cruise speed by adding weight

This is basic aerodynamics.  To get stability (AKA, hard to flare), you move

the CG forward.  Think Lawn Dart.  In this mode, the tail actually pulls

down, not up.  This results in the wing needing to produce more lift (weight

+ the tails negative lift so that total lift = weight).



If you move the CG back, you have less and less down-lift on the tail,

letting the wing work less, thus getting less induced drag.  Faster.



A heavier airplane, with the same CG, should fly slower than the same plane,

same CG, when it is lighter. The reason is the heavier airplane must have a

higher angle of attack to produce the needed additional lift.  That means

more induced drag.



The CG limits are set to give reasonable handling.  The forward CG is

limited by the ability to flare.



The Aft CG is limited by the need for positive stability. If we moved the CG

back to where the tail was lifting, we would have no stability.  In the

normal range, as the plane slows from the trimmed AS, the wing and the tail

both lose some lift.  But the tail is now lifting less in the downward

direction, letting the nose fall down a bit and resume the trimmed AS.



If we move too far back, we lose the stability feature making the airplane

difficult or impossible to fly.  



Rule of thumb:  smooth air, load for aft CG (within limits).  Rough air,

keep a more forward CG.



So your data does make sense.  (But the faster/heavier combination is

probably just due to aft CG.)



Dan

--

Tim(at)MyRV10.com
Guest

Posted: Tue May 30, 2006 9:15 am Post subject: How to add 15kt to cruise speed by adding weight

I don't see it as an especially big risk, providing it's a fairly

simple load move.  People don't think twice about putting another

person in the rear seat, which is basically what we'd be talking

about here, or putting some baggage in the tail.



I first noticed this phenomenon in my Sundowner when I brought

along a CFI buddy and his girlfriend, who was a pretty chunky chick.

It was weird that we had a noticeably higher cruise speed with

her, than without.



So sure, if you're going to fly X/C somewhere, do the load

calculations and if you're way forward on CG, feel free to

NOT put that big pile of bags in the rear seats, but

move them to the baggage area if it fits there and weighs

out properly.   Just don't go aft of CG.  That's what the

range is there for.



Tim Olson - RV-10 N104CD - Flying

do not archive

Lloyd, Daniel R. wrote:

[quote] 

 

 This has been discussed before on the 10 list. The heavy lift planes of

 the military will shift loads in flight to do this same thing. How much

 and to what degree, you would have to talk with a loadmaster, and they

 tell real interesting stories!

 So, yes it is possible, but to do it reliable in a small aircraft,

 without getting your CG stuck in an aft position, is the big risk, and

 not worth it in my book.

 YMMV

 Dan 

 RV10 40269

 N289DT

  

 Do not archive

 --

jjessen

Joined: 22 Apr 2006
Posts: 285
Location: OR

Posted: Tue May 30, 2006 9:53 am Post subject: How to add 15kt to cruise speed by adding weight

Someone, on some list, suggested a system of water bottles that you empty /

fill as needed.  I know about the Bonanza gaining aft CG as fuel burns, but

not sure about the RV's.  I think the -10 might.  One might have full water

on take off and empty as the CG naturally moves aft with fuel burn.  The

amount of water dependent on the load calculations.  



John Jessen - Tailcone - RV10

  do not archive

 



--

Ed Anderson

Joined: 10 Jan 2006
Posts: 475

Posted: Tue May 30, 2006 10:09 am Post subject: How to add 15kt to cruise speed by adding weight

Hi Dave,



I discovered the same effect in my RV-6A which is a bit nose heavy.  Best 

top speed was when I had the baggage compartment load to the max with 

camping gear and full wing tanks.  I suspect you are correct in that the aft 

CG results in less down component from the horizonal stabilizer resulting in 

less drag.   The wing incident may also be a bit more optimum (less) also 

resulting in less drag.  As best I could determine my speed increase was 

between 8-10 MPH.



Ed

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson(at)carolina.rr.com



---

_________________
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com

smittysrv

Joined: 10 Jan 2006
Posts: 67

Posted: Tue May 30, 2006 12:18 pm Post subject: How to add 15kt to cruise speed by adding weight

Funny that this topic came up. Over the weekend, My wife, 10 year old son

and I went a short one hour flight in our C172. My 10 year old pitched a

royal hissy, wanting to ride in front with me. Rather than argue, my wife

reluctantly volunteered to sit in back seat. This was the first time in

many pounds since we have done this. The take-off felt a little weird but

manageable. When we leveled off I noticed that the ground seemed to going

by a lot faster than usual. We were going 15kt faster!

