Static Thrust calculations Jab 60 X 42 prop

[lee-wave]

While researching performance and static thrust on my 60x42 Jab propellor I came across this fascinating article.  The Jab propellor under test happens to be the exact same type as fitted to the factory built UL-D.

It is hard to believe that a small wooden prop rotating at around 3000 rpm can push (or pull) the aircraft along with a thrust of approximately 130 kg.

 

Static Thrust.pdf

[facthunter]

STATIC thrust is not maintained when you move. If it did at a common (and low) L/D ratio of 10:1 it would be capable of (just)  driving a 1300 Kg's plane in flight A jet engine pretty much maintains it's thrust during the take-off and at 100 knots  a pound of thrust equates to a horsepower. The 120 horsepower driving a bigger prop (at lower revs) would give more static thrust. TWO average to strong people could hold a Tigermoth with a 130 HP Gypsy 1-C so that matches pretty much what you have there. A static thrust check is not difficult to arrange. Where people made their own props, it's pretty much necessary before a test flight to have a minimum specified figure achieved. Nev

[lee-wave]

First sentence absolutely agree, second sentence I do not fully understand...but I would like to know how a static thrust on a fixed pitch propellor is carried out without the special equipment as was used in the article. 

[old man emu]

Am I correct in saying that Thrust is a force acting along the fore/aft axis of the plane? Therefore a very simple and rough way to measure Static thrust would be to tie the plane to a building with a weighing scale included between the building and the plane. Then apply thrust at various engine RPM from idle to full power and record what the scale is showing.

[facthunter]

It's just done with a spring balance  attached to something solid ( Parked land cruiser?) Usually just to get the maximum to guarantee performance at take off is adequate. Getting results at various revs is of limited value. What happens when you move is of more importance. The whole thing is basic and only one part of the equation.  The PITCH must be related to the cruise speed. The check for that is WOT  RPM at a certain height..  Naturally a slower plane will have less pitch. Like picking your final drive ratio in a car or bike but there's SLIP with a prop and blade AoA efficiency considerations. A bigger finer pitch prop will give a high thrust figure but won't be any good once you get speed up. Like being stuck in a low gear. Nev 

[old man emu]

Compromise, compromise. It's always a compromise.

 

And for the slight performance advantage of a CSU, the weight and extra complexity are hardly worth it in a plane that cruises below 100 kts.

[facthunter]

 You are pretty safe with the speed you have chosen, OME. I'd add safety to the consideration as it's not generally discussed.

    The Avro Anson had fixed pitch props and the proto type spitfire and plenty of earlier racers. Prop reliability affects safety as much  as the engine does. (perhaps even more as it has caused engines to detach from the plane.).

  Just to respond to lee-wave a few back. Jet engines are rated in thrust. Turboprops and pistons are in shaft horsepower. Propellers turn HP into thrust. What creates thrust in a pure Jet is a small volume of high speed efflux, usually above the speed of sound, and VERY noisy. A FAN (bypass) jet has more thrust but the thrust producing bit is lower velocity but more of it, so it gets you off the ground better and is quieter.. Nev

 

Edited September 29, 2020 by facthunter
more content.

[Methusala]

Years ago my plane, a VP-2, came with a de-Havilland 60"x30" prop. Its performance was so poor (produced max. 2850 rpm) it was dangerous. I went to Mangalore airshow and measured every prop attached to a VW there. They were about 56" max dia with far less chord and a flatter airfoil with max thickness about 1/4 of the chord. My prop had a Clark-Y type section with max thickness about 1/5 and a blunt leading edge. I attacked, taking 4" off the diameter, removing the brass L/E and savagely reducing the bulk. I placed an epoxy L/E and then had it sprayed white and with red tips and balanced it. When I ran it, tied to a tree with a spring balance, I found the static thrust had reduced from about 150lb to around 120! Well, time to run it along the strip to explore the result. I was shocked to find that the revs had increased to 3600 and it accelerated like a racehorse. I then committed to the air and found that the miserable climb performance had improved to 500fpm. It cruised at 3200 /60kts.

I believe that the old prop was acting similar to a fan. It moved more air at zero airspeed giving a thrust figure of about 160. However, the purpose of a propeller is to accelerate the air to a high velocity. It therefore produces more reaction force to the airplane and an increase in performance. The engine was obviously producing more hp and the prop converted this to a more energetic airflow.

I'm not trained in physics so am unable to explain in more technically precise language. Simply my experience. (Don't be afraid to experiment, most satisfaction comes from positive experience {or, as Facthunter explains,"Never stop learning."}) Don

[facthunter]

Unloading the engine so as it can develop all it's (low)HP is a first step. Match prop to engine. The extra revs means more propwash velocity. Jabiru always warn against "over propping" their motors. Nev

[tillmanr]

Facthunter, for the less experienced, what is under or over propping? 
Thanks.

Edited September 30, 2020 by tillmanr
Misspelling

[Old Koreelah]

2 hours ago, facthunter said:

... Jabiru always warn against "over propping" their motors. Nev.

I was led to believe a major reason is to keep revs up and avoid detonation. Luckily I’ve only experienced piston rattle in an old Falcon car, when loading up the engine at low revs.
Nev maybe you could comment on my hazy understanding of this; if the prop’s pitch is too course it causes the engine to work harder to make revs, allowing the flame front to propagate across the combustion chamber before TDC is reached.

