I have some numbers for the crunchers

[Fl90]

Hello, I have some numbers from a machine I just finnised. I will fly it this Saturday (hopefully), and I'll post the rest of the numbers after that.

It is mostly a Bensen with a rotax.

AUW = 490LBS.
Static thrust = 300LBS.
Thrust line is 4" above COG. COG determined by double hang test.
Thrust line 0 deg from keel.
HS is flat plate 3sqft surface, -3 deg AOI, prop blast measured at HS while pulling 300LBS is 90mph.
HS is 4' from vertical cg.



I have taken parts from my experimental collection and tried to put together a safer gyro. I have been flying the Bensen marginaly powered, with a four stroke, and that has removed the option of selling it. Now I'm attepting to make it a machine for a wider variety of enthusiasts, so that I am more comfortable with the idea of selling it, some day.


The numbers are here for those of you that have the ability and inclination to calculate the effectiveness of the HS to counter the thrust above CG.

 

[Friendly]

Do yo have a picture of the new set up. Which engine and prop are you using

 

[Bob]

FL90:
I just poked in the figures into HSCalc.xls and there are the results.
total lift force of HS= 24.14lbs
total force imparted to airframe =96.56ft lbs of torque.
PPO force = 100


what is shooting you down is the small size of the HS being only 3 sqr ft.... I'ed double it myself!
as you can see from the calculations it will PPO on you the way you have it if your not extreamily carefull !
if you change your headding at full throttle you will probly ppo... so back that throttle off as soon as you can !
....
I'll attach the HS Calc. XLS for you it does not figure in the AOA of the HS as Doug Riley told me , the way its calculated is at maximum angle of attack just before stall.... so the actual down force on your gyro's HS is probly substantually LESS !
I personally wouldn't fly it till I doubled the HS size !
...
hope that helps !
...
Bob......

p.s. HS lift Calc.xls is a microsoft excell spred sheet file. very handy for these types of things !
...

 

[Fl90]

Bob, explain? From your numbers, 24lbs at 4' is 96 ftlbs. 100 ftlbs of force from engine is 4 ftlbs left for PPO.

 

[Bob]

ok , even useing those numbers ...which I mentioned are very consirtive.... due to the fact that the programasumes optimum AOA of the HS....
As you say 4 ftlbs left for PPO ... that means there is still 4lbsof torque turning your airframe when everything is accounted for .... another words your airframe will turn about its COG because of that 4 lbs of torque...
the entire point of the HS is to beat the PPO force SOUNDLY ! trounce it GOOD !..... not just barely cover it... to prevent PPO... as it is you can't even beat the PPO force the figures say IT WILL PPO on you.... that there is nothing to stop it...
perhaps as soon as you leave the ground it will start to roll foward . nose down in a PPO no matter where your rotor is pointing...,though I know that playing with the throttle will limmit the torque imparted to the frame( backing down from 300lbs of thrust to say 250lbs of thrust instead) will alow the HS to over come the torque of the engine and stablize the airframe. that changes the figures for the good
where the Torque on the frame from the HS is 96 ftlbs of torque
the engine's torque on the frame is only 83.33 ftlbs of torque
now leaveing you 12.67 ft lbs to the good,
not 4 ftlbs to the negitive!
Do you see what I'm saying here ?

anotherwords if you have a 100 ftlb torque figure that your engine is putting on to the airframe
you will then need at the bare minimum of say 110 ftlbs of torque imparted to the frame by the HS ! ...
this is CRITICAL ! its not something that can be guessed at !
you have to know what it is, or you could die.
PPO is the single most incidious thing a gyro can do it almost alwayse ends in fatilities... but we know what causes it !
and there for we can fix it....
if we take those figures you gave and change the HS size to
6 sqr ft. of Airea. the results are...
193.11 ftlbs of torque by the HS
and 100 lbs of torque for the engine thrust.
leaveing 93.11 ftlbs torque as a safty margin.. which is wize.
there is no way your craft will PPO then it simply CANNOT.
because the stablizing torque is larger than the PPO force.
....
you can do other things besides change the size of the HS too.
say we move the HS back further to give it more leverage by one foot.... the results are.
120.70ftlbs of torque from the HS
and 100ftlbs of PPO force
it works but its marginal at best
Personally I think a safty margin of 50% should be maintained
but that is my opinion too.
these figures are done at the 1000ft ASL air densisty if your altitude is diferent then plug that in instead.
....

