Request for Info: Static Thrust Measurement

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Rockiedog2

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I went back and skimmed thru this thread
the thing that caught my eye was VB's statement in post 77
" The tail comes up fast and I have to hold full back stick to keep from nosing over on takeoff."
and then about lowering the stab back to where it was when flipping the gearbox

lowering the stab may do the trick but I would assume the opposite til I had tested/ proven it. It's gonna be mostly outa the propwash with the gearbox upside down, right? and so no airspeed over it when it needs it the most, on initial power up, especially if going smartly to full power.

So we got 2 warnings; the Kolb dude's and your personal experience that it wants to go over. Add the unknown of flipping the box but assume the worst(always) and we got 3.

Last thing I would do VB, is cob it on takeoff. And I would be very careful and slow about it all. Maybe it will be fine, but it look to me like a possible(probable) setup.
 

Victor Bravo

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OK, two guys whose knowledge/experience I respect have weighed in on the side of NOT taking off with full power. I will definitely take that into serious consideration.

But one of the things I am trying to NOT have happen is to take off using partial power, get the airplane up to 500 feet, then apply the rest of the power and find that I don't have sufficient elevator authority, yank the stick all the way back, and get a tail stall, where the airplane wants to go hard nose-down. Then I'd instinctively pull the throttle back and all that aft stick would suddenly create a strong nose-up pitch.
 

Toobuilder

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Much better to find that out at 500 feet than 50!

Really, if it makes it up to 500 on part power, what's the problem? You have a
"known" safe condition to fall back on if it doesn't like full throttle. Anyway, I'm not suggesting you avoid full throttle entirely, just take it easy.
 

lr27

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VB:
My impression of those comments is that you should bring the power up slowly, not that you can't use full power for takeoff once you're moving at a good clip. The latter assuming that you still have some elevator authority left.
 

lr27

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Chocking the tires makes the moment larger than it is on takeoff, because the wheel axles are much further from the thrustline than the center of mass is. OTOH, it's not quite as bad as when running up the engine with the brakes on. You also need an attachment point at the back of the airplane that can handle the load.
 

Rockiedog2

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OK, two guys whose knowledge/experience I respect have weighed in on the side of NOT taking off with full power. I will definitely take that into serious consideration.

But one of the things I am trying to NOT have happen is to take off using partial power, get the airplane up to 500 feet, then apply the rest of the power and find that I don't have sufficient elevator authority, yank the stick all the way back, and get a tail stall, where the airplane wants to go hard nose-down. Then I'd instinctively pull the throttle back and all that aft stick would suddenly create a strong nose-up pitch.
No, I didn't mean to indicate don't use full power, just bring it up slowly like lr27 said...as the speed builds and you get some air over the tail. But you may find that it won't like full power, I dunno.
We know you're dealing with 2 separate unknowns, the runway handling and the handling when airborne.
On the ground the gear of course carries the weight and is our pivot. A number of factors affect that, gear position, 3 point cg, vertical and horizontal distance from the prop, etc etc. The engineers can do better with all that than I can. I don't know anything about the Kolb and it's noseover tendency, pitch command etc, all that should be available from the Kolb guys. Surely somebody has flipped the box on a Kolb; I expect you're on that...the head Kolb dude may be your best source but like you said he's got all that liability and stuff to think about and may be talking defensive. Regardless, I'd take every precaution even to pump the tires up hard and wouldn't touch the brakes with power much off idle til I had gradually checked that out. And no chocks. And maybe ballast aft. Etc.
So when we get airborne the pivot shifts to the cg and and that whole new set of factors. Vertical cg, stab and all that, the engineers again. You already flew it so you got the best info there; maybe the stab change will take care of it.
You a real test pilot VB!
 

