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Static Thrust Question

hevansrv7a

Well Known Member
In a recent conversation with Sam James he said that if I could do a full power runup with only the brakes to hold me back that I don't have enough power. This does not accord with my common sense meter, but I thought I'd ask the group. I can do this and will often do it briefly before takeoff.

1] What should static thrust (not RPM) be for a FP prop? In my case, a Superior 180 HP injected. Suggestions for how to measure it are also solicited.

2] Can you do a runup to full throttle with just the brakes? This may only apply to the -A model owners - I'm not sure what will happen with the tail dragger - would it pull you over onto the prop? Sam's is a tail dragger.
 
I can get to about 2300 rpm with my C/S 180, 6A, before it becomes difficult to hold it back. Don't know what the MP is at that point, but there is no way that thing is going to sit still with full power. I think if the brakes were more powerful, maybe. Don't know about a FP prop.

As for how to measure static thrust, some sort of load cell and harness would have to be ginned up. I did find some data for a O470 on a 180/182, at 2600 rpm static thrust was about 1000 pounds (see here).

To get an educated guess we would need to know about what sort of rpm you are getting with the FP prop.
 
scale

There was an article in Kitplanes a few months back about how to make a simple hydraulic scale to measure static thrust. Basically you tie the plane to the scale and the scale to something solid.
 


2] Can you do a runup to full throttle with just the brakes? This may only apply to the -A model owners - I'm not sure what will happen with the tail dragger - would it pull you over onto the prop? Sam's is a tail dragger.

It will not pull a taildragger over if the flaps are up and the stick is held full back!
 
So this means you CAN do full power runup?

It will not pull a taildragger over if the flaps are up and the stick is held full back!
Mel, thanks. Does this mean as it seems to that you CAN apply full power against the brakes without tying the plane to a stump?
 
I've heard of guys using a spring scale, like a fish scale only a higher range. They are often used to measure the weight of materials being lifted by a hoist. If you have any millwright friends perhaps you could borrow one. Attach it between the tail wheel of the airplane and a tiedown or a stump.
 
In a recent conversation with Sam James he said that if I could do a full power runup with only the brakes to hold me back that I don't have enough power. This does not accord with my common sense meter, but I thought I'd ask the group. I can do this and will often do it briefly before takeoff.

1] What should static thrust (not RPM) be for a FP prop? In my case, a Superior 180 HP injected. Suggestions for how to measure it are also solicited.

2] Can you do a runup to full throttle with just the brakes? This may only apply to the -A model owners - I'm not sure what will happen with the tail dragger - would it pull you over onto the prop? Sam's is a tail dragger.


I check mine by applying brakes prior to rolling and use that number as static. I'm also running the Superior IO360. In my conversations with Craig Catto, using his prop, he likes to see static +500 rpm as a prop range.
 
Tie her down, real down

In a recent conversation with Sam James he said that if I could do a full power run-up with only the brakes to hold me back that I don't have enough power. This does not accord with my common sense meter, but I thought I'd ask the group. I can do this and will often do it briefly before takeoff.

1] What should static thrust (not RPM) be for a FP prop? In my case, a Superior 180 HP injected. Suggestions for how to measure it are also solicited.

2] Can you do a run-up to full throttle with just the brakes? This may only apply to the -A model owners - I'm not sure what will happen with the taildragger - would it pull you over onto the prop? Sam's is a taildragger.
I am cringing. However once or twice when I was doing some extensive prop balancing on my hartzell, my prop guy want to run it up to flight RPM and get the blades off the low ptich stops. Long story short, we had it chocked and tied down to beat the band (RV-4 taildragger). It made a difference in the quality of prop balance we got on the constant speed Hartzell.

