REFLECTOR: Takeoff roll - fuel burn

[Al Gietzen]

Al, would moving the CG forward shift additional load onto the front of the
plane (canard) which would need to be overcome by generating more lift with
a higher t/o speed? IOW did you happen to notice the t/o speeds for the last
2 tests? Were they the same, or was the last test a higher speed?

Yes; I'd say that's it exactly. Although I didn't record the speed; the
liftoff speed is pretty much directly proportional to the length of the roll
- all other things being equal.  The required speed on final for the forward
CG case was at least 10 -15 knots higher. I guess it would have been
interesting to do gear-down stall speed check.

 

 Also as we know, the canard is pointed downward slightly which creates more
pressure on the nose wheel increasing rolling resistance which decreases
acceleration resulting in a longer t/o roll. 

I think the incidence of the canard is close to neutral when the plane is
level - I'd have to check. I generally trim the elevator about 1" trailing
edge down for the takeoff roll, so the overall effect is positive incidence.
As I get to liftoff speed, say roughly 70 kts, I get a 'feel' for when the
nose is ready to lift - if so, I'll apply a bit more back pressure and start
'flying' the canard with the nose wheel off until the mains lift.  Applying
too much elevator too soon just adds drag and extends the roll.

 

What's your analysis of these results?

First; these results are specific to my airplane and my technique, so it is
not to be assumed that they are generally applicable.  Secondly; I'm sure
there would be a fair amount of scatter in the data if one were to do 3-4
runs with the same condition. The first case is a far more common condition
for me, and I have a number of consistent observations, and for which I have
good technique.  I have never before flown with that forward CG (and
weight); so it is likely with a number of attempts and a better feel for the
behavior, that roll may be reduced. It was definitely different on both
takeoff and landing.

 

The data does give me some information about the effect of weight and CG; so
combined with the use of the Koch chart, and a bit of a safety factor, I can
have a pretty good idea of runway requirements.

 

Al

 

 

Chuck H

 

From: Al Gietzen <mailto:ALVentures at cox.net>  

Sent: Thursday, May 21, 2009 11:47 PM

To: reflector <mailto:reflector at tvbf.org>  

Subject: REFLECTOR: Takeoff roll - fuel burn

 

Here's a bit of data from today's measurements.

The question came up here awhile back about fuel economy vs CG.  I did
flights today in my SE RG with CG at 120", total weight about 2060; and at
116", total weight about 2270.  The CG was shifted by adding a person in the
right seat. Surprising; within the accuracy of the measurements, both cases
were the same: 160 KTAS burning 9.5 gph at 2500 rpm on a fixed pitch prop
(rotary 20B, lean cruise). At pattern speed of about 100 KIAS I noted the
heavier, forward CG took more power; but I didn't take any data.

 

Takeoff roll - 85F; 1400' field elevation, roughly 8-10kt wind component
down the runway, fixed pitch cruise prop.

 

Weight - 2100; CG - 120"; about 1630 ft

Weight - 2300; CG - 120"; about 1730 ft

Weight - 2300; CG - 116"; about 2300 ft

 

These are all single run measurements, taken by spotting relative to taxiway
exits and edge lights, and taken from a rolling start - rolling on to the
runway, getting it straight and adding power. Moving the CG forward made a
more dramatic difference than adding weight, and I didn't like the way it
handled at pattern speed, or landing.

 

The first case above was repeated using so-called 'Short field takeoff'
procedure - getting lined up near the end of the runway, and applying power
while holding brakes; then release.  Takeoff roll was the same as the
rolling start. My rotary spools up very quickly when I push in the throttle;
things may be a bit different with a Lyc.

 

Al

 

 

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