Rockiedog2

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VB I got somethin for you man
Thinking waaayy back I remember flying a friend's some sort of pusher that had a pitch problem so I called him up and he's surprisingly still alive so got it that the plane was a Panther Plus 2 pusher and a Rotax 582 or one of those. They had just done a buncha work on it and got it ready to go they thought and I flew it; it's all come back now. The pitch/power response was insane, barely manageable, soon as I pushed the power up it instantly near went over so I reflex pulled it back and off the ground we went and then the wildest PIO imaginable. I instinctively brought the power back slowly and managed to get it on the ground a couple hundred yards down the runway. They investigated and FINALLY found the problem. The gearbox was up instead of down.
I got an old friend that was a very early Kolb dealer. Last I heard he was about to keel over(most of us have already) but I called him up anyway and he made a miraclous recovery and was back working. Ran your problem by him and he allowed as to he knew exactly what you're dealing with; had the same on one of his Firestars, wouldn't go over about 50. He says your gearbox ratio is correct he remembered it precisely. He also says don't flip the box, down is where it's sposed to be. He says it's the prop, 65 is the correct length and yeh it's close to the boom. They put a different prop on it he didn't remember what prop but that cured it, the speed near doubled.
I think you need a prop VB.
 

Rockiedog2

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uhoh
Jimmy just called me, said he'd been thinking back and wanted to amend some of his recollection about your problem. This is a MS State aero engineer who has built and rebuilt dozens of airplanes I got every confidence in him except his memory does need joggin occasionally funny how that works.
Yes the box is sposed to be down don't flip it, I agree with that from personal experience. The HKS wasn't the original engine you already knew that I think. The gear ratio on the one he had the problem with was 2.68 or about that, on a 447 and yes it was the prop problem and the symptoms were same as yours. When he got it right it ran about 80. He says the 3.47 and wide blade prop sounds like a a setup intended for an airboat, never intended to go over about 40 or so. He doesn't think a prop change is gonna fix your problem. He says you need a gearbox that will turn the prop 2500-2800. That makes sense; same as a Cherokee or the like. And probably a diff prop
Sorry for what turned out to partially be misinformation but I think we got it right on this end now. Dunno how any of that jives with other info you got. Hopefully will help you make a decision.
If it was mine I wouldn't flip the box, would look for a trade on a box with the right ratio and whatever prop turned out to be right. Or maybe a trade on a proper Rotax/prop setup. Or sell that airboat setup
Or just fly 50.
 
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Marc W

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I know this is a little off the wall, but sometimes you have to think out of the box. Can you turn the gearbox 90 degrees? It would move the thrust line to the side a few inches or you might be able to move the engine over to compensate. It would keep you below the magic 40 inches and give a little more room for a larger prop.
 

Victor Bravo

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OK, back at home after another test flight this afternoon. I had increased the pitch of the prop blades by 2 degrees after the last flight, and flew it in that configuration today.

Same takeoff roll but noticeably higher rate and angle of climb. However this was at lower RPM. 5300-5400 RPM on initial climb, a far ways off from the 6000 or 6200 I am allowed for 5 minutes.

However there was no significant improvement in cruise speed. Still 50-51 MPH IAS at 5700 RPM with full throttle.

Today's flight was conducted during the hous of tower operation,a nd my slow speed managed to irritate everyone form the ATC people in the tower to the other folks who were Cessna-ing and Piper-ing around in "normal" airport operations. The fact is that this aircraft is simply not going to be able to mix in with the normal traffic here. A J-3 Cub will barely mix in well enough not to p*** off anyone. The Taylorcraft will mix in fairly well. This Kolb with its big tires is not going to be welcome and fit in here for very long.

So the combinaiton of an engine/prop combo that's not putting out the thrust it needs to, plus an "open ultralight" that makes everyone stop what they're doing, plus the fact that everyone else is clearly inconvenienced by my presence.... is writing on the wall I cannot ignore.

The aircraft and engine will be put up for sale separately. It's a good engine and a good airframe, but they just don't like each other and won't work together. Like two "alpha dogs" in the same territory, they just need to not be together.

