RAF H-stabs

[Doug Riley]

I finally got a chance to measure a couple typical after-market RAF H-stab units at Bensen Days. I don't know brands, but this one looked like most that I've seen in recent pictures. The two units I measured were essentially identical and were installed in the same way.

Viewed from above, it looks like a flattened-out version of a child's drawing of a house. That is, it's a five-sided figure with swept leading edges, a straight-across trailing edge and squared-off tips. Total surface area is about 6.35 sq. ft. It has small vertical fins extending up from its "wingtips."

It has an airfoil section with, however, a sharp leading edge instead of a standard L.E. radius. The unit is mounted on a rearward extension of the tail tube, using a section of 2x2 square tubing which projects beyond the end of the tail tube about 18". The lever arm from the estimated aerodynamic center of the HS to the approximate location of the aircraft CG is 7.5 feet. The HS is mounted low enough that it probably does not receive a great deal of propwash.

An airfoil with a sharp L.E. behaves like a flat plate and is limited to a lift coefficient of around 0.4. At 100 mph in standard air, this translates into a maximum lift per square foot of HS area of 10.24 lb. Multiply this by the HS's area and by the lever arm, and you get a maximum righting moment of 488 ft.-lb. If the RAF has a 12" thrust-CG offset and produces 500 lb. of thrust, the upsetting moment is 500 ft.-lb and the HS can't save you in a worst-case PPO at wide-open throttle, even at quite a high airspeed.

Fix the sharp L.E. problem and you can easily double the lift potential. Effective tip fins can increase it further, though for this purpose they ought to droop down below the HS rather than stick up above it (i.e since the HS is a down-lifting wing). Some experimenters with gyro tails have reported lift coefficients of over 1.0 with such devices.

Things get more depressing at more realistic airspeeds, though. At 50 mph, the aerodynamic forces on the HS are 1/4 as great, but engine thrust will probably be greater than at 100. With the sharp leading edges, the maximum righting moment the HS can produce is down to 122 ft.-lb. -- only about a quarter of what you need in a full-throttle, zero-G situation. If you use a correct leading edge radius and get the lift coefficient up to 1.2 with the tip fins, you're still shy by 122 ft.-lb. With a lift coeff. of 1.6, you're JUST BARELY THERE. 1.6 is very optimistic.

This all proves what we already know from the recent Virginia RAF accident -- that a low-mounted H-stab of a realistic size by itself does not prevent PPO in a stock RAF. Stabs can and do help provide more margin against PPO and also damp out oscillations that lead to PIO, which can lead in turn to PPO. You might be able to cancel all of the PPO moment with an immersed HS, at a high cost in lost performance.* A complete and sensible cure, however, requires reducing the enormous thrustline discrepancy and the consequently large PPO moment.

* the down-load on the HS loads the rotor up just like carrying an equivalent weight in lead.

** this discussion doesn't account for the effects of the body pod, for good or for ill. You'd generally expect a pod shape with a sloping windshield and a flat bottom to produce a "down-wedging" effect that would increase the pitchover moment as airspeed increased.

[StanFoster]

Doug: Good post. In fact all your posts are just a wealth of knowlege and have been very educational for me. Also...your attitude makes all your posted information easy to digest. I personally thank you for all your effort.

My main point here is my personal experience with my Parham stab. I know it does not have the perfect airfoil shape that would give it the numbers for better performance...but let me give my version of what my experience is with it on my RAF.

I have a long trail of posts seeking advice on what negative incidence to put my Parham stab with the keel. I received several e-mails and I received suggestions from parallel to the keel..to as much as 5-6 degrees nosedown.

I thought about it for awhile...and figured if the guys flying theirs parallel were having good luck..I would try two degees nosedown and thus causing even more tail down force.

I cautiously explored different airspeeds in my RAF as I was also learning to fly the thing at the same time. I have a post where I put a Warp drive protractor on my dash to varify what my keel angle really is doing in flight.

My dash is exactly 19 degrees from plumb when my keel is level..so any reading greater than 19 degrees is how much my keel is running nosedown.

I tested it at 55...65...75...and 85....and all these speeds showed a consistent 21 degrees angle for my dash...thus translating this as a two degree nosedown attitude for my keel.

I was very satisfied that the higher speeds did not show any of the nosedown tendencies I read about. My Air Command that I had last year had a stab and a pod. I never measured how much...but the nose definately was running lower at higher speeds.

