Morning Speed run / scare on final

Part 1

I decided this morning I was going to see how fast my -7A was as it was smooth and I was by myself and I wanted an excuse to fly. My gross at takeoff was about 1100 empty + 40 gals (240) + me (230!) = 1570 lbs. I have all fairings on and I'm unpainted.

I made a 4 way speed run at 2500 ft on the altimeter, OAT 58 F, altimeter setting 30.03. This worked out to a density altitude of about 3000 ft. I was WOT, 2600 RPM. I have a Superior XP-360 180 HP carbureted, mags, and the Hartzell 74" blended airfoil prop.

GPS
Heading Groundspeed

180 189 kts
090 184 kts
000 168 kts
270 174 kts

using the true airspeed calculator here http://www.csgnetwork.com/tasgpscalc.html it came to 181 kts true. This is almost exactly 208 MPH just like Van's web site says for top speed. Would it be faster down low (500 ft off the deck?). At any rate I believe Van's numbers.

Part 2

With a smile on my face, I headed back to base only to turn final, pull the power and ------ POP, POP, BACKFIRE, ALMOST DEAD ENGINE! Pushed the power back in and found that it would run smoothly above about 1800 rpm.

Got it back on the ground, and had to jockey with the throttle to get it to keep running to get back to the hangar. Uncowled the beast and lo and behold - the spring held in place idle mixture set screw had come off in flight and departed the airplane! I have a new Precision Airmotive MA-4-5 carb. I called them and they said they would send a new one all expenses paid, I should have it back in a couple of days. Great customer service.

Has anyone else experienced this?

Thanks!

This is almost exactly 208 MPH just like Van's web site says for top speed. Would it be faster down low (500 ft off the deck?). At any rate I believe Van's numbers.

Click to expand...

TAS increases with density altitude.

True but

horsepower also decreases. It just seems like everytime I read testing numbers for top speed of an aircraft, it is wide open, max rpm, at sea level. Either way, I'm totally happy!

Congrats nice job

Rusty: Nice job, you should be happy. What matters is you gave all the data, altitude, baro, temp, gps speeds. Nothing wrong with that, but MAP would be a good number to also report to calculate HP (estimate).

From your numbers if you could fly at sea level and 2700 RPM you would be about 3-4 mph faster, say *209 to 210 mph, which is better than the 208 mph top speed spec Van reports for a solo RV-7A (180HP) @ sea level. As you already said, Van's numbers are right and your plane is as fast or better than Van's specs.

One note is at 2600 rpm you where not making full HP; the extra 100 rpm, 2700 rpm, would have increased speed about 1 or 1.5 mph.

BTW I have O-360-A1A with a MA-4-5, never heard of the set screw coming undone. Bet I will take a look at mine.

George

*PS: Just looking at your raw numbers, using any three legs, I got 179 kts or 206 mph. I could be missing something, but if you go to sea level at 2700 RPM your speed is faster than book. Well done

My mistake

I used 189, 184, 174 in the calculator, which were not contiguous left turns. You are correct George, if you use contiguous left turns, it is about 178 - 179 knots true. Thanks for the input! BTW I mistakenly left out I was running about 27.5 inches of MAP. I also noted an anomaly with the RPM. If I run 2700 RPM, I lose about 1 knot over 2600 RPM. I don't know why unless it has something to do with the Engine Power curve and the Prop efficiency coinciding at that point for my airplane and the drag it produces. That is why I chose 2600 RPM.

Thanks!

The important question...

Rusty, glad to hear you made it back safely without any mishap. Now the important question, will you be able to get the part in time to get to Waxahachie?

I'll look forward to seeing you down there.

Stewart

TAS increases with altitude, and HP decreases with altitude. The magic number is 8000 feet, where a normally aspirated engine at wide open throttle will make 75% power (plus or minus a bit for conditions.) This is the condition at which Van's reports their 75% cruise numbers, and this is the condition you should test at to get a true apples to apples comparison.

With the numbers you're getting down low, I'd expect to see a few mph gain at 8K, which is reasonable given the blended airfoil prop. In the recent past there have been some posts questioning the validity of Van's published performance numbers and it is good to see people posting as good or better.

Speed demon!

Rusty,
We did the same thing but at 7500 feet and WOW! You'll definitely go faster and maybe pick up some RPM in the thinner air, even tho' it'll only be making 74% or so.
Fun, ain't it??

Lost Idle adjustment screw

Lost mine on a flight about 3 or 4 years ago. Only had a few hours on the plane at the time. It too was a brand new carb. Noticed it when I pulled the power going into downwind. Definitely a wakeup call!!
Replaced it and never had the problem again.
Glad you made it down OK!

Rusty, Im not familiar with all my carb adjustments yet but is it the spring/screw just to the left of the mixture lever in this photo?

Thats the one!

