information "Airmaster" Propellers

[Mhalc]

The only cautionary comment I’ve had was from a LAME who warned to keep a spare set of carbon brushes handy as they do wear down in service. Apart from that, the quality of mine is superb and all ground testing has been perfect. I checked blade tracking yesterday and the total difference is less than a quarter of a millimeter or less - too small to even measure accurately unless I wanted to use a dial gauge.

 

You MUST fit a MAP gauge AND a fuel flow gauge. The first is required by Rotax because you CAN overload the engine if the prop is mishandled. SL - 912 -016R1 section 3.1.3 refers.

 

The second is a matter of safety. It is possible to have an engine producing takeoff rpm and correct MAP but producing SFA power if fuel flow is restricted. You must see correct fuel flow and MAP to assure required power.

Sounds great, I just wish I could see for myself. Still no definitive ship date, and the window Airmaster provided is quickly passing :(.

 

When I built my Lightning I included MAP and fuel flow gauges, but I think for the Jabiru all I would need is the MAP sensor.

 

Good tip on the carbon brushes for the slip ring.

[facthunter]

ref. Post# 25 There have been instances where a fuel pump has failed completely and MAP and RPM remains constant. Engine speed and throttle position affect MP. Read the report on the DH Dove at Essendon. That even applies to direct driven supercharged motors. Double magneto drive failure on Merlins is the same .Nev

[M61A1]

Please explain how you can have TO RPM & correct MAP if fuel flow is compromised??

That’s what a constant speed prop does. It varies the pitch of the prop to maintain a set rpm. If the engine is losing power it will fine up the pitch to maintain rpm

[Downunder]

After reading this, which has excellent explanations......

https://www.propellor.com/Performance#takeoff

 

There is mention of 75% and 65% throttle.....(cruise and eco-cruise)

I'm guessing manifold pressure would play an important part in determining these figures?

[facthunter]

Yes . In a Normal situation they work. Normally aspirated doesn't give you a wide choice of combinations especially as you go higher. It's usually a % of POWER. A throttle restricts by throttling (reducing the mass airflow). when you want less than full power .Nev

[Downunder]

Thanks Nev. I guess normally aspirated, 75% (sea level) power requires more throttle the higher you go.

Which poses the question. Is a quoted 75% Figure based on a fixed sea level point or or a variable point based on altitude? (For prop setting purposes)

 

And turbo normalised, manifold pressure remains the same (sea level) as you climb?

[facthunter]

Sorry Downunder. I made an extensive explanation by editing but it has exceeded the time allowed.

With any supercharging, you will reach it's effective limits somewhere as you climb. This was over FL 300 on the R 1830s which were TURBOcharged on the Liberators. Some direct coupled superchargers actually change gears (Manually) as they climb to spin the compressors faster.

Supercharged engines can be overboosted at below full throttle height for the power figure you want. so you are in effect throttling back the supercharger below that level to save the engine from exceeding it's safe output. Maintaining RATED power to higher levels is the aim there. In the war you might have other reasons. (catch the enemy) if you are a fighter, not a bomber. Nev

[Thruster88]

Thanks Nev. I guess normally aspirated, 75% (sea level) power requires more throttle the higher you go.

Which poses the question. Is a quoted 75% Figure based on a fixed sea level point or or a variable point based on altitude? (For prop setting purposes)

 

And turbo normalised, manifold pressure remains the same (sea level) as you climb?

 

Yes more throttle the higher you go, this chart for a 180hp carburetor lycoming shows the altitude when FT full throttle is required. A % figure is based on max power for the engine, in this case full throttle 2700rpm at sea level.

 

As for manifold pressure remaining the same for a given output and rpm you can see less manifold pressure is required as the aircraft climbs, that bit I don't understand.

 

[M61A1]

less manifold pressure is required as the aircraft climbs, that bit I don't understand.

There is less manifold pressure available as altitude increases.

If you think about the atmosphere like water, on the ground at sea level you are at the deepest point with the highest pressure available to fill your cylinders, as you rise the pressure available to fill the cylinders reduces.

[Mhalc]

That’s what a constant speed prop does. It varies the pitch of the prop to maintain a set rpm. If the engine is losing power it will fine up the pitch to maintain rpm

I’ll track down the report you recommend, sounds interesting. It is true the prop pitch will “fine up”, but to maintain RPM the engine will still need to pump an amount of air equal to 1/2 its displacement times the RPM (for 4 cycle engines). If the fuel air ratio drops below the minimum stoichiometric ratio the engine will be so lean as to quit.

[Thruster88]

There is less manifold pressure available as altitude increases.

If you think about the atmosphere like water, on the ground at sea level you are at the deepest point with the highest pressure available to fill your cylinders, as you rise the pressure available to fill the cylinders reduces.

Yes I fully understand pressure drops with increasing altitude. If we look at the 55% power, 100hp at 2400rpm column, (#33) ar sea level 19.3in of manifold pressure is required. We climb to 10,000 feet, the rpm is same 2400, the prop has increased pitch in the less dense air and the engine is still producing 100hp and the same torque but now only 17.2in of manifold pressure is required. Why?

[walrus]

Thruster.....because you open the throttle more as you climb.

[M61A1]

Yes I fully understand pressure drops with increasing altitude. If we look at the 55% power, 100hp at 2400rpm column, (#33) ar sea level 19.3in of manifold pressure is required. We climb to 10,000 feet, the rpm is same 2400, the prop has increased pitch in the less dense air and the engine is still producing 100hp and the same torque but now only 17.2in of manifold pressure is required. Why?

Apologies, I misunderstood what you were asking. Walrus explained it I think,

[facthunter]

Air is not like water as it expands when under less pressure or more temperature as per the universal gas equation P1V1/T1 is a constant. Mass airflow equals power so as it gets thinner less mass for a given volume. Nev

[eightyknots]

Sounds great, I just wish I could see for myself. Still no definitive ship date, and the window Airmaster provided is quickly passing :(.

 

When I built my Lightning I included MAP and fuel flow gauges, but I think for the Jabiru all I would need is the MAP sensor.

 

Good tip on the carbon brushes for the slip ring.

 

Which fuel flow gauge (or system) did you use?

[Mhalc]

Yes I fully understand pressure drops with increasing altitude. If we look at the 55% power, 100hp at 2400rpm column, (#33) ar sea level 19.3in of manifold pressure is required. We climb to 10,000 feet, the rpm is same 2400, the prop has increased pitch in the less dense air and the engine is still producing 100hp and the same torque but now only 17.2in of manifold pressure is required. Why?

Which fuel flow gauge (or system) did you use?

I have a GRT avionics system, EIS and EFIS. The EIS has a “Red Cube” fuel flow sensor attached and measures fuel flow and totalizes usage. I did take a bit of effort to get the calibration right, but I find it to be very accurate, much more so than the fuel tank gauges :).

[M61A1]

Air is not like water as it expands when under less pressure or more temperature as per the universal gas equation P1V1/T1 is a constant. Mass airflow equals power so as it gets thinner less mass for a given volume. Nev

Yes Nev, I was only referring to it in a depth/pressure relationship.

[walrus]

Mhalc, rememyer that an engine is a pressure device where hot gas flows to low pressure - the exhaust. The pressure drop from piston to atmospheric produces the power. At altitude there is less exhaust back. pressure.