so you see your current 96ftlbs of torque from the HS is not enough to over come the 100ftlbs of torque of PPO force you are UNDER it by 4... and that means it will PPO !

Hope that helps.

Doug Riley could explain it much better than I ever could
perhaps he will add his 2 cents worth to confirm or denie those figures I've given. I Hope so!
....
if you've any other questions Please ask, I don't want you doing a PPO on your first flight !
....

Bob......

 

[scottessex]

Sounds like a KB-3, Did you raise the seat? Seat tank or Boat tank?

 

[Bob]

I am also courious of the "margnial proformance 4 stroke engine "
you mentioned, I'ed like to switch to a 4 stroke and get rid of the horrindious scream of the Rotex 447, what engine did you use anyway Mark ?
...

Bob......

 

[PW_Plack]

HS is 4' from vertical cg...

your airframe will turn about its COG because of that 4 lbs of torque...

( backing down from 300lbs of thrust to say 250lbs of thrust instead) will alow the HS to over come the torque of the engine and stablize the airframe...

Guys, guys...

The CG is a point; the "vertical CG" is a line. FI90, what is the arm of the HS measured from the CG? (The point?)

Bob, IMO, four pounds is inconsequential. That's smaller than the error in the process.

If you back off the throttle to 250 pounds of thrust, you're also reducing the downforce on the horizontal stab, and since that force decreases by the square of the decrease in the HTL moment, reducing thrust actually makes the numbers get more unbalanced.

I doubt that the math model is good enough to predict things within four pounds, but if you wanted to make it perfect "on paper", why wouldn't you just shim another degree of negative AOA into the H-stab? If it's a flat plate, you don't have to worry as much about stalling it.

Or, replace it with an airfoil-section stab of the same size, and you're there.

 

[Bob]

Oh HAY !!!!

I found version 2 of that calculator.
My memory must be getting purdy bad as I thought it gave you the option of the HS Angle of attack but it doesn't... sorry.
...
I have tried the same calculations in both programs but didn't find any diference...
but just to be on the safe side here is version 2 of
" HS Lift Calc-2.XLS"

hope it helps!
...
Bob......

 

[Bob]

Paul
Are you saying it is ok for him to fly his machine when the math says it will PPO on him ?
I sure hope not !
though I agree that the idea of the HSliftcalc program is not to get the forces equilized to the 3rd didget...the idea is to have OVERKILL with your stablizeing force so there is no question weather it will PPO on you.
but you made a good point Paul reduceing the throttle will also reduce the airspeed accross the HS and therefore reduce the HS's downward force substantually... good point indeed ! I didn't think about that in my example, thank you !
...
as I stated before its WIZE to have at least a 50% OVERKILL on the HS's torque over the engine and thrust torque on the frame.
to not have this LEE WAY would to me be very unwize !
....
changeing the Coefecient of lift of the HS by changeing the AOA of the HS is an option, but it will not change the figures in the calculator because the HS is already asumed to be at a point just shy of stalling as Doug told me .
what that angle is I have no idea... but if its already asumed to have Max AOA for a flat plate adding more won't help it will just stall the HS. so in that regard the program is found lacking quite a bit !
Doug Riley posted a chart for the AOA of a plate HS a while back that might be of some help FL90,
...
I wish you luck !
Bob.......

 

[Fl90]

Bob, I have been flying a Bensen with no HS. I have tried many engines. The 4 stroke you're asking about only pulls 245lbs. static. If 4 lbs of force over the CG bothers you, you do not want to try a marginal machine.