On the return fight, my wife made the kiddo sit in the back. Man, that was

a long (and slow) trip back. Hmmm, in the future, how do get the wife in

the back seat without being obvious? I guess I'll keep my mouth shut and

fly slow.

Ya'll have a goodun!

Smitty

http://SmittysRV.com

[quote] 

 I don't see it as an especially big risk, providing it's a fairly

 simple load move.  People don't think twice about putting another

 person in the rear seat, which is basically what we'd be talking

 about here, or putting some baggage in the tail.

 I first noticed this phenomenon in my Sundowner when I brought

 along a CFI buddy and his girlfriend, who was a pretty chunky chick.

 It was weird that we had a noticeably higher cruise speed with

 her, than without.

 So sure, if you're going to fly X/C somewhere, do the load

 calculations and if you're way forward on CG, feel free to

 NOT put that big pile of bags in the rear seats, but

 move them to the baggage area if it fits there and weighs

 out properly.   Just don't go aft of CG.  That's what the

 range is there for.

 Tim Olson - RV-10 N104CD - Flying

 do not archive

 Lloyd, Daniel R. wrote:

> 

>

> This has been discussed before on the 10 list. The heavy lift planes of

> the military will shift loads in flight to do this same thing. How much

> and to what degree, you would have to talk with a loadmaster, and they

> tell real interesting stories!

> So, yes it is possible, but to do it reliable in a small aircraft,

> without getting your CG stuck in an aft position, is the big risk, and

> not worth it in my book.

> YMMV

> Dan

> RV10 40269

> N289DT

>

> Do not archive

> --

_________________
Smittys RV-9A
SmittysRV.com

Dale Ensing

Joined: 11 Jan 2006
Posts: 571
Location: Aero Plantation Weddington NC

Posted: Tue May 30, 2006 12:33 pm Post subject: How to add 15kt to cruise speed by adding weight

Hmmm, in the future, how do get the wife in

the back seat without being obvious? 

Smitty, just think back a few years. How did you do it then?

anonymous RV-6A

do not archive

_________________
Dale Ensing
RV-6A
Aero Plantation
Weddington NC

rick(at)n608ef.us
Guest

Posted: Tue May 30, 2006 12:41 pm Post subject: How to add 15kt to cruise speed by adding weight

I like Dan's explanation, but will add a bit more.



Specifically, we require the CG to be forward of the center of lift (CL) in

order to have a stable aircraft (and one whose nose drops when the wing is

stalled).  The closer the CG gets to the CL, the less induced drag, but the

less stable the plane will be.  If the CG should go aft of the CL, all bets

are off in terms of controllability.



Another e-mail commented on Bonanzas and how their CG moves with fuel burn.

Whether the CG moves aft with fuel burn should depend on the CG of the tank

and its fuel relative to the CG of the plane.  I fly an A-36 Bonanza, and

when weight is up (hence, realistically, with a CG farther aft than when

lightly loaded), I have indeed noticed greater pitch sensitivity after a

long flight.  When I plan flights at higher weights, I check my loading by

calculating CG for zero fuel, just to be on the safe side.  For extra

margin), I try to land with fuller tanks, too.



Rick

A-36, RV-7 in process



--

jpl(at)showpage.org
Guest

Posted: Tue May 30, 2006 1:00 pm Post subject: How to add 15kt to cruise speed by adding weight

Hand her a camera and point out she can take photos out both sides on  

the way home 



-J



do not archive



On May 30, 2006, at 3:15 PM, Smitty wrote:



 	  Quote: 	
		   



 On the return fight, my wife made the kiddo sit in the back. Man,  

 that was

 a long (and slow) trip back. Hmmm, in the future, how do get the  

 wife in

 the back seat without being obvious? I guess I'll keep my mouth  

 shut and

 fly slow.



 Ya'll have a goodun!

 Smitty

 http://SmittysRV.com

low pass

Joined: 10 Jan 2006
Posts: 32
Location: Houston

Posted: Tue May 30, 2006 1:35 pm Post subject: Re: How to add 15kt to cruise speed by adding weight

Couple of points on this topic:



1.  A friend said they used to (probably still do) add steel weights to the tail of AT-6's that race in Reno for speed improvements.  Can't recall, but I believe they have minimum weights and are probably working with that number but just shifting the CG aft.



2.  A few years back I started trying to quantify this situation with adding weight aft.  I never got a conclusive answer, but wondered if the benefit of adding ballast to move the CG aft offset the penalty of added overall weight.  