With that said I cringe thinking about doing really high power run-ups on any RV with just brakes. Yes you can over come the brakes (or skid on grass); yes you can put it on the nose with a taildragger if you don't have the stick in the gut. (edit: as James Freeman said even full elevator is not enough sometimes.) On top of this, high power static run-ups pick up all kinds of pebbles and rocks, sand blasting the prop and your plane. No thanks. IF YOU HAVE TO DO A FULL POWER RUN-UP, TIE THE TAIL DOWN AND I MEAN DOWN.

What is the purpose of static RPM? It is a traditional criteria or benchmark, but it has limits to its meaning or importance. You can get a good idea of static RPM on initial takeoff run with full throttle, before speed builds up. Also what is your takeoff run and Rate-O-Climb? These are more useful to me. Most important to me is wide open throttle RPM in cruise flight.

I'm not a FP prop expert but the criteria of fixed pitch goodness that I learned, besides decent takeoff RPM, reasonably short ground run and good rate of climb, is the cruise test. The cruise test is wide open throttle (WOT), 8,500 feet DA, leaned for best econ (EGT 125-150F ROP). (Leaning above 75% pwr is forbidden by Lycoming, so you might need to climb a little higher or do the test at full rich or lean for max rpm.)

With the cruise condition above, you should be at Van's spec air-speed or better while not over revving past 2,700 rpm red line (*too much). If you have more of a climb prop, you'll be over 2,700 rpm, up to say 100-150 rpm and need to throttle back for cruise. A good overall general purpose climb/cruise prop, you'll be under 2,700 rpm, say 50-100 RPM. If your prop's WOT cruise RPM is lower still, closer to 2,500 rpm than 2,700 rpm than its getting to be more of a econ/cruise prop, at the sacrifice of takeoff & climb. Some people like this. If both are way off, speed (low) and RPM (high), its just the wrong prop or you don't have the fairings on. The cruise test means nothing unless you have all the fairings on.

Who cares what static is, if you're happy with takeoff, climb and cruise performance. Keep in mind the prop blades are totally stalled during a static run-up. It is an OK criteria or bench mark but it has to live in the real world of the plane moving through the air. We don't fly parked with the brakes on. Also with wide varitions in props and engines in RV's it's harder to compare notes apples for apples.

If you want what the normal or typical static or wot cruise RPM than find folks to compare notes who have the same prop, HP and RV model. There's a ball park static RPM range, but guessing, you want at least 2,200 rpm on initial takeoff roll. If the prop is a cruise prop static will be lower, climb prop static will be higher, +/- 150 rpm? RV's tend to have lower static RPM than a factory planes because of the much wider range of operational speeds, ie, RV's are fast.


** Note: Lyc says RPM over 5% of red line is forbidden and failure to observe this limit requires extensive engine inspection, including tear down! Just the messenger, since we all know many fly around at 2,900 rpm all the time, and you know who you are. :rolleyes:
 
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It will not pull a taildragger over if the flaps are up and the stick is held full back!

Mel--

I hate to disagree with you but I don't think that's true of all RVs.

It depends on static thrust, CG relative to the wheels, and how well the brakes work.

We had a -4 locally with an injected motor that stood on its nose with a hot start--the engine "caught" suddenly after a vapor lock and the airplane went over in spite of full aft elevator.

My RV-8 will usually start to lift the tail before the wheels slide (solo anyway). I can't use anything close to full power without tying it down. I have 200hp/CS but my CG is slightly more aft than seems typical for -8s.

I'm very careful to keep the stick fully back when starting (I put the starter on my stick grip) and taxiing unless I have a strong tailwind. I'm still careful with the left hand though...
 
Lots of Info, no answer

It's all good info, but all I wanted to know was if Sam James was right or wrong because I can go to full power static with my FP prop with only the brakes to hold it. It's a -A, so all that stuff about tail draggers is helpful to others, but not to me. I may have missed it, but I did not see a direct yes or no answer. The only reason I care if Sam is right or wrong is that he think that therefore I don't have the power that I should have. I agree that top cruise speed is a better test, but there is no solid benchmark for my airframe due to the mods such as SJ stuff.
 
