Measuring Thrust
 

I have a Magnum XL .40 in my trainer, and my intructor wasn't pleased with how well it pulls the plane out of jams so he loaned me an OS max .46 fx to replace it. Before I change out the engine I want to do some thrust measurements before and after. How can I do this at home. I'd like to make my own home made thrust meter like something I could hook on the tail. I'd go buy a thrust meter or whatever you call them but I can't find any on line. I'd rather just make something though. I have an old fish scale I could use but I think the smallest increment it measures in is a pound. Any Ideas or anyone already dione this let me know.

Crash

RE: Measuring Thrust
 

Digital fish scale.

RE: Measuring Thrust
 

Totaly forgot they make them digital now. Thanks!

Crash

RE: Measuring Thrust
 

There is (or was) a test stand on the market that had a thrust measuring device on it, but I haven't seen it advertised in a while. Probably among the combat/racing/speed guys. The measuremenys you get with a make-shift rig will be suspect for real accuracy, but at least you can get some comparison data. BTW, have you experimented with props on your trainer? A prop change can make all the difference in the world.

RE: Measuring Thrust
 

And we all know how accurate fish scales are, don't we ;)

RE: Measuring Thrust
 

Hi!
Your Magnum .40 XL not having enough pulling power in a trainer....?? Strange? Put on a 11x5 or 11x6 APC and see if that works better. OS .46 is way too much power.
Most ball bearing .40 engines are usually just too much power for any high winged trainer airplane. Choosing the right prop and propsize is what makes airplanes perform.
Been training people fly for over 30 years!

Regards!
Jan K
Sweden

RE: Measuring Thrust
 

Thrust measurements are a waste of time and tell you nothing. Do as Jaka says and try different props while flying the airplane. To measure the performance of a prop you fly the airplane in the air, not tied to a pole.

Ed S

RE: Measuring Thrust
 

Some lessons we learn come easier than others. As I am in the age minority here I'm sure this is of no great revelation to any of you. I've only been involved in this hobby for only a few months and the knowledge I gain from my hard lessons grows by leaps every day. Yesterday was one of those lessons.
Walt has been instruciting me teaching me to fly these things. We flew my Kadet the other day and had some kind of onboard malfunction that brought us down after a few minuites. Walt seemed to think it might have been the engine dying. Whatever the case, he felt that the Magnum .40 wasn't enough engine to pull us out of a jam and certainly not enough power to have any fun with. So Walt loans me one of his engines, an OS .46 FX. He brings me the engine wich had a 10x9 apc prop on it and suggests tying off the plane and running the old engine then swapping the engines and running it to get a comparison. That got the wheels turning in my head. So I decided to build my own Thrust-o-meter, using an old fish scale I had in my tackle box. I had now completed the make-shift meter, time to make some noise!
I ran the Mag .40 on the 10x6 and got 4.75 lbs Thrust. I ran the OS .46 on the 10x9 that Walt loaned me with the engine and got 4.25 lbs thrust. Ok.....So I swapped the props. I started the engine up, brought it to idle, checked over my rope and other things before opening it up all the way. I inched up the throttle little by little adjusting the air mix along the way until I found the sweet spot....prepare for lift off.
I ran the throttle all the way open gradualy and managed 6.5 lbs thrust on the 10x6 prop before the tail section let go and sent the plane crashing through my carport/garage area. The prop shattered on the wheel of an old car I'm working on. and the tail section was damaged pretty good. As I stood there wide eyed and disbelieving, I was vaguely amused. As no body was hurt and no property was damaged(except my pride) The whole thing became funny.
Walt said the other day"either you become a good pilot or a good builder, cuz if you aint a good pilot you better be a good builder." seems very true for me. Thats ok though. Maybe God wants me to be a good builder before I'm any kind of pilot.
The incident reminded me of when I put a .45 ACP hollowpoint into momma's new carpet, but thats a different story for a different board.
So whats the lesson?....I'm not sure exactly But I'll be alot more careful next time I whip out the old Thrust-o-meter and when I do I'll get it all on video!

Crash

RE: Measuring Thrust
 

Too bad about the experiment! One thing to keep in mind is to carefully re-check the CG after your rebuild/repair. You are likely to be tail heavy as a result of the materials and epoxy used for the repairs, and I don't want your next trip to the field to end up with a tail heavy plane that goes in!

Good luck.

RE: Measuring Thrust
 

The lesson is exactly as I posted previously. When checking the performance of props and engines fly the airplane in the air. Do not tie it to something.

Ed S

RE: Measuring Thrust
 

The problem with the staic thrust IMHO anyway. The 10x9 is going to unload in the air. Not that the 10x9 should ever be run on a trainer but in the air it would build more speed with time than say a 12.25x3.75 apc. Thats what it's built for is speed. The longer lower pitch prop is a lot better for a trainer. While it wont have the speed the throttle response will be much better. This is what gets you out of trouble with a trainer. When I buddy box someone I want instant response when I let go of the trainer switch.