So someone will have a use for an advanced and efficient little 4 stroke engine, and someone else will have a use for a bush-modified Kolb. I'll have to go back to being happy flying my 172 and/or find some other little fun E-AB to tinker with.

I thank everyone who put their time and brain power into helping me try to solve this puzzle. I hope that my friends here do not feel that I have wasted their time.

Bill / VB
 

lr27

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Couldn't you just sell the engine and use the money to get a more appropriate engine?
 

Victor Bravo

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Yes I could, but that would leave two problems. First, I am flying out of a city airport, and I didn't want to fly with a 2-stroke over solid city and houses. Second, the 2-stroke does not resolve the problem of the airplane becoming an issue for the other "normal" air traffic.

If I had the money to have another hangar out at another airport in the desert, this would be aperfectly good option. But right now I'm squeezing a nickel so hard the buffalo farts.
 
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Dana

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Hmm, well, that's a different problem. I never had a problem with my Kolb, but my home field has 1/10 the traffic of yours, and no tower. Though I did fly my Fisher 404 (which cruised at 55mph) into a class D a couple of times, on one occasion the tower called me out to some other traffic and described be as "a bi-wing moving very slowly."

One thing you can do (and ATC eventually gets used to working with you) is to fly a smaller or alternate pattern. When I was helping a guy transition to a Quicksilver, which is a lot slower, the tower quickly figured out to put us in a right traffic pattern while everybody else was left, and then just fit us in when we got to a position to turn base. But still, KWHP certainly isn't a Kolb's native territory, unless it was a Slingshot, which is basically a clipwing Firestar with a big engine (I know a guy selling one with a 105HP Simonini, he wants to buy my Starduster but can't until he sells the Kolb...).

But before we're done here... you've never said just what the actual propeller pitch was at any point. Not degrees, pitch in inches. Or, what is the blade angle at 75% radius (24 3/8" from the centerline)?
 

proppastie

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selling a working aircraft might be easier than selling engine and air-frame separately. But then again often people get more money by parting out an aircraft than they can selling the whole aircraft. Good luck.
 

Victor Bravo

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But before we're done here... you've never said just what the actual propeller pitch was at any point. Not degrees, pitch in inches. Or, what is the blade angle at 75% radius (24 3/8" from the centerline)?
I measure the pitch at the end of the prop blade, maybe 3/8" from the very tip, because that is the most convenient place for me to put a protractor with repeatability. I made a tool that straddles the tailboom tube and has a rotating plastic protractor, with a reference mark. Not a high-end Starrett metal gauge by any means, but probably more than good enough.

So 3/8" form the very tip, on a 65 inch propeller diameter, the pitch is now 13 degrees on the last flight (Wednesday). It was at 11 degrees for most of the flight testing.

The "blade angle" I used is a straight edge across the face of the blade (bottom side of the airfoil).
 

Dana

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13° at the tip works out to 47" pitch (40" for 11°), but depending on the blade twist, especially for one designed to be adjustable, measuring the angle at the tip may not be the best place, which is why 75% of radius is preferable. That gives you an average for the outer half of the prop, which is where most of the work is done. I'd be interested in knowing what the angle is at 75% (24 3/8").
 

plncraze

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VB, thank you for sharing this process! The kolb is one of the few choices for config that are affordable for pushers. If you have issues with this then other's must have or will have the same issues.

On the subject of gear ratios: why do the higher ratios (3.47 sounds like an airboat ratio as stated by Rookiedog2)keep the overall speed down? I know this is sort of a "why don't cars go faster in first gear question" but don't props prefer to turn slowly or is this what is leading to Dana's question about pitch?

The other question is about thew effects of thrustline. Is some of this discussion about how much up elevator it takes to deal with the higher thrustline as compared to a normal Kolb?

These may seem a little ignorant but I am trying to put together all of what is being discussed here. In recent posts it seems that there have been discussions about a lower thrustline and a smaller prop.
 
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