I posted the numbers for anyone that can calculate the equilibrium with the angle of incidence given. The moment arm for the HS is 3.5 feet from the COM, thanks Paul Plack.

If the make, model, engine size, or color of the interior is necessary, please explain.

 

[Bob]

Yes I'ed like to know the make and size of the 4 stroke engine even if it only puts out 245lbs of thrust...
...
I know Bensons have been flown for years, decades even with no HS at all, thats not the point...
the point is without an HS you can PPO in a benson or any HTL machine for that matter, that is a fact of life. Yes You the Pilot can if your good enough , get away with it, thats been proven by people like yourself that fly them.
However if you want to make the machine stable so its a safer machine you have to do certain things ....

Unfortionatly I don't know how to calculate the AOA and down force of the HS, I know Doug Riley can, you might ask him.
infact I'm sure its in one of his posts of the past.
i believe Al Hammer also covered it extreamily well too.
...
to each their own ... if you want to fly your gyro without a properly sized HS be my guest, the risk is yours to take and no one elses.
I thought you could use the calculator programs is all
if not no biggie !
...
Have a good one !

Bob.......

 

[Doug Riley]

I guess posting this chart once more won't overload the Forum.

The goal should be for the aircraft to be at equilibrium at all throttle settings and airspeeds -- WITHOUT having to pitch down to reach that equilibrium. If the HS only counters the PPO moment when the HS reaches 12 or 14 degrees AOA, then the gyro only (maybe) reaches equilibrium after it's well into the PPO. That's not exactly optimal.

Follow the chart. Across the top are angles of attack. (Let's assume the head plates are set so that the keel normally flies level; then AOI equals AOA.) Down the left column are airspeeds in MPH. Let's assume that the 90 mph is a good number for WOT at any airspeed (i.e the HS is fully immersed).

To use the chart, pick an AOA across the top and and airspeed down the left. The result is lift in pounds per sq. ft. of HS area.

Lift is proportional to AOA. Therefore, the lift for 3 degrees is halfway between the numbers for 2 and 4 degrees. For 90 mph and 3 degrees, it's 3.5 lb./sq. ft. A 3 sq. ft. HS yields 10.5 lb.

Mutiply 10.5 by the lever arm. We've heard 3.5 ft. and 4 ft. In either case, it's less than half of what you need.

The errors in this process will generally not be in our favor. Airspeed in a prop slipstream is typically lower in the center than near the edges. Small HS's not only have fewer square feet (obviously) but they also are less efficient PER square foot. The chart is derived from actual tests of a smooth, formed sheet-metal wing with a NACA 0012 airfoil, not a flat plate.

Bottom line: You need more HS area or CLT. Probably double the HS area you have, if your are not going to reduce the HTL.

 

[Bob Gregory]

I have some numbers for the crunchers

This has an enclosed text file attachment.
There are what if's that show some of the
data being discussed.

 

[Doug Riley]

Guys, don't lose your perspective in irrelevant detail. This does not have to be complicated, and making it that way won't necessarily improve your results.

The mass density of air doesn't matter. Use .00238 (59 degrees F, sea level) and move on. The prop blades and the HS are subject to the same percentage of variation in their forces as the air density changes. The HS and the prop blades are both wings.

Lift per square foot varies roughly proportionally to angle of attack. IOW, if the lift is X pounds at angle of attack A, it'll be 2X pounds at angle of attack 2A.

The HS ought to do its job at a relatively low angle of attack, 2-3 degrees. Don't compute HS power at 12 or 14 degrees; the HS is about to stall at those figures, if it hasn't already.

A lift coefficient of 1.2 isn't achieveable with a small, simple HS; forget about it. Work with what you know has been done.

Fudge the whole project with a generous safety factor.

Or build CLT and make the HS design much less critical.

 

[PW_Plack]

Paul
Are you saying it is ok for him to fly his machine when the math says it will PPO on him ?
I sure hope not !