As far as how my RAF handles and feels in winds...gusty winds,..I am very satisfied. I have had absolutely no problems training myself to transition from my Air Command. I actually feel it very relaxing to fly and wind is not a concern when I fly....except for making some poor groundspeed on half my flight.

Doug...after reading your post...I have no doubt that if my stab had a true airfoil cross section...then I would probably need less negative incidence...maybe even none...and my machine would gain a few miles per hour. But right now...my main concern is to have a machine I am comfortable flying in.

Again...good information that you just posted.

Stan

[Doug Riley]

Stan: I guess what I was looking at down at B-Days was a Parham stab. I couldn't get an edgewise view of the airfoil section because the winglets were in the way. The surfaces looked like a true airfoil, though, except for the sharp leading edge.

I imagine that this sharp edge comes about because the 'glass HS is made of upper and lower half-skins which are then glued together. You need a lip for a glueing surface, hence the turned-out edge of each half-skin. The consequence of this is probably NOT so much that the stab deosn't make as much lift at X or Y angle of attack. Rather, the sharp leading edge leads to a premature stall at a rather low angle of attack... 4-5 degrees vs. the 12 or so of a blunter airfoil.

The sharp-edged HS has a lower maximum power in a potential PPO in the sense that it stalls sooner in the pitchover sequence. A HS that doesn't stall so soon will experience higher and higher down-lift as the nose pitches farther over, giving it a better chance of arresting the PPO. But at any given AOA (as long as it's lower than stalling AOA), the sharp and blunt airfoils probably won't be that far apart in that amount of force they generate or the trim effects they produce.

To balance out the PPO tendency produced by the engine-prop system WITHOUT also using rotor thrust ahead of CG, the HS would have to make a variable down-load that's (1) proportional to prop thrust and (2) NOT affected by the craft's airspeed. This pretty much requires that the HS be stuck inside the prop blast -- only then will it vary its force in response to throttle changes.

If the HS can't completely balance out the prop's nose-down moment, the rotor thrust must take care of the rest... this means that the rotor thrust line will end up ahead of the CG by some distance. That, in turn, usually means that the craft will fly slightly nose-down. More ominous, though, is the fact that the rotor's thrust can disappear in a zero-G event. This leaves you with a HS that's not offsetting the prop thrust completely. The nose will drop abruptly, though not as abruptly as if you had no HS at all. Whether this nose-drop is far or fast enough to amount to a full PPO depends on throttle setting and on how much force the HS can make at that airspeed.

[davreich]

Hi,
I've been looking at those too. I tried to put pictures up of all the stabs on one webpage. Sorry, no links because I made the name part of the picture.
www.geocities.com/gyromaniac2001/Horizontal_stabilizer.html

[Gary_in_Orygun]

This discussion is a good reminder for me to pay attention. I feel fortunate that I trained with Scott Malone, who was not used to stab'ed or CLT machines. Therefore, whenever the nose even hinted at pitching down in a gust, he retarded the throttle (and he taught me to do the same).

After he cut me loose I started letting the machine pitch nose-down more and more (but very slightly) with gusts, only to find out that it would right itself. I have a personal limit that I can't describe: when it reaches a certain nose-down point I retard the throttle "right-now!"

I think student pilots of high-thrust-line machines still need to have it drilled into them that the throttle needs to come back if the nose is "going down". Even if you have a stab, as mentioned here there may be that one set of conditions where the stab can't handle it all. I know my stab is helping me out, but I never forget that one day only retarding the throttle will keep me upright.

How else could pilots like Duane Hunn and Doffin Fritts have survived all those hours in unstab'ed, high thrust line RAFs? I'm sure they used throttle many times (automatically) to save their butts.

So, while I love my RAF, and love my stab, I will never forget that thrust may push me over one day if I'm not ready to get rid of it.

On another similar note...Ron Menzie mentioned to me that about 2000 RPM is equivalent to 0 thrust in a glide. Anything under 2000 RPM is like hanging a parachute off the back of the engine. I verified this one day when my engine quit. Earlier in the day I had been practicing idle-engine landings. The decent rate was extreme. When my engine quit later that day (I didn't know it had) after going to idle during one decent I was surprised at how much slower I was descending than previous engine-idle descents. Then I noticed that was because the engine was not running. (I got it restarted with 200 feet to spare.)

If I want to practice an engine-out landing, I set the throttle to 2000 RPM, which simulates an engine-out glide rate.

