Static Thrust vs Actual Thrust

[Cantona156]

Hi

Need some help here. :D

Came across this useful utility called ThrustHP.
I've this Saito 72 running with APC 13x7 @ 9400rpm.

According to Thrust HP, the prop speed is approx 62mph.
Horsepower is approx 1.7hp. Static Thrust is 10.6lb.

I realised that unlike jet engine, the thrust will be lower as the
airspeed increases. Infact, the thrust 'decays' slowly.

1) For my case, will the thrust decay to zero as the airspeed increased to 62mph (assuming my RPM is still 9400rpm ....) :confused:

2) If the thrust has decayed, are there any ways to determine the actual
thrust during forward flight? :confused:

3) Assuming I have a plane that flies at 60mph.
The total drag experienced @60mph is approx 10lbs.
Would this Saito 72 setup be suitable? :confused:


Any help will be useful!
Thanks!
:D

[vintage1]

Thrust is momentuim change of the air.

So if you suck in static ar and accelerate it to 60mph, the thrust will be the diameter of the cylinder of air that is acceleretade times the speed (I think)

As you approach pitch speed the thrust decayes to zero at some point: Argumenst abound as to whether the zero thrust point is teh 'true' pitch speed, or whether that is teh geometrical speed ontianed from measuring teh blade pitch witha protractor and multilying by ROM, but let's not go there (again)

Other effects arte loss of prop efficiency at low airspeed due to partial stalling, especuially near the prop root. And nasty approaches to transonc speeds of the blade tips at high speeds. These reduce teh thrust at these points.

If teh enegine were held at constant RPM there is almost a linear degradtain of thrust as speed increases, but drag goes up faster than that, so enormous power is needed for hogh speed.

A motor that produces 10lb of thrust static at geometric 60mph pitch speed will most likely produce no more than a 1lb of thrust at the pitch speed, even with the engine unloading and speeding up.

Its very hardto predict te final speed of a model, and almost as heard to measure it wthout a radar gun.

[gkamysz]

The anwser to that question is pretty complicated. However if you really need 10Lb of thrust at a 60MPH flight speed there is no prop that you can put on that engine that will do it. To make 10 Lb of thrust at 60MPH the HP required will be much higher. I couldn't find an accurate rating for the .72, but it's less than 1.3HP. By my math a 13x7 at 9400 RPM is about 1HP and 8.5Lb of thrust.

The real question is how much thrust do you really need to get your model to 60MPH. It's probably not 10Lbs, so you are probably OK.

Where did you get the 10Lb drag figure?

Greg

[Cantona156]

The anwser to that question is pretty complicated. However if you really need 10Lb of thrust at a 60MPH flight speed there is no prop that you can put on that engine that will do it. To make 10 Lb of thrust at 60MPH the HP required will be much higher. I couldn't find an accurate rating for the .72, but it's less than 1.3HP. By my math a 13x7 at 9400 RPM is about 1HP and 8.5Lb of thrust.

The real question is how much thrust do you really need to get your model to 60MPH. It's probably not 10Lbs, so you are probably OK.

Where did you get the 10Lb drag figure?

Greg
Hi

Thanks for your reply.
10lbs is an arbituary figure for illustration purposes.

To make 10lbs at 60MPH will require what kind of engine power?

[gkamysz]

Try this.

http://www.mh-aerotools.de/airfoils/javaprop.htm

Greg

[gkamysz]

Looks like it wouldl be about 2.2HP give or take depending on RPM and prop diameter.

Greg

[Cantona156]

Great! Thanks a lot for your help!
;)

It more or less resolves the problem that has been bugging me for the longest time. :D

[peterangus]

www.rcgroups.com/forums/showthread.php?t=288705