David

RE: Measuring Thrust
 

Re. the Magnum XL 40 --- it is only moderately powerfull at mid-level RPMs (just a bit more than a TT 42 GP, or OS 46 LA), which is still more than adequate to fly just about any 40-size trainer. However, it really starts to go as the revs wind up. Dave Geirke's dyno tests list this motor as having torque and power peaks at 16,500 RPM & 17,500 RPM respectively. This is not very well suited to a trainer, but you can help it by getting the revs up into a more effective range for that engine, or by improving thrust through a larger diameter prop.

Jaka's suggestion of an 11-5 will certainly help somewhat through better efficiency from the greater disc area, but it will still hold the revs down to about 12,000 RPM. An 11-4 will start to get you somewhere (~13,500 PRM) & retain that usefull 11" diameter. A 10-5 will compromise somewhat by reducing diameter, but the revs will really start to climb (~14,500) & may adequately compensate for the reduced efficiency.

Before changing the engine I suggest that you try some of those sizes.

RE: Measuring Thrust
 

I really appreciate the tips and advice. keep them coming. I'm really new to this and I'm a slow learner as my mom used to mix lead pain chips into my frosted flakes when I was young. Then Child Services came and took me and my 8 brothers and sisters away but the damage was already done. Anyway, so why not use the .46? Wouldn't the .46 be better than the .40?

Crash

RE: Measuring Thrust
 

When I started I had an OS 46FX on a .40 size Hobbico SuperStar Trainer, and never felt it was overpowered. In fact, I have never thought that any plane was overpowered, since you can control the engine in the same way that you can control the control surfaces - you don't have to use full deflection during normal flight, but it sure is noce to have more (power or elevator deflection) if you need it.

Since the Magnum and the OS 46FX probably weight about the same thing, go for it! By the way, whenh I indicated that overpowering is a good thing I am not talking about putting a .91 on a .40 size plane, due to weight and other structure related issues. Just sue some good judgement, including that of your instructors. If you are trusting your instructor to teach you how to fly, why not listen to what he/she says about other aspects of the hobby - at least until you have enough experience to be able to pick and choose what suggestions to use and which ones to discard?

Another "By the way" - If you have the .46FX on the plane you may well like the flight characteristics of the trainer enough that you won't be too anxious to move on to another plane/engine combo. In this way you can have a lot more fun with the 'trainer' than just learning the basic of flying. Most trainers are discarded too soon by new pilots, when they are quite capable of some basic aerobatice and therefore will be a lot of fun as you learn more about flying.

Since I seem to be bucking 'conventional wisdom' of some, I might as well go deeper...I disagree with those that tell you and others that thrust measurement on the ground is meaningless. I know that it is a valuable tool for making initial comparisons and decisions, while flying is the ultimate tool to fine tune those decisions. When making your thrust comparisons, just make sure that you are making good comparisons by having the plane on a solid and level surface, and that you have eliminated as much friction of the tires, wheels, axles, etc.

RE: Measuring Thrust
 

There is a lesson here about props. That 10-9 prop is the wrong choice for a mild 40 engine, flight training, or accelerating out of a jam. High pitch props do one thing, basically, and that is generate straight line speed. A high pitch prop will lessen the plane's rate of climb and lengthen the takeoff roll. Lower pitch props generate less speed, but climb and accelerate better. And that's what you want in training and sport flying. Modern 40s do very well on 11-4 and 11-5 props. APCs are generally more efficient than Master Airscrew, for instance. Experiment until it flies the way you like.

You didn't say which Kadet you have. There is quite a bit of difference between the Mark II, the LT-40 and the Seniorita/Senior designs. But the power you've got with either of those engines, should fly the socks off any of those planes if the engine is running well and it's propped right.

I believe, too, that the idea that you just strap on a &*^& load of horsepower to whatever you fly and depend on it to keep you out of trouble is short sighted. What happens when you get yourself into some odd flight attitude and that SuperGrunt 77 craps out and dies? Now you have to know how to use the wing and the controls to get it down in one piece. Learn to fly the airplane, not just the engine. I've been futzing with RC aircraft since 1970, and I still do dead stick practice nearly everytime I go out. The game is to climb nearly out of sight, kill the engine, trim the plane for a glide, and land it so that you can turn it and stop at your feet. Then you really do know how to fly and not just point the thing and open the throttle. I'm still trying to make a landing that nice.....

RE: Measuring Thrust
 

I appreciate all your input. I like the OS .46, and since my instructor suggests I use it, I'm going to listen to him for now. A differnt prop like an 11x6 or 11x5 might be a better choice for it through training. I will hang on to the Mag .40 and put it back on down the road when I'm out of training and my instructor wants his engine back. Any other tips, suggestions or comments are welcome.

