Triebwerk
Topic Author
Posts: 91
Joined: Tue Sep 02, 2008 12:29 am

If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 4:24 am

Suppose that a Cessna 172 makes a 60 degree roll to the left with only the left fuel tank selected. Having read in a book that the Skyhawk's fuel system is gravity-operated, would this steep angle of turn cause the plane to lose fuel flow to the engine?

On a similar vein, imagine that our troubled Skyhawk yaws hard to the right at level flight. Considering that the pitot valve is placed on the left wing, would this cause the Airspeed Indicator to display a higher airspeed than the nose is actually traveling at?
 
sccutler
Posts: 5556
Joined: Thu Jan 27, 2000 12:16 pm

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 6:00 am

It is, one supposes, hypothetically possible for an uncoordinated turn to last long enough to "unport" the fuel pickup in the wing, causing an interruption in the flow of fuel, but it would have to be a severely uncoordinated turn, and the tank, nearly empty.

Typical, however, would be a coordinated turn, in which event the force acting on the fuel in the tank would still be "down" relative to the airframe, and the fuel would flow like always.

As for your other question, the pitot tube can experience airflow which differs from the rest of the airplane, especially in a turn, but as long as the airplane's parts remain attached to each other, the difference will not be enough to materially affect the pilot's use of the airspeed information.
...three miles from BRONS, clear for the ILS one five approach...
 
Triebwerk
Topic Author
Posts: 91
Joined: Tue Sep 02, 2008 12:29 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 6:22 am

Ah, that makes sense. I had forgotten that the plane would still experience a downward force.

What about yaw, however? I know we're crossing into the hypothetical, but if you applied severe rudder to a Skyhawk operating on one fuel tank, could that force fuel to the side of the wing and prevent it from heading into the engine? (I suppose this all depends on the location of the tube connecting the fuel tanks and the engine. I'm assuming that it's close to the fuselage, but...)
 
wingscrubber
Posts: 806
Joined: Fri Sep 07, 2001 1:38 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 7:10 am

Gravity still applies during yaw, and a short hard yank to one side wouldn't interupt fuel flow for long enough to cause a problem - also don't forget there's quite a bit of fuel in the line line and filter. I think the scenario you're describing would only happen if you were continuously flying knife edge. (I'd like to meet a pilot who can knife-edge a 172!)
Gravity fed fuel systems usually come unstuck during negative G aerobatic flying, obviously because that's when the fuel flows the wrong way.
Resident TechOps Troll
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 8:02 am

The 172 POH says something to the effect of "Avoid extended slips", and I found out myself why one day.

I had planned a cross-country fuel stop at Coolidge, Arizona.

For kicks and grins, did a slip down from a cruise at 9500' to pattern altitude at P08 (Coolidge, Arizona), 3500' and when I turned base to final in the traffic pattern, the engine sputtered a little. Well, when I landed, one tank was bone dry, and all of the fuel had cross-fed from the tank which was banked up to the tank which was banked down (I still had a legal quantity of fuel on board, about an hours worth of fuel, it just worked its way into one tank and out of the other). To top it off, everyone at the airport had gone home for the evening. No one on the field to re-fuel me, and this was in the days before self serve avgas pumps (It was about 6:30 in the evening on a Sunday evening in the late spring, the later half of May ). I had to get creative, so I rocked the wings by hand (I grabbed a wing tip and pushed up and pulled down) on the ground for about 30 minutes to get the fuel level to even out somewhat (at least get about 2-3 gallons over to the empty tank), and then proceeded to E60 (Eloy, AZ), which was the nearest airport with fuel. I used a payphone (A few people I knew then had cell phones, but it was still for the most part a rare thing in the mid 1990's) at Coolidge to call the FBO at Eloy and verify that there was fuel available and someone around to fuel me up, and then made a beeline for Eloy.

Moral of the story: it pays to read and understand the POH  embarrassed 
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 8:27 am



Quoting Triebwerk (Thread starter):
Suppose that a Cessna 172 makes a 60 degree roll to the left with only the left fuel tank selected. Having read in a book that the Skyhawk's fuel system is gravity-operated, would this steep angle of turn cause the plane to lose fuel flow to the engine?

It could conceivably cause some trouble, yes...you would want to watch the level of the tank's fuel feed (down to the gascolator) versus the level of the carburetor. If the tank outlet port for the fuel line dips below the level of the carburetor, then things will flow backwards.

However, in a banked, co-ordinated turn, centrifugal force will create an artificial "gravity" force working downward (in a 60 degree bank, that would be 2g's, twice your normal weight), so the fuel flow should continue uninterrupted. Where you would worry is when you are not co-ordinated (as you would be in a slip-see my previous post Wink ).
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
zappbrannigan
Posts: 231
Joined: Wed Oct 01, 2008 7:41 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 10:37 am

As has been mentioned, in a *coordinated* 60 DoB turn, the aircraft will not "know" that it's banked - and the fuel will not slop to one side of the tank(s). Since the aircraft is now feeling 2 x the normal force of gravity (i.e. 2g), the gravity-fed system will work perfectly well. Sustained negative-G may interrupt it.

