102
votes
Accepted
Why do most radial engines use an odd number of cylinders?
An odd number of cylinders is required by the combination of the single-crank radial design, the four-stroke (Otto) work cycle, and the desire to keep the power strokes evenly spaced in time.
To keep ...
- 3,988
73
votes
Accepted
Why isn't the APU a standard diesel generator?
Weight.
Piston APUs for trucks are designed for frugal and quiet operation. This one produces 5.2 kW electrical power and weighs 375 lbs.
The APU for the A320 and B737 is a noisy screaming unit that ...
- 61.2k
73
votes
Accepted
Why are two-stroke engines nearly unheard of in aviation?
Several issues:
As you mentioned, high specific fuel consumption. About to that of turboprops, but without the reliability. If you're going to live with that SFC, you might as well as go with ...
- 125k
54
votes
Accepted
Why is water-contaminated fuel bad, but water-injection is not?
Because of the quantity of water in the fuel, as opposed to a careful introduction of water into the combustion process. Typical water contamination is bad in fuel tanks as water is denser than ...
- 71.9k
47
votes
Why increase the number of cylinders in an engine instead of increasing their volume?
Constraints
Different applications have different constraints:
Aviation: very light weight, highly reliable
Marine: very high endurance
Automotive: moderately light weight, responsive
Motorcycle: ...
- 10.7k
46
votes
Accepted
What is the metal bit in the front of this propeller spinner?
It is a fitting for a Hucks Starter.
Photos from Vintage Wings Canada
- 37.3k
45
votes
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Why do aircraft piston engines consume so much oil?
Automobile engines are not similar. They are liquid-cooled and therefore can be built to much tighter tolerances with regard to thermal expansion and contraction.
Air-cooled aircraft engines must ...
- 37.3k
43
votes
Accepted
Starting on the left magneto only - why?
The reasons seems to be that only the left magneto is equipped with an impulse coupling which delays and intensifies the spark during engine starting:
Many opposed reciprocating engines are equipped ...
- 53.5k
42
votes
Accepted
Why do propeller driven planes have intakes/inlets, such as this one on the P-51 Mustang?
Propellers are driven by engines, and engines burn fuel, and burning requires oxygen from the air. The intake supplies air to the engine. Burning creates heat, and air intakes also direct cooler air ...
- 52.8k
40
votes
Is a turbocharged piston aircraft the same thing as turboprop?
They are both internal combustion engines that have a turbine in their exhaust that is used to power a compressor to pressurize the air before it is used for combustion.
In the turboprop, the turbine ...
- 783
38
votes
Accepted
Why is carb icing an issue in aircraft when it is not an issue in a land vehicle?
Carb icing can occur in car engines, I know of at least two cases where it caused rough running, and that's just my own personal experience. If I remember right VW Beetles are susceptible to this, ...
- 52.8k
36
votes
Why do propeller driven planes have intakes/inlets, such as this one on the P-51 Mustang?
The design of the P-51 means the engine radiator (a component of the cooling system) is situated behind and below the pilot, as in this diagram (from here):
- 9,132
35
votes
Accepted
C-152 carb heat on before landing in hot weather?
You should always pull the carb heat when throttling back no matter the conditions for 3 reasons:
Ice forms from moisture (duh!), and there's much more moisture in hot tropical air than cold arctic ...
- 52.8k
33
votes
Why can we not use full throttle in a C172 when cruising at lower altitudes?
These engines are not designed to run at maximum horsepower output all the time with a lean mixture. They are designed to cruise at 50-75% of their rated max power, and deal with that level of ...
- 125k
32
votes
Accepted
Why did the turbojet replace the piston engine?
Two more reasons the gas turbine supplanted the piston engine for aircraft use:
Power output. Aircraft piston engines have a practical limit on how much power they can put out, before becoming ...
- 8,717
31
votes
Accepted
What is an aircraft magneto?
A magneto is a gear driven electric generation device connected to the crankshaft of the engine. It supplies the ignition system (spark plugs) with power.
Each engine has two magnetos. Each cylinder ...
- 16.5k
30
votes
Accepted
Why do piston engines in aircraft burn fuel at a higher rate than a comparable car engine?
As others have noted, airplane engines and car engines have very different duty cycles. An airplane engine will typically run at full power for a few minutes during takeoff & climb, then at a ...
- 2,577
29
votes
Why did the turbojet replace the piston engine?
There are several benefits:
piston engines are best for driving propellers. At the same shaft horse power $P$, propeller thrust $T$ varies with the inverse of air speed $v$: ($T_{Prop} = \frac{P}{v}$)...
- 228k
28
votes
Why is water-contaminated fuel bad, but water-injection is not?
Water-injection in jet engines and piston engines is mixed with fuel
before it enters the combustion
This is actually somewhat wrong (or at least imprecise) and I think it is adding to the ...
- 381
28
votes
Why increase the number of cylinders in an engine instead of increasing their volume?
Your reasoning is correct if engine mass is not important. Ships use huge engines, because increasing the number of cylinders beyond 8 will have diminishing returns in terms of smoothing out the ...
- 228k
28
votes
Why do older pistons crank for a while before starting?
On many aircraft engines, there is a significant distance between the carburetor and the intake valves, so it takes several turns of the crank to get the fuel-air mix all the way out to the cylinders ...
- 20.4k
27
votes
Accepted
Is there any legitimate reason for adding momentary power when descending at idle with full carburettor heat on?
straight out of the Airplane Flying Handbook, pp 9-4
Operating the engine at idle speed for any prolonged period
during the glide may result in excessive engine cooling, spark
plug fouling, ...
- 71.9k
27
votes
Accepted
Why cycle the magnetos before engine shutdown?
This is a hot-mag check to verify that when you shut the engine down, you are really putting it in a safe(r) state. Think about how the mag-switch works, when you switch to R or L, it grounds the P-...
- 36k
27
votes
Did people really hand-start big bombers in WWII?
When you shut down a radial the unscavenged oil in the case (oil coating the surfaces that didn't get pumped back to the reservoir tank) runs down and seeps past the rings of the cylinders directly ...
- 125k
27
votes
Accepted
What does "spark plugs appear yellow at the ignition point" mean?
Use the tip residue to evaluate the combustion process
The yellow the manual is describing is the normal condition found in the top left corner:
If the plug is not the yellow/tan shown in the top ...
- 4,722
26
votes
Accepted
Why do propellors have white markings on their tips?
It is so you will see a circle when they are turning. Without contrasting bright tips turning propellers are invisible, and pose a grave risk for someone who is deaf, wearing heavy hearing protection,...
- 24.1k
25
votes
Accepted
Why would a C150 tachometer bounce so much that the needle fell off?
Cessna 150s have a mechanical tachometer driven by a mechanical tach drive cable geared to the engine.
The tach needle is moved by sensing spinning magnets driven by the tach cable.
They do wear ...
- 37.3k
24
votes
Accepted
Why do reciprocating aircraft engines have more than one spark plug?
Bold Method has a great post today about why aircraft engines have more than one spark plug per cylinder. There are a few basic reasons.
Reliability
Having two sparks plugs is more reliable. If one ...
- 14.8k
24
votes
Accepted
Why have propeller engines never been mounted on the tail in production transport aircraft?
There are two really big issues with tail mounted powerplants with propellers; weight distribution and Foreign Object Damage.
Tail mounted engines move the empty C of G aft. This forces you to put ...
- 125k
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