Static Thrust Question

[hevansrv7a]

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.

[AlexPeterson]

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.

[wingtime]

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.

[Mel]

Originally Posted by hevansrv7a

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!

[hevansrv7a]

Originally Posted by Mel

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?

[CESSNADON]

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.

[roadrunner20]

Originally Posted by hevansrv7a

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.

[gmcjetpilot]

Originally Posted by hevansrv7a

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.

[flyeyes]

Originally Posted by Mel

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...

[hevansrv7a]

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.