Technically, during a hard yaw to the right, the dynamic pressure in the pitot tube will probably increase for a split second by a small amount not noticeable on the ASI. Since a hard kick to the rudder will result in uncoordinated flight, the ASI, if anything, may under-read due to alignment error, i.e. the relative airflow not hitting the pitot tube head-on, causing a small drop in dynamic pressure and an under-read. The same effect is seen on rotation, where for a few seconds the pitot tube may form a 5-10 degree angle to the relative airflow.
 
cptspeaking
Posts: 567
Joined: Mon Apr 24, 2006 2:49 pm

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Tue Dec 30, 2008 6:04 pm



Quoting Triebwerk (Reply 2):
What about yaw, however?

I the air, you'd have to be in a slip for quite some time to make any difference.

On the ground, however, it can happen pretty quick because you slosh all the fuel away from the pickup. When we're relatively low on gas in the 172 with the G1000, it constantly yells at us about low fuel as we make turns on the ground. The level can drop from 10 gallons all the way down to 1 or 2 very quickly as you make a turn. This has a lot to do with the fuel sensor, and you'd have to be WAY low on gas anyway to get the engine to shut off by doing this. BUT, if you're in that situation, you have bigger problems anyway  Smile

JB
...and don't call me Shirley!!
 
dw747400
Posts: 1091
Joined: Wed Aug 15, 2001 8:24 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Fri Jan 02, 2009 3:52 am



Quoting Triebwerk (Thread starter):
On a similar vein, imagine that our troubled Skyhawk yaws hard to the right at level flight. Considering that the pitot valve is placed on the left wing, would this cause the Airspeed Indicator to display a higher airspeed than the nose is actually traveling at?

I don't believe this is a major problem. However, as you may know, certain maneuvers take the longitudinally axis of the aircraft away from the direction of flight--so the plane is no longer pointed in the direction it is going. In these instances the pitot tube is no longer pointing directly into the relative wind, and reads incorrectly. Many maneuvers can cause this, but its generally most noticeable in slips. Your CFI will most likely brief on the need for careful attention to airspeed during a slip for this reason.

As for gas, if you fly uncoordinated, you can draw more fuel from one tank then the other. More than once as a student pilot I found myself loading unbalanced amounts of fuel. However, you'd need to be dangerously low on gas (or involved in some extended uncoordinated flight, as KELPKid alluded to) before this becomes a real safety of flight issue.
CFI--Certfied Freakin Idiot
 
YWG
Posts: 1055
Joined: Wed Feb 07, 2001 11:29 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Fri Jan 02, 2009 4:45 pm

Do some zero g dives without the fuel pump on, then you will find the answer to your question.
Contact Winnipeg center now on 134.4, good day.
 
Fly2HMO
Posts: 7207
Joined: Sat Jan 24, 2004 12:14 pm

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Fri Jan 02, 2009 8:13 pm

Quoting KELPkid (Reply 4):
No one on the field to re-fuel me

Reminds me of a time I flew to DUG. A Sunday, around noonish, place looks like a ghost airport literally. Despite I was there in business hours per the AFD, there was not a single soul around the airport, aside from the several hundred in the state penitentiary next door. There was self serve but it was shut off. And there was a paper in the FBO terminal entrance with a number scribbled on it to get service, but it was disconnected. So I had to fly back to TUS to get gas, then I realized I left the aircraft board in DUG with the fuel cards   , so I called my flight supervisor, who amazingly didn't slit my throat, gave me a card number over the phone, fueld up in TUS, flew back to DUG, grab the damned cards, then proceeded back to PRC. A 4.5hr cross country turned into almost 8   

Quoting Triebwerk (Thread starter):
would this steep angle of turn cause the plane to lose fuel flow to the engine?

You'd loose fuel flow to the reservoir tank first, IIRC it's 2gal capacity, then when that runs out then yes you may get some protesting from the engine.

[Edited 2009-01-02 12:15:38]
 
meister808
Posts: 924
Joined: Sat Jan 01, 2000 11:45 am

RE: If A Cessna Rolls Left W/ Only One Fuel Tank...

Fri Jan 02, 2009 11:46 pm



Quoting Dw747400 (Reply 8):
In these instances the pitot tube is no longer pointing directly into the relative wind, and reads incorrectly.

In a Skyhawk with one static port forward of the LH door, the static pressure will get all wonky in the event of a slip as well. Again, not really enough to ruin your day except in the absolute worst-case scenario, but a consideration nonetheless.

-Meister
Twin Cessna 812 Victor, Minneapolis Center, we observe your operation in the immediate vicinity of extreme precipitation

Who is online

Users browsing this forum: Dalmd88 and 22 guests