Example:  I am flying (racing) with a near empty airplane.  Would the plane be faster with 50-lbs of ballast in the rear baggage compartment than without the 50-lbs?  Or faster with 50-lbs of ballast and if I lost 20-lbs??    



Seriously, it sounds like there may be some conditions where the added overall weight to move the CG aft may help.



Bryan

Jerry Grimmonpre'

Joined: 09 Jan 2006
Posts: 144
Location: Huntley, Illinois 60142

Posted: Tue May 30, 2006 2:11 pm Post subject: How to add 15kt to cruise speed by adding weight

Passed on for what it's worth ...

I've seen it demonstrated, in the big iron, when the airlines loaded the a/c 

tail heavy, the elevator would raise the tail and thereby, create an airfoil 

shape.  The hs/elevator would sort of act like an airfoil and create lift. 

The up movement of the tail would reduce the wing incidence and shed some 

induced drag.

Jerry Grimmonpre'

RV8A Electrical



 	  Quote: 	
		   I discovered the same effect in my RV-6A which is a bit nose heavy.  Best

 top speed was when I had the baggage compartment load to the max with

 camping gear and full wing tanks.  I suspect you are correct in that the 

 aft

 CG results in less down component from the horizonal stabilizer resulting 

 in

 less drag.   The wing incident may also be a bit more optimum (less) also

 resulting in less drag.  As best I could determine my speed increase was

 between 8-10 MPH.



 Ed

 Ed Anderson

 Rv-6A N494BW Rotary Powered

 Matthews, NC

 eanderson(at)carolina.rr.com

jpl(at)showpage.org
Guest

Posted: Tue May 30, 2006 2:27 pm Post subject: How to add 15kt to cruise speed by adding weight

You could do some very careful calculations, then try adding a much  

smaller weight in the far aft tailcone area somewhere.  You wouldn't  

need remotely as much ballast as something in the baggage compartment  

- a few pounds would be pretty significant.



Not something you would want to do casually.  If remotely in doubt,  

you'd want to reweigh the plane afterwards and reverify the CG  

information.



As you say, 50 pounds of ballast could easily offset the advantages  

of pushing the CG back.  This would at least require far less than 50  

pounds.



-Joe



On May 30, 2006, at 4:35 PM, bdjones1965 wrote:



 	  Quote: 	
		   



 Couple of points on this topic:



 1.  A friend said they used to (probably still do) add steel  

 weights to the tail of AT-6's that race in Reno for speed  

 improvements.  Can't recall, but I believe they have minimum  

 weights and are probably working with that number but just shifting  

 the CG aft.



 2.  A few years back I started trying to quantify this situation  

 with adding weight aft.  I never got a conclusive answer, but  

 wondered if the benefit of adding ballast to move the CG aft offset  

 the penalty of added overall weight.



 Example:  I am flying (racing) with a near empty airplane.  Would  

 the plane be faster with 50-lbs of ballast in the rear baggage  

 compartment than without the 50-lbs?  Or faster with 50-lbs of  

 ballast and if I lost 20-lbs??  



 Seriously, it sounds like there may be some conditions where the  

 added overall weight to move the CG aft may help.



 Bryan

T.gummo(at)verizon.net
Guest

Posted: Tue May 30, 2006 3:03 pm Post subject: How to add 15kt to cruise speed by adding weight

Simple, get an RV-4, RV-8 or Harmon Rocket II.  If she isn't a pilot, she 

has to sit in the back.  



Tom Gummo

Apple Valley, CA

Harmon Rocket-II



do not archive



http://mysite.verizon.net/t.gummo/index.html



---

ronlee(at)pcisys.net
Guest

Posted: Tue May 30, 2006 3:30 pm Post subject: How to add 15kt to cruise speed by adding weight

 	  Quote: 	
		  Simple, get an RV-4, RV-8 or Harmon Rocket II.  If she isn't a pilot, she

has to sit in the back.  


	




Possible problem is that if she has any hint of claustrophobia she

may never fly with you again (at least in a -4).



Ron Lee



Do not archive

khorton01(at)rogers.com
Guest

Posted: Tue May 30, 2006 4:04 pm Post subject: How to add 15kt to cruise speed by adding weight

--- DAVID REEL <dreel(at)cox.net> wrote:



 	  Quote: 	
		   

 <dreel(at)cox.net>

 

 I've been exploring the CG range in flight testing

 of my RV8A and have found

 an unanticipated cruise speed effect.  At 85.9

 inches, I was getting 155kias

 with 2300rpm, 23 in mp, 5,500 ft, and 42 degrees F

 outside.  At 79.6 inches,

 I got 142kias with 2300rpm, 23 in mp, 4,500 ft, and

 64 degrees F outside.