Bob, this is all moot point after Doug's posting, but two things...

A four pound net nose-down force is not going to cause a problem for anyone. The rate at which the nose-down rotation occurs is calculated from the machine's moment of inertia (how quickly it accelerates into the rotation) and the torque. This thing would rotate so slowly you'd never outrun the H-stab's increasing AOA.

(The big two-place everyone argues about produces a net nose-down force of 500-600 lbs. We're talking less than 1 per cent of that force.)

The other issue is that this number is so small as to become insignificant at speed. The parasitic drag on the airframe will be generating numbers bigger than this, which makes a four-pound HTL force just statistical noise.

As Doug points out, the way to attack this problem is with big safety margins. If the formlula says you're left with four pounds in any direction, even nose-up, your design has left you without an adequate margin.

 

[Bob]

Thank you Doug for your post !
Paul:
You make a good point in the rotation would be slow giving the forces involved... but I think it would be alot faster than you think.... in actual flight of a single place goint into a PPO that the guy actually flew out of ( only one I know of) it happened so fast that one second he was at proper attitude the next he was almost 90 degrees nose down....
in short it will happen extreamily fast though the math says it will happen slowly beware of what actually happens in the real world... as it doesn't alwayse match the theory !..
as you say though I think Doug Riley saved the day by posting his chart and explaining how to calculate it once again !
and I am pleased to see that he also thinks the HS should be doubled in size...at the very least...
.....
remember However Doug's chart is useing a NCA0012 airfoil shape and Not a flat plate... so you must compensate... another words make the down word force less because you are useing a flat plate instead of an airfoil shape.
don't forget that detail or it will all be messed up !
<GRIN>
...
Bob......

 

[dragonflyerthom]

Bob

What's wrong with this demonstration?


http://www.youtube.com/watch?v=JZgpjdeVTwA

 

[Bob Gregory]

I have some numbers for the crunchers

Spreadsheet to set the incidence of horizontal stabilizer using data from Doug Riley. Indicates PPO power push over prone situations.
Seems to correlate to the HS liftcalc.xlr file.
Add larger stabilizers, special airfoils, longer moments, lower propeller thrust lines to enhance stability.

 

[Bob]

Nothin at all Thom... I think that man could fly a broom if he wanted to ! hehehehhe
you'll notice that that machine he was flying had about as miniml a HS as you can get and still have one but it was being flown by a master.... did you knotice from the sound when he throttled down and back up? expertly done...don't try this at home folks ! <GRIN>
...
Bob Gregory:
Nice try on the XLS program but I think you missed something... in Dougs chart the figures given are for a 0012 airfoil... not a flat plate therefore useing those figures doesn't realy help unless you have a airfoil shaped HS.( the one in question doesn't, its a flat plate)
I love stredsheet programs and I used to be real good with one a long time ago, but I'm not very good with Excell yet.
Yes it does seam to conferm the HS Lift Calc.xls file but what we need is some figures for a flat plate so we can stop guessing. thank you for posting your rendition of the figures!
I will no doubt use it in the future!
....
what I'ed like to see in an XLS program is one that will calculate a flat plate AND OR a airfoil shape at a pictular angle given....
like perhaps 3 sqr.ft. at -3 degrees at 90mph on a 4ft arm
for a flat plate... or a 0012 airfoil...
if 2 programs are needed then so be it ...
Name it FlatplateHS.xls if ya want <GRIN> I'ed write it myself If I think I could do it ! hehehehehhe
....
I think almost everyone now has the Microsoft Excell spredsheet programas it comes with windows in alot of cases.

Can You write that type of thing Bob ? it sure would help alot of guys out ! post it here if ya can and I for one will be indebted to you ! as I am sure others will be too !
...
problem is I dunno where you can get a chart for a flat plate
if 1 sqr ft in size at diferent angles for diferent air speeds
....
I am sure that would be realy helpfull though as there are alot of gyros out there with flat plate HS's

thanks
Bob.....