Max. RPM can be required at run-up...

I am cringing. .....
Who cares what static is, if you're happy with takeoff, climb and cruise performance. Keep in mind the prop blades are totally stalled during a static run-up. It is an OK criteria or bench mark but it has to live in the real world of the plane moving through the air. We don't fly parked with the brakes on. Also with wide varitions in props and engines in RV's it's harder to compare notes apples for apples.
.....:rolleyes:

George... not quite....

If you have a Fixed Pitch prop, then the Lycoming instructions require a leaning for max. rpm on the ground (at run up) if the DA is above 5,000 ft.

http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1094D.pdf

I believe the original post was for FP props, and the max. static rpm is a sign that power is being generated.

gil A - with a runway that is frequently above 5,000 ft. DA...:)
 
It's all good info, but all I wanted to know was if Sam James was right or wrong because I can go to full power static with my FP prop with only the brakes to hold it. It's a -A, so all that stuff about tail draggers is helpful to others, but not to me. I may have missed it, but I did not see a direct yes or no answer. The only reason I care if Sam is right or wrong is that he think that therefore I don't have the power that I should have. I agree that top cruise speed is a better test, but there is no solid benchmark for my airframe due to the mods such as SJ stuff.

Whether or not it can be held with brakes is more a brake check than an engine check. Not all brakes are the same depending on disk and pad condition. It proves nothing. If you are worried about whether or not the engine is up to snuff, check rpm during the first 15 seconds of take off roll and if it isn't what it should be, abort.

For what it is worth the last time I did it a full power run static with the airplane tied down, it cost my $1800 to repair a rock dinged blade. I won't do that again.

Also, for what it is worth, I've learned you can get a very good dynamic prop balance at 1900 rpm rather than 2300 or higher. Just set a course pitch and run it up. It seems if a the thing is balanced at the lower rpm it is smooth from there on up, but not always vice-versa. I bet this is true with a fixed pitch prop also.
 


1] What should static thrust (not RPM) be for a FP prop? In my case, a Superior 180 HP injected. Suggestions for how to measure it are also solicited.

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According to the book "Modern Propeller and Duct Design" by Martin Hollman and Mark Bettosini, static thrust, lb, = 7.38 X (HP X D)^ 2/3 X (rho/rho0)^1/3

where: HP = actual HP, D = prop dia, ft., rho/rho0 is the density ratio.

I recently did a static thrust measurement on my Lancair 235; I got 220lb. This with an O-235 of 125 HP turning my FP three-blade prop at 2230rpm, 28.25" MAP, 14C. Calculated power for this run was 99HP. This measured thrust is quite a bit less than the 477lb this formula would give. But then this particular prop design has reduced static thrust, but excellent climb and cruise efficiency.
 
Good to Know, but ...

According to the book "Modern Propeller and Duct Design" by Martin Hollman and Mark Bettosini, static thrust, lb, = 7.38 X (HP X D)^ 2/3 X (rho/rho0)^1/3

where: HP = actual HP, D = prop dia, ft., rho/rho0 is the density ratio.

I recently did a static thrust measurement on my Lancair 235; I got 220lb. This with an O-235 of 125 HP turning my FP three-blade prop at 2230rpm, 28.25" MAP, 14C. Calculated power for this run was 99HP. This measured thrust is quite a bit less than the 477lb this formula would give. But then this particular prop design has reduced static thrust, but excellent climb and cruise efficiency.
That's good info and I thank you for it. However, the point of the question was to find out if I have enough power for my engine and prop. I guess the answer will have to be determined by experiment and measurement.
 
I measured mine by connecting 1/8" cables from my gear legs to a spring gauge attached to one of the front hooks on my car. The test I always use on takeoff is to look at rpm the moment I bring my throttle up to max; for me it's 2210-2230 rpm. Cessna in their 172 handbook recommended the same test, and they said somewhere about 2200 rpm. 'Course this is only for fixed-pitch props. 'Hope this is what you were looking for.
 
