THROTTLE AND MIXTURE AT ALTITUDE

another question. i was looking through the ra-aus website about CRUISE SPEED and how engine performance drops at altitude so the best performance is a altitude where at full throttle the engine is developing around 65% to 75% of its rated power

1 : is this what most do ?

2: do the carbs / EFI adjust the mixture as altitude increases ?

( im assuming they would have to ):;)3:

There's lots involved with the theory on this topic...I'll have a go at a simple explanation.

Power for piston engines varies with altitude, the higher you go the less power is produced for a given throttle setting. The manual with the engine will describe what power is produced at various altitudes with respective % throttle settings.

With respect the mixture control, this again varies with engines. The Rotax 912/914 series, Jabiru engines etc. for example have an auto-mixture control or pressure calibrated mixture control. With Electronic Fuel Injected mixture is automatically controlled but older Mechanical Fuel Injection systems (like in our CT4) have a manual mixture control and mixture is set based on a combination of fuel flow settings for a given altitude from the flight manual and EGT (Exhaust Gas Temperature).

There's a lot more to this and it's worth reading the manual for each aircraft / engine you're flying and understand it thoroughly...for this and all systems operations.

Cheers,

Matt.

so there would be a mixture control either manual or automatic but how about flying on full throttle ?

If it's auto there's no mixture control, just a throttle. If it's manual then there's a mixture control and throttle.

As for cruising at full throttle, as an example as your altitude increases you'll move from 75% power / 75% throttle setting at sea level to 75% power / 100% throttle setting (wide open throttle) at 7500' (as an example). Power is generally measured using manifold pressure where a given manifold pressure setting will represent a given power setting or percentage for a given altitude.

The cruise power for our CT4 is 23 inches of mercury (intake manifold pressure) at 2600 RPM. At sea level, this will represent roughly 75% power at a throttle setting of about 75%. At 7500 feet for example, 23 inches equates to full (wide open). So effectively you are flying at full throttle (but not full power) at 7500 feet.

Piston engines with a turbo overcome this issue (reduction of available power as altitude increases) by forcing higher (than ambiant) pressure air into the intake manifold.

Hope that helps a little.

Most of the old type GA aircraft, such as Cessna, Victa, Piper etc have a mixture control. This is needed at altitude and in mountaneous countries is needed at ground level in high country, ie Denver USA. Normal practice is to start leaning above about 3000'. Climb out at full rich except in high altitudes, this keeps the mixture rich and the excess fuel aids in cooling. When cruising at altitude you lean the mixture gradually and watch the EGT readings. There will be a point where the max EGT is reached as yiu lean the mixture and you need to change the mixture to reduce this by about 50 deg F. This can be done by either leaning or enriching the mixture, but if you are running at less than about 80% power it is economical to lean to the 50 deg point. It is possible to richen the mixture to the same 50 deg point but all you re doing is burning more fuel. If running at greater than about 80% power setting it is better to enrich than lean , to avoid detonation, but at less than about 80% you will do no damage to the engine and save a lot of fuel. At high altitude airports it is considered a good idea to lean the mixture while taxying to the runway until the engine runs rouggh, then enrich a little to avoid plug fouling. If you forget to enrich for take off the engine will be so low on power that you will soon know.

Full throttle cruise at altitude.

In a normally aspirated engine the manifold pressure is controlled by the position of the throttle,(think of it as a restrictor) up to the point when it is as far open as possible, where the air pressure available at your altitude determines the manifold pressure. (ignoring ram effect)

At sea level you will get around 30" and provided your propellor allows the revs for max. power to be achieved and the air temp is appropriate, You should get the rated power of your engine.At altitude, unless you have a manifold pressure indication, you will have to go off the performance tables that should be in your Pilot's Operating Handbook. As a rough rule of thumb, you would be only be able to get about 75% power at full throttle above 4000' and this would be provided the revs remained the same as for max power.

Full throttle at cruise height is particularly desirable where you have a geared supercharger, as it's pretty wastefull to throttle the airflow off ,then compress it later to force it into the engine, Otherwise, it's a bit academic. Note that with a CV. Constant Vacuum carburettor, Full throttle only lifts the dashpot piston ALL the way up if

1 the engine is in good condition.

2 the carburettor is the right size for the engine.

3 you have standard sea level pressure or higher

4 the engine has achieved the correct rev figure.

The fact that the needle is attached to the dashpot piston & moves with it , and is responsive to altitude effects (as well as the other factors listed) is utilised to achieve some measure of automatic altitude mixture compensation, if the taper on the needle is constructed appropriately. Also note that there must be some pressure drop across the dashpot, as this is inherent in the design. Nev..

thanks for that everyone:big_grin:

up to now ive been associating 75% power with 75% throttle etc at all altitudes

so the turbo's a good thing just as long as you watch your Vne........read that somewhere else...thanks again Troy

Just a note, running Lean of Peak should be done only with an accurate EGT per cylinder and preferably with tuned injectors. See GAMI for more info.

Of course most of us fly with a Rotax/ Jabiru engine and Bing carby's so its not possible to do this.

Cheers

J;)

Flight above 5000 feet

Sorry to re-hash this point, but the foregoing discussion is a bit academic for ultralight aircraft.

(a) the aeroplane may be flown 5 000 feet above mean sea level or higher:

(i) only if it is flying over an area of land, or water, the condition, and

location, of which is such that, during the flight, the aeroplane would

be unable to land with a reasonable expectation of avoiding injury to

persons on board the aeroplane; and

(ii) only if it is equipped with a radiocommunication system

David