[facthunter]

Basically putting too big a load on the motor as you would if you stay in a high gear too long. OK the flame front is fast if the fuel is vapourised and mixed evenly even across the range of octane ratings. You are talking about micro seconds. While it might seem anti intuitive to burn all the fuel before  TDC that's pretty much the aim and that's why you see ignition timings of around 35 degrees before TDC common and 28 on aviation motors with dual ignition. You retard these for starting or the engines will kick back. Nev

[RFguy]

For general consumption.... :

The velocity of the reaction determines maximum speed.

* Just like if you are pushing a car on the (flat road) , the car will not go any faster than how fast you can run ! If you can only run 25km/h ,  that's as fast as the car will go. 

and of course  Jets have a higher reaction velocity than props.

Props gave way to jets, which gave way to rockets, which give way to particle propulsion systems like ion drives. Rockets in space will never get us to the next star system because their nozzle velocity is only maybe 25,000 m/s. The Ion drives can generate nozzle velocities far higher, enabling far higher (ultimate) speed.

Edited September 30, 2020 by RFguy

[Old Koreelah]

9 hours ago, facthunter said:

... that's why you see ignition timings of around 35 degrees before TDC common and 28 on aviation motors with dual ignition. You retard these for starting or the engines will kick back. Nev

Having no retard mechanism, Jabs need to spin over 270 rpm to fire (unless you have the Cold Start kit fitted).

So what exactly is happening when they get detonation?

[facthunter]

It may be built into the electronics.( Hall effect.) Starting from cold  detonation is not an issue. Detonation is where the flame front is not the only way  the fuel is ignited. Some may self ignite under excess pressure hot spots or fuel not meeting spec. This issue is not fully resolved Jabiru reduced compression ratios and retarded the ignition to alleviate  the condition. Never have I seen a suggestion to use colder plugs. for instance. Anyone with very white insulators in any engine may be running too hot a plug and risking engine damage. Nev

[old man emu]

1 hour ago, facthunter said:

Never have I seen a suggestion to use colder plugs. for instance. Anyone with very white insulators in any engine may be running too hot a plug and risking engine damage. Nev

Whoa! Not that there is anything wrong with what you have said, but you've opened the hangar doors on a confusing subject when talking about hot and cold spark plugs. 

 

The term hot/cold is commonly used to describe whether a spark plug itself does not conduct heat away from the tip, and heats up easily (hot) or whether it does (cold). The numbers in the diagram are determined arbitrarily by the spark plug manufacturer. They are not standard in the industry. You have to use a comparison chart to see if, say an NGK plug is in the same heat range as a Champion. The manufacturer's recommendation for my bike's plus is a Harley-Davidson Number 4, but since I can't get the OEM ones, I used a Champion Number 14.

 

 

An interesting note about my bike's spark plugs. Since the cylinders are in line, one behind the other, it is recommended that a cold plug is used in the rear cylinder, and a "hotter" plug in the front when the bike is used for highway speeds.

 

I wonder what effect that would have on the operation of an aero engine if "hotter" plugs were fitted to the cylinders that showed higher CHT's.

[facthunter]

Why is the subject confusing? You have explained the basic principles .A loose plug will run hotter because it's not conducting heat away  as well and putting a temp sensor there  will do likewise. The consequences of getting it wrong are considerable.. Nev

[old man emu]

9 minutes ago, facthunter said:

Why is the subject confusing

Not confusing when you have learned what "hot" and "cold" relate to, but what was you understanding when, as a pimply faced 1st Year apprentice, you first heard people taking about hot & cold plugs. The descriptors are counter-intuitive.

 

Methinks this topic needs a thread of its own.

[facthunter]

Respectfully, that would be your perception from where you see things. A "HOT" plug runs hotter.  Kids learn at a young age what hot and cold mean, often the hard way. Ooh Burney as Homer Simpson would say.. . Over to you. What would you have called them? (In lieu of Hot/cold when we are dealing with a "heat range" which probably would be better called temperature range.)  Today you just order them as fitting CAR "X". and I think Jabiru recommends one plug. Since aircraft engines don't spend a lot of time idling  and get more problems at the higher end of the power spectrum a conservative approach to hot plugs might be justified  and run one a bit colder to be safe (er).Nev

Edited October 1, 2020 by facthunter
correction

[old man emu]

But do you run a hot plug in a cylinder that runs hot, or do you run a cold plug in a cylinder that runs hot. 

 

When you first encounter the terms as applied to spark plugs, you have to delve a bit deeper into learn what these terms mean for  the engine. Initially it's not quite quite the same as "I can touch something that is cold, but will be hurt by something that is hot".

[facthunter]

You run a plug SUITABLE to the engine  and the way it's operated, so the PLUG insulator bit inside the combustion chamber doesn't run too hot and cause pre ignition. ie act like a glow plug. IF it doesn't run hot enough it will get dirty and may be conductive of the high voltage  the plug handles and may misfire under load. It's a matter of MATCHING the plug to the situation...Design, condition and operating factors.  Other sources of pre-gnitionare incandescent carbon and red hot exhaust valves  and combustion chamber sharp edges. Nev

[old man emu]

11 minutes ago, facthunter said:

You run a plug SUITABLE to the engine

Too right. You know that, and I know that, but are we going to start a new thread to educate the masses about spark plug design considerations?

[facthunter]

I've brought it up a few times and virtually no response so I'm trying again and being as clear  as I can be. It doesn't need a whole separate thread. These charts used to be everywhere  in auto shops . It's not legal to fit colder than recommended in your car as it mustn't foul when idling in traffic. Then you pull a caravan with it and it's not right for that. Most use diesels or now direct injection petrol which cannot detonate even on rotten fuel.. Nev