Thanks, yes it is, sort of.

I measured mine by connecting 1/8" cables from my gear legs to a spring gauge attached to one of the front hooks on my car. The test I always use on takeoff is to look at rpm the moment I bring my throttle up to max; for me it's 2210-2230 rpm. Cessna in their 172 handbook recommended the same test, and they said somewhere about 2200 rpm. 'Course this is only for fixed-pitch props. 'Hope this is what you were looking for.

The original issue was SJ telling me that if my brakes could hold me back then I didn't have enough power. This was in the context of trying to figure out why my airplane doesn't go as fast as I think it should. The answer I got from the forum was "it depends". Since mine is FP, it probably has less static thrust than a CS. I know the -6 180 HP CS that I did transition in had a much better take off and climb (mine is very good; his is spectacular). My takeoffs are generally well under 700' even with gradual application of throttle and I'm usually doing well over 100 kts and 1000 fpm by the end of the 4000' runway.

So I was just trying to evaluate SJ's comment. Since then I gained 6 kts more or less by just aligning one main wheel fairing. Since nobody thinks static thrust, especially in a FP, means much, I think I'll just keep looking for aerodynamic issues and assume then engine is OK. My climb rates are quite OK and that should be a better measure of HP anyhow.

Thanks again.
 
Is actual static thrust important? (yes kind of)

George... not quite.... If you have a Fixed Pitch prop, then the Lycoming instructions require a leaning for max. rpm on the ground (at run up) if the DA is above 5,000 ft.

http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1094D.pdf

I believe the original post was for FP props, and the max. static rpm is a sign that power is being generated. gil A - with a runway that is frequently above 5,000 ft. DA...:)
100% right about leaning. I missed the high DA. I know the Lyc Service Instruction No. 1094D well. It's a great document.

And yes he wanted to know static thrust. Than we all got off on the wisdom of doing FULL power run ups.

My comment or thought is do we care or need to know the static thrust? Really need it. If I'm trouble shooting yes it's good to know. Having flown enough RV's, I can tell on initial takeoff roll, climb and cruise what the prop/engine is doing, compared to other fixed pitch RV's (and Van's specs), but is it good to note static thrust? Is it worth measuring? Well if you are having problems it can be........

It's useful to compare static thrust of other fixed pitch RV's, especially if they have a similar prop and engine. If your RPM's are down, you don't need to know static thrust, I can tell you it will be sub par. If you know the static thrust of other RV's with the same prop and engine, low static thrust might point to low engine power, so get the compression test out. If your top speed is off but static RPM/thrust is close to the bench mark's or other RV's, your airframe drag is higher. Static thrust can be useful for trouble shooting engine and performance issues, if you have other RV's to compare with. Other wise if your RV performs as advertised, smile, be happy and don't worry.

I have no problem doing a first flight with a new fixed pitch RV, with out a static test. If initial t/o RPM and performance is good, than why do a static test? Static thrust is great, but I know the plane is performing to Van's bench marks. Most F/P prop pilots know if their prop is over/under pitched.

Engine power loss is harder to measure, but if compression is up and static RPM is up on initial takeoff roll, you are going to be close. When you start messing with unknown props, engines and airframes static gives you are warm fuzzy to start from. We kind of know the score and use similar engines, props and airframes. So just knowing airframe performance is validation of prop and engine. Usually top speed issues are mostly from airframe drag. I suppose if you do a static test after your new engine is broak-in, you can do one 1000 hours later and see if its still putting out the power.

My past RV project and current both use c/s props. All I needed to know, I got 2,700 rpm and good t/o acceleration. As David said prop balancing a c/s prop does require higher power that can damage your prop. So just be careful.
 
