RE: [Dragonflylist] Re: Cherokee 140-150
> Based on the Cherokee, it all makes perfect sense, I never thought ofthe
> engine as derated, but my instructor who was Piper all the way trainedme
> that there was no problem with cruising at full throttle with theCherokee
> which was 2500 rpm. At altitude and leaned out, with two people aboardI
> used to figure 10 gal per hr. but it actually burned about 9.6 andmade 143
> mph.Cool! Some better numbers to play with.
9.6 GPH would work out to 132.4 hp assuming
.435 lbs of fuel per HP per hour.
If the plane makes 133 hp @ 2500 RPM, (assuming .435 BFSC,
the torque works out to be 279.5 ft-lbs. Bump the RPM's to 2750
(supposing a flat torque curve) and 146.3 would be the new HP.
Factoring power loss from altitude (.5" MP per 500')
these numbers seem to jive.
...Which just goes to show you, you don't make HP w/o
In other words, folk who claim 3 gph @ 75% power are
either lying, or don't understand how things work...
and this is a pet peeve of mine, and here's why:
Say for example, we have a 2180 VW engine rated at 80hp
WOT (wide open throttle, 30" MP) at 3400 RPM.
Joe home builder figures that 2550 is 75% of 3400,
so if he throttles back to 2250, he's making 75% power,
or 60 ponies, and low and behold, he's sipping 3 GPH!
So the boast goes something like, "I cruise at 75% power,
and if I lean it way back, I get 3 GPH."
Well... at 3 GPH, he's only making 41 ponies!
Here's the flaw in the thinking...
Without knowing manifold pressure (MP) you really have no
way of knowing HP, unless you figure fuel burn. If your
2180 is making 75% power, (60 ponies) you should be burning
4.5-5 gph, depending on your "brake specific fuel consumption"
(BSFE), which up till now, I've been using .435 lbs per hour,
per HP... but in all reality it's probably closer to .5
.5 BSFC would be a solid 5 gph fuel burn at 60 hp.
But I digress...
You should know that when you are in your driveway, sitting
in your car, you are not burning near as much fuel (per hour)
holding the RPM's at 3500 RPM, as you would if you were
running down the highway at 70. Additionally, you will
burn even MORE fuel at 3500 RPM, if you have to floor the
car to maintain 3500, going up a hill.
3 different 3500 RPM scenarios, 3 different HP settings
3 different fuel burns. Same RPM, what's the difference???
Manifold pressure! In the first scenario, the throttle is barely
cracked, 14" MP for sure, In the second scenario, we're
probably up to 22" MP. In the last scenario, we are seeing 29"!
Now remember when I said that we loose approximately .5" MP for
every 500' we climb? Assuming 30" (29.92) at sea level, a
4000' climb gives you 26" at best, 8000', we only have 22" MP
available with WOT... The engine now makes the same HP at 8000'
WOT than it would at sea level, with the throttle reduced to 22".
22 is 75% of 30, so 22" is probably 75% power with the throttle
wide open. So... if you are at 8000', and want 75% power,
don't move the throttle, you are there! But!!! Some fine folk
think that even at 8000' they need to reduce the RPM's 75% to
reduce power... hence the low fuel flow numbers reported by
Another smaller issue is that MP is affected by the barometric
pressure (not all days are 29.92) and by temperature. So the
8000' = 75% power isn't always true, BUT!!!! 22" MP is always
22" MP, no matter the conditions or altitude!
So I'll sum up this rant by advocating that EVERYONE who is
interested in the performance of their engine, run out and
get a manifold pressure gage. Even if you don't install it
in your panel, take it up with you and watch the pressure
go down, as your plane goes up, and then ask yourself why you
are reducing the power.
MP gauges are REAL cheap on e-Bay.
I think you are assuming the speed brake is hinged in the front. It
is actually hinged in the back and once the panel is open a little
the airflow will be trying to open it further. Drew no longer has
speed brakes on his Raptor because with a 38% elevator he does not
need a whole lot of help slowing the plane down. I now have one of
the Raptor's two old speed brakes mounted on the belly of my
Dragonfly (thanks Drew).
Go to http://www.fly-raptor.org/~jeff/Latest.htm and scroll down to
the 24 AUG update. The picture with the spped brake deployed is
from the back of the plane looking forward.
We discussed several speed brake options and I almost started
cutting on the fuselage more than 1 time. I am not an engineer and
do not pretend to be an expert with this stuff, but I do know what
effect I want to have. I want to slow down the plane and I do not
want the fuselage angle to be more nose up when doing so.
Some designs that we tossed around were the knife type device you
are talking about. It would have been attached to the backside of
the seat bulkhead, but with a center stick control there would have
been a big horseshoe shape cut out right in the middle. In addition
I have a bunch of stuff going on in this area in my modified
My favorite design was a clam shell type device that would have come
out of both sides of the fuselage aft of the wing. Drew was
concerned about disrupting the airflow around the rudder and
ailerons so I trashed that idea.
I also came up with something similar to the Q guys speed brakes,
but Drew was concerned about the cable system and buffeting.
When I flew with Rich I noticed a nose up pitch change when he
deployed his speed brake. I would suspect if you missed the
position of the speed brake a little it could have a serious impact
on the attitude when deployed. I would also suspect that it could
have a different effect with a forward CG and aft CG (single pilot
and dual pilot). I would not think it would be very good to deploy
the brake solo and have the nose pitch up and then have it pitch
down when you have a passenger.
With all that in mind....what I hope to do is create lots of drag
and not change the attitude of the aircraft in doing so.
p.s. as Pat said, the upper part of my engine mount is still in the
--- In Dragonflylist@yahoogroups.com, Dave Morris <dave@d...> wrote:
> Drew, how much force is pressing on that air brake while you're
> push it down against the airstream? And why not design something
> slides down into the straight in a linear slicing action through a
> slot, rather than requiring so much force to push it into the
> slipstream? Your motor could be tiny, and the difference would be
> you'd need a strong frame.
> Dave Morris
> P.S. That engine mount only has 2 bolts holding the whole thing
> firewall?? No attachment to the top of the firewall?
> At 11:06 AM 11/3/2003 -0800, you wrote:
> > > Rich and I are waiting for Jeff to hurry up and show us how to
> > > aluminum tank so we can make a 17 gallon fitted one for the
> > > fuselage that is our tank frame.
> >No reason to wait... it's on his website now:
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