SO....If I did experience an extreme nose pitch over, pulling the throttle to idle would be like putting out a drag chute above the horizontal center line, thus helping to pull the nose up. Well, at least that is how it seems from my experience.

[C. Beaty]

“I love my RAF”

Sort of like being addicted to strychnine which is a stimulant in small doses. Addicts gradually build a tolerance, same as with an RAF.

[LARRYEBOYER]

Several months ago I changed my LM stab to a LM ultamate stab. I was an am truely amazed at the difference. Well worth the money. With the old stab, I got the ossilations in pitch that made me uncomfortable at first. I learned early on in my training and experiences that the best defence for pitch up and down moments was to simply load the blades by slightly pulling back on the stick. I left the throttle setting alone. This technique made all the difference to me plus eventually it became an autiomatic response. With the new stab, I have experienced no pitch ocilations at all. It is becoming quite comfortable for me to fly in winds that before would have kept me grounded. I understand the Parham stab gets rave reviews also.

[Doug Riley]

I learned through trial and error (luckily not fatal error) to use a touch of back stick in turbulence in my old unstable Air Command. If the turbulence wasn't too bad, the gimbal head would sometimes do it for you if you loosened your grip on the stick (NOT the same as letting go). In the more powerful vertical turbulence, I also reduced power.

It is disgraceful to have to do this. You are spending your time attempting to survive in a craft that is trying to kill you.

The numbers don't lie. A bolt-on H-stab (even if it's located on a stinger extending back from the main tail tube) alone is not enough to restore pitch stability to a stock RAF. Something must be done about the thrustline-CG relationship. Otherwise, you're still in basic-survival mode. Even if it feels more comfortable than before.

[birdy]

Doug.
I too reckon that the clt is more effective and important than H stabs.

BTW,Am after an oppinion,
when I put the 914 on the RAF with a 62" Ivo prop it dropped the thrust line down about 5 1/2 ".
With the @140bl less weight in the engine,how much closer would the machine be to clt.??????????

[Doug Riley]

Birdy -- you're changing three variables at once.

Moving the axis of the prop down (while changing nothing else) closes the thrustline offset by exactly the amount you moved it -- 5 1/2".

Moving ballast (e.g. relocating an engine of a given weight) moves the CG by a distance that equals the distance the ballast moved, multiplied by the fraction of the gross weight represented by the ballast. Example: move the 200 lb. engine on a 1200 lb. machine 6 inches. You've moved 1/6 of the gross weight, so the movement of the aircraft CG is 1/6 of the movement of the weight, or 1".

With the 914 being both lighter and a different shape than your old engine (Sub?), you have to do a more complete weight and balance check, however. For VCG, it's done the same as in a FW plane for fore-aft CG, just in the other axis.

Pick a handy datum plane -- say the plane that contains the teeter bolt, is parallel to the flight path and is level in the roll axis. Determine the vertical CG location of the entire craft (loaded, with blades), but MINUS engine. Express this location as a distance ("arm") down from the datum. Call it "Arm #1." Then determine the CG location of the new engine (they show it in the CD manual that comes with the engine). Find the arm of the engine's CG with reference to the datum plane. Call it "Arm#2." Call the complete loaded craft, less engine, "Weight #1." Call the engine's weight "Weight #2." Add Weight #1 and Weight #2 to get gross weight.

The overall CG location, or arm, with reference to the datum, then is

[(Arm#1 x Weight#1) + (Arm#2 x Weight#2)] / Gross Weight.

Once you know this distance, you can figure the remaining thrust offset, based on your new, lower prop location.

[birdy]

Thanx Doug.
I'v never been good with maths,[can't evan remember me age] but would what you stated above bring the same results as Paul B's pic method???[I can use a camera.]
Coz I'v also changed the rotors and the suspension and other things which would affect the weight distrabution.

[birdy]

I forgot to mention,it's set in with light rain here now and there's no action on the mustering scene for a while,so I'v got time to take the pics.

[Doug Riley]

Birdy: The pic method is foolproof if you're careful with the process. The math method is just a way to predict the results you'll get, before you cut the metal.

[birdy]

I took all the pix of both machines this afternoon with a digital camera thing and because I'm a computer dropout I got me 8 year old daughter to print them off and she blew the lot,so I gotta do it all again tomorrow,good thing it's still rain'n.

[Doug Riley]

Birdy: I assume you're doing a double hang test, with a plumb line in the picture?

[mceagle]

And yourself in the seat with half fuel?

[birdy]

Yup...................