 My whiz wheel gives true airspeeds of roughly 169kt

 and 154kt.  Quite an

 improvement just by adding some weight in back. 

 Yes, the fast flight was

 with a little bit higher weight!

 

 There's lots of discussion of speed improvements in

 the archives, but

 nothing related to CG location that I could find and

 certainly nothing so

 easy to get and of this magnitude.  So, my question

 to everyone is have you

 had similar experience?  Do you know anywhere this

 has been quantified or

 further detailed?  After all, I'm just comparing two

 test flights here, but

 theoretically, I'd guess the decrease in drag due to

 decreased pitch

 stability requirements as the CG moves aft would

 explain the speed increase.

 

 Excited!


	


A few comments:



1. If you keep the rpm and MP the same, the power

produced increases as you increase altitude (due, I

think, to the lower exhaust back pressure helping

improve the engine's breathing).  So, the test at

5,500 ft was with higher power than the test at 4,500

ft.  The air temperature was also quite a bit lower

for the 5,500 ft test point.  I don't know what engine

you have, but if you have an IO-360-A series engine,

the power at 5,500 ft was about 67.2%, and the power

at 4,500 ft was about 65.1%.  If you had an O-360-A

series engine, the power at 5,500 ft was about 75%,

and at 4,500 ft it was about 72.6%.



As Alex Peterson mentioned, the mixture setting could

also play a role, as you can change the power by

changing the mixture, at the same rpm and MP.



2. ASIs have instrument error, and the error will be

different at different indicated airspeeds.  Static

system position error is also different at different

indicated airspeeds.



Given items 1 and 2, it would be very interesting to

repeat these tests at the same altitude.  It will also

be interesting to record the fuel flow, to be sure it

is the same for each test.



3. At a given weight, there is less drag as you move

the CG aft (with the CG in approved limits).  The tail

has to produce a down force to balance out the nose

down pitching moment from the wing, and the fact that

the CG is ahead of the wing's centre of pressure.  The

total lift that the wing has to produce is equal to

the aircraft's weight, plus the amount of down force

that the tail is producing.  If you keep the aircraft

weight the same, and move the CG aft, the amount of

down force that the tail needs to produce decreases. 

This reduces the drag from the tail, but more

importantly, it also reduces the amount of lift that

the wing has to generate.



So, if you move some item of weight aft (baggage, a

body, etc), that will help reduce drag.



4. If you can't move an item of weight aft, some

people are interested to know if you can improve

performance by adding weight aft.  Ideally, the weight

you add would be in the area of the horizontal stab,

or further aft.  In this case, the amount of down

force that the tail needs to produce is decreased. 

This reduces drag, as there is less induced drag from

the tail.  But, the amount of lift that the wing has

to generate has not changed, for as far as it is

concerned, we have simply replaced some down force

from the tail by down force from gravity on our added

weight.  So the wing's induced drag does not change.



5. If you can't put the added weight at the HS, what

about putting it in the baggage compartment?  In this

case, some of the additional weight is supported by

the wing, and some by the tail.  So the amount of tail

down force decreases a bit, but the amount of lift

that the wing has to generate increases.  Depending on

how far aft the baggage compartment is, the total drag

may be increased or decreased.  My gut feel is that in

most cases the total drag would be increased.



6. it is very difficult to obtain repeatable speed vs

power data.  Any one test point may be in error due to

poor air quality, not waiting long enough for the

speed to stabilize, errors in setting the power or

mixture, or errors in recording the data, etc.  I

wouldn't draw any conclusions from one set of tests. 

If you can do the same test on three or four different

days and get the same speed vs power, then you know

you have something.



Keep us posted as you do more tests.



Kevin Horton

RV-8 (Finishing Kit)

Ottawa, Canada

http://www.kilohotel.com/rv8

T.gummo(at)verizon.net
Guest

Posted: Tue May 30, 2006 6:12 pm Post subject: How to add 15kt to cruise speed by adding weight

Of course, marrying wisely helps a lot too.  My wife watched me fly an F-4 

for twenty years and never even had one thought other than the Pilot sat up 

front and the other person sat in the back.  She picked the RV-4 as the 

starting point of my building a plane.  The RV-4 changed to a Harmon Rocket 

very early and didn't require reworking.



Gummi

 	  Quote: 	
		  >Simple, get an RV-4, RV-8 or Harmon Rocket II.  If she isn't a pilot, she

>has to sit in the back.  

 Possible problem is that if she has any hint of claustrophobia she

 may never fly with you again (at least in a -4).



 Ron Lee



 Do not archive

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