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An accurate thrust gauge is easy to make. You just need a pull cylinder and a decent pressure gauge. These came from a surplus sales outfit:


Static thrust is going to be poor in the case of a fixed pitch prop designed for a fast airplane. Here's an example; Paul reports 220 lbs for his Lancair with an engine rated at 125 hp. On a past project I had 220 lbs with 68 hp....but the prop was pitched for a 65 mph cruise. The best static thrust number among 6 samples we checked was the Subaru on an RAF gyro....but again, consider the appropriate propeller pitch for the slow expected flight speed range.
 
Any Hartzell static thrust #'s floating around

An accurate thrust gauge is easy to make. You just need a pull cylinder and a decent pressure gauge. These came from a surplus sales outfit.
Nice Dan. I hate to poo-poo on experimentation. If you want to know static thrust go for it. Dan is 100% right, a prop pitched for a fast plane like a RV is going to have poor static thrust.

With that said has anyone done a static full throttle thrust in a RV with 180HP Lyc and Hartzell prop? I seem to recall Dan you or someone did a static thrust on a bunch or planes? Thanks
 
Another source to get suitable device might be from the people installing power lines. At least some local guys has borrowed scale from them to measure their statics...
 
Thrust measurement

Static thrust measured on my RV-7A with XIO 360-M1B and Hartzell 74" prop was 2974N @ QFE 957hPa and 16 deg. It was measured with load cell and plane anchored by Honda Civic, just after the governer limited RPM to 2700. (See Youtube: RV-7A thrust measurement)
Note: in Switzerland every Homebuilt has to be subjected to thrust measurement and it is usually done by EAS.
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Really

Static thrust measured on my RV-7A with XIO 360-M1B and Hartzell 74" prop was 2974N @ QFE 957hPa and 16 deg. It was measured with load cell and plane anchored by Honda Civic, just after the governer limited RPM to 2700.
Note: in Switzerland every Homebuilt has to be subjected to thrust measurement and it is usually done by EAS.
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By the online calculators 2974 N = 13,229 lbs Perhaps I am not interpreting the 2974 Newtons quite right as I would normally expect a healthy IO 360 would be 700 -800 Lbs static thrust.

I think 13,000 lbs thrust would drag a civic with the wheels locked and take off in less than 500 feet, with the civic still tied on the tail wheel. You could quickly transition to vertical and accelerate past VNE before circuit altitude. Even with the Civic, at say 2500 Lbs, plus the RV at say 1200 lbs you are still at 3:1 thrust to weight.

Perhaps I need to study how the N really converts to Lbs of force.
 
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My apologies!
I should be energetic enough to walk and refer to some physics text books vs the online lazyboy method. The following link was the basis of my blunder. http://howmanyconvert.com/2974-newtons-to-pound_force (it must be a website set up to assist North Korean missile engineers) the Civic should be safe.

I think the practice of actually measuring the static thrust is a great idea. It would have saved a few aircraft over the years as fuel flow and power issues would have been found on the ground.

Thanks for the conversion. Enjoy your RV7
 
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Physics 101: braking force, assuming the brakes can lock the wheels, equals tire friction times weight. So there is a huge difference in the force it takes to overcome braking force between a light airplane and one at gross weight.

Remember that A340 at the airbus factory that went through a wall and was destroyed on a runup? They decided to run all 4 engines at takeoff power at light weight. They are not the first ground crew to make this mistake and they won't be the last.

So a blanket statement that you need enough thrust to overcome the brakes is meaningless. In fact static thrust is pretty meaningless too. It gives no indication of flight performance. The instant you start to accelerate the thrust decays. It is the thrust at flying speed that matters. The prop could be partially stalled at static conditions but start to unstall at 20 kts. In my humble opinion measuring static thrust is a complete waste of time. Save your fish scale for fishing.
 
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Btw if you are troubleshooting and you want to measure power, not thrust, use manifold pressure and rpm. That will be much more useful and accurate than static thrust and it is independant of the prop used.
 
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