Crash

RE: Measuring Thrust
 

Just a note re. your Magnum -- it is really much more suited to a small high-speed sportster than a trainer. It hits its peak horsepower at 17,500 RPM with an 8-6, which is pretty useless on a large, blunt, draggy plane like a trainer (this translates to 100 MPH airspeed, assuming slippage is balanced by unloading). You may want to keep it in your stable of engines for a little screamer of some description.

RE: Measuring Thrust
 

That's precisely what I was thinking. I'll hang on to it. It'll make a nice engine for soe theing smll and quick.

Crash

RE: Measuring Thrust
 

Yeah, but I'm not sure you understood what the lesson _was_.

Pulling the tail feathers off a model which is tied down for engine testing does NOT mean doing so is a bad idea, it means the tail feathers weren't up to the task of holding the model back under full power. The 'lesson' is that models must be built so they CAN BE tied down during full power runs, not because you will always do that, but because you should tie the model down somehow while starting the engine to be _safe_, and you don't want to get chewed up by the prop if you happen to go full throttle _and your model restraint fails_.

Four or five years ago My Dad decided he'd jump back in the hobby after a 20-year absence (he soloed back in the 80s unsing my old student trainer, but never did build his own model). He bought a trainer, flight box, starter, yada yada yada. Borrowed my old but rebuilt K&B .60 and an old Futaba gold-face FM radio system. The engine had not been run since it was rebuilt by K&B a decade or so ago, so I gave him the instruction sheet for breaking it in all over again. He got it all about right, when he clamped the engine to his flight box for a break-in run on the ground. He set the 20-odd pound flight box (with 7AH lead battery inside) on the concrete patio deck, filled the temporary fuel tank, primed the engine, etc., etc., etc. The engine lit up on the first flip just like I told him it would. The bad thing was that Dad had the throttle open about two clicks too far, and the old K&B proceeded to TOW THAT FLIGHT BOX across the patio. He got one shot at the Tx and missed. He showed up at my front door a few days later, handed me all of the stuff he'd bought and borrowed, showed me the 23 stitches in his index finger, and declared that model airplane flyers were lunatics.

Accidents _will_ happen. You just had one, and because you're just getting started in this nutty hobby, there are no doubt more in your future. Do not have an 'accident' because your model could not stand being tied back during ground engine runs.

RE: Measuring Thrust
 

Excellent guidance.

RE: Measuring Thrust
 

I didnt read every post ,But I read an article on how to do this. clamp a 1x4 about 3 feet long onto a work bench and tie a heavy string arond the tail then make a loop to go over a scale which you tape onto the end of the board. then hang the plane over the scale which is on the diving board set up. O out the scale then hold the wing so the plane wont twist then throttle up slowly,probably better if you have two people.
This works with small planes not so sure for a 40 size.

RE: Measuring Thrust
 

For what ever you think it is worth here are some of my thoughts. As you have already seen the damage of a tail section that could not withstand the thrust of open throttle, does anybody think it is a good idea to hang a plane from a "divingboard and scale" to measure thrust? I would not do that with any size plane regardless of the size or source of power. I saw the method that NJ Georgee is refering to being used on a cd rom motor foamy and I don't think I would trust even that to be safe with only 12 oz. of thrust and 10oz. of aircraft weight.
I agree with kdheath that power is not always the answer when in trouble. I have been teaching rc flying for ove 15 years and still believe proper control surface commands work better than brute power. I teach novices at no higher than 1/2 throttle untill they can land consistantly.
the_plumber is right about it's better to learn that the tail section was not up to the task while you were still on the ground. If the plane had been in the air when the tail let go the plane would be toast and someone could have been hurt due to the lack of control. Thank what ever Great Powers you believe in that no one was in front of the plane when this accident happened.
I hope you can make proper repairs soon and get back to learning to fly. As one of the guys pointed out, be sure to rebalance after the repairs
I'm not saying your instructor is wrong about wanting more power. Just remember that the Sig Kadet series of plane are a design that has a high ratio of design drag. I have had a lot of time with the Kadet Junior, MkII and the Senior and all of them are a great trainer BUT are not fast planes. They are made to float along and give you plenty of time to figure out your attitude condition and make a correction.
Good luck with your learning to fly. I hope you learn to be both a Great Pilot and a Great Builder.

RE: Measuring Thrust
 

I understand what you guys are saying and I don't disagree. The tail section was weak due to a crash a couple days earlier. As you say...It's better to find this stuff out on the ground than in the air. I have pretty much completed the repairs, just need to cover the nekkid spots:) By the way, with this OS .46 on my Kadet MKII, what prop should I be using?

Crash

RE: Measuring Thrust
 

11x6 or 12x4 MA would be my .02.



David

RE: Measuring Thrust
 

11-6 APC, best bang for your buck