Yes, that is the culprit. I would have never dreamed it could come out on its own. I intend to put a drop of torque seal orange at the top of it when I get it set, and you can guarantee it will be on my preflight checklist to look and verify that it hasn't moved! You can see this screw from looking up thru the exhaust stack from the bottom of the airplane. I figure as long as I can see the orange torque seal on the top of the screw head, it hasn't moved.

Wow I cant see it coming out either. It must have been unscrewed a bunch before flight to have vibrated out. I bet once you replace it and have it adjusted properly you'll never have that problem again. Its a good thing to know that if it did fall out that the engine will still get you home. I assume its for the idle circuit right?

-Jeff

Yes, thank goodness! Above about 1800 rpm it ran smoothly and you would have never known anything was wrong.

Make sure the spring is under compression when the needle is at the desired setting.

Stretch the spring a bit if needed.

The side force on the threads from the spring is what keeps the needle in place.

Mike

The devil is in the details

sprucemoose said:

TAS increases with altitude, and HP decreases with altitude. The magic number is 8000 feet, where a normally aspirated engine at wide open throttle will make 75% power (plus or minus a bit for conditions.) This is the condition at which Van's reports their 75% cruise numbers, and this is the condition you should test at to get a true apples to apples comparison..

Click to expand...

TAS verses Altitude

Jeff: What do you mean TAS increases with altitude? I think I understand but
TAS does not automatically go up with altitude, in fact TAS goes down with
altitude with normally aspirated engines. I think you mean TAS increases (as
a percentage) relative to IAS, at approx is 2% per 1000 feet. I also suppose
if you can maintain thrust with altitude you will indeed increase TAS, and
those are called jets (or turbocharged) airplanes.

For use TAS goes down 1 mph and change for every thousand feet, and at
some point the speed drops even faster with altitude as drag increases.
Because speed vs. altitude involves thrust, drag and lift, it is hard to define
an exact curve unless we do individual flight test. However if you look at
Vans data from sea level to 8,000 feet you loose 10 mph, or about 1.25 mph
per 1000 feet. So if you take the flight test data as 206 mph @ 3000 ft, than
top speed is about 210 mph or a little better than Van's 208 mph top speed
at sea level for a RV-7A/180HP. The fact is with RPM restricted from 2700
rpm continuous he is giving up about 1 mph, may be 2. So his absolute top
speed is 211-212 mph, about 3 to 4 mph faster than Spec.

With all flight test data there is error. GPS satellite postion can contribute to
this in part. For example using the 4 data GPS speed, there is scatter or
deviation when comparing any three legs. The trick would be to take more
tracks and get better correlation. I like the track method better, Kevin h talks
about. When doing heading we have compass error to take into account. The
point is the GPS data was not consistent. In part winds aloft are not
consistent. May be doing it at the Crack-O-dawn in calm conditions. Reading
winds aloft forcast to get calm conditon. So even with a 1-2% the data is
right where Van's numbers are and likely a tab better. Than ain't bad.

BENCH MARK? ITS A MOVING TARGET

The other point about 8,000 feet being "The bench mark", I agree. However
you would think 8,000 ft, even at WOT, would assure approx 75% power or
less. That has always been the case in planes I have flown. However it's a
little misnomer, because in many RV's they can and do make more than 75%
power at 8,000 ft. So just as a comment we still have to specify all
parameters.

With all of our RAM air, cold intakes, free flowing exhaust and fairly high
speed (more intake pressure) we gain the efficiency. So if able you have to
reduce RPM (constant speed prop) or reduce MAP with throttle (fixed pitch).
That is a good reason to have a MAP gauge, even on a fixed pitch RV. Other
wise you don't have a good idea what your power is. The other option is fly
higher than 8,000 ft but that defeats the purpose of a bench mark.

Here is an example where Van specifies 2,500 RPM (for the constant speed
prop planes). He had to correct the fixed pitch prop speeds, which was
over 2,700 RPM at WOT / 8,000 ft!

http://img82.imageshack.us/my.php?image=prop200415me.jpg

In a Cessna with lousy intake/exhaust, going 120 mph @ 8,000 ft, may be
can only muster 70% power.

8,000 feet is magic but does not define enough info alone, but as long as we
report the parameters: MAP/RPM/Alt/Baro/Temp, we have enough info to
apples and apples. Fuel flow would be nice to know also. It is like this flight
test data at 2,500 ft indicated. With enough data defined you can "normalize
it". The nice thing about 8,000 ft is that is a normal cruise condition and
everyone can safely fly it, even in high country. Flying WOT full RPM on the
deck is a little sporty. Bottom line we still need all the data to compare,
which Rusty did an excellent job of.

Cheers George


PS: Got a screw loose?
Is it just me, but I find it funny that all the nuts and bolts on the Carb are
safety wired or tab locked and this one screw is just loose with spring
resistance to hold it in? Hummmmm I guess it is not "critical" it can enough
and is OK this way?