Finding best L/D

[hevansrv7a]

I'm in Phase 1. I have a Lift Reserve Indicator. The LRI is just a kind of AOA meter. So, instead of a long series of iterations and imperfect observations, I tried this: Find the airspeed (this day, this altitude) at which the airplane will remain in level flight with the least power. Note where the LRI is pointing. That is now my indicator for best L/D and thus best glide. The speed will likely vary with air conditions and load, but the AOA will not. I asked the LRI distributor if he agreed and he did.

To my surprise, the airspeed for this test was about 75 kts. I expected it to be higher, based on the POH's I've seen from other builders. I don't have wheel and leg fairings yet, so the speed and angle will likely be different when they are installed.

My GRT power meter said 17%, for whatever that's worth. I plan a separate posting on the power meter issue.

If this is an erroneous approach or if you have any other comments, please jump in. Thanks.

[ProCoach]

Originally Posted by hevansrv7a

Find the airspeed (this day, this altitude) at which the airplane will remain in level flight with the least power.

I've wondered about Vbg too. What do you mean by "level flight with the least power"? Least power would be idle. Level flight could be mantained with different combinations of AOA and power. You lost me...

Anyone know the RV4's typical best glide speed?

[hevansrv7a]

Level flight cannot be maintained below a given power - below that power, the airplane will sink, no matter what AOA. Referring to the charts we have all seen, the bottom of the power required curve is the best L/D. That is where induced and parasite drag intersect best. If you go slower, the drag rises; if you go faster the drag rises. You need just enough lift to keep from sinking and just enough power to maintain that lift while overcoming all forms of drag combined. If you were gliding, you'd still want that AOA, but you'd be supplying the power by using up stored (gravitational) energy in the form of altitude. In a glide, the speed might be different because the prop would not be turning, but the AOA should be pretty similiar. You can't get any closer to the perfect measurement without shutting off the engine and I'm not ready for that!

I'm being windy here because I'm really not sure what the question was. Sorry!

[az_gila]

Hevansrv7a (huh? )

There is a way of doing these tests for L/D without switching off your engine.

Jack Norris gave a presentation a long time ago to our EAA Chapter (Van Nuys, about 10 years ago..), and he did testing for the CAFE foundation then. It involved a microswitch and the location of the fore-aft play of your crankshaft in flight....

His methodology (Zero-Thrust) is described here, about half way down the long page...

http://cafefoundation.org/v2/main_story.php

A more compact report is here, but needs EAA membership to log in...

http://members.eaa.org/home/flight_r...echnology.html

gil in Tucson

[fodrv7]

I think Min Drag speed would give you Min Sink (Gliding speak for Minimum Descent rate.)

(Additionally, in the real world there are sometimes benifits gained by not operating at quite the theoretical optimum speed. Long range cruise for example is usually given as slightly higher that optimum to avoid the back of the drag curve.)

But back to your procedure, what you might try is flying at different airspeeds and noting the Pitch Angle and subtracting the Angle of Attack and this would give your actual glide angle. Not sure how acurately you could read the scales, but it should work.

But, where's Kevin.
Pete.

[hevansrv7a]

I remember that zero-thrust micro-switch article. Great stuff, if you are fanatical about it. I'm not, so this is, perhaps, next best.

As to lowest drag giving minimum sink instead of best L/D, then where on the curve would best L/D be? Now I'm really interested! My undersanding of minimum sink is that it's usually a slower speed than L/D, but which is which? And, why?

Great contributions to the group's knowledge, hopefully mine, too.

Thanks.

p.s. - Not that anyone cares, but my "handle" does not mean: H.E. Van's RV-7A. It's HEvan's RV7-A. The pun was accidental. Oops. Just call me "H".

[fodrv7]

It's all explained here. With good graphics.
Pete.
http://home.att.net/~jdburch/polar.htm

[gmcjetpilot]

Ball park Vx about 90 mph or 78 kts works pretty good, but......... we want in general best penetration speed or distance.

Definitions:
Vmd - min decent, most time in air, about 90% Vbg
Vbg - the best power-off , greatest (air) distance

Don't forget about wind, fly slower with tail wind faster with head winds. Usually take half of the wind and add or subtract as needed. (Obviously there is a practial limit to the max wind correction.)

Don't forget weight, heavy fly faster, light slower. The variation is about +/-7 mph.

Windmilling propeller - Windmilling reduces Vmd and Vbg speed over a stopped prop. Just keep in mind, if you really do have a dead engine your Vbg will be a little slower than what you got testing with an idling engine. I agree with the other poster there is no need to simulate a real emergency with a real one by intentionally shutting the engine down.

Fixed and constant speed props also have different effects on glide . It's subject to debate wheather stopping the prop is worth it? It's academic if the engine dies right after takeoff. Also some recommend going to high pitch or low RPM if constant speed prop equipped. I say it can't hurt; put the throttle full forward and prop full back. (thanks larry and pete for the correction, but I meant prop back, course, low rpm, and throttle fwd wide open. It's debatable if it makes any significant difference. In theory it sounds good. The reason for open throttle, fwd is less engine force needed in the intake (suction) stroke, ie reducing pumping loss. Less force to turn the prop over equals less drag. This is small stuff but some swear by it. I have not tested the PROP in low RPM (lever back) with a dead engine. With it idling it does make a difference. I suspect the low RPM prop trick works when testing it, is because the idling engine is making residual power/thrust, which is enhanced by going to low RPM. I don't know if you even have enough oil pressure with a dead windmilling engine make the prop move? Depends on the constant speed prop, governor and how fast your are windmilling I suspect. This comes under nice to know, it can't hurt, but it is not top priority in an emergency. Like they say fly the plane, aviate, navigate, communicate.)


Attitude is more important than speed?

More important than the speed is the attitude to me. Know what the sight picture is and have it memorized in your head. There always is going to be vertical gust, winds and other factors you can't account for. Best L/D is an angle of attack thing than speed, right.

Taking off we have a significantly high nose up pitch; it's a big push to get from nose up Vx or Vy climb to nose low best glide attitude. It's a foreign idea for most shove the nose down right after take off near the ground, but that is what you need to do if the engine stops. I know of one guy that did not get the nose down fast enough and mushed into the ground with a high sink rate. That is why I probably carry extra speed in a practice guide. I may not go as far, but I have more energy (and lift) to play with at the last part of the flight, near the ground and most important place. It does not help to get to your landing point if you are sinking like a rock.


When debating this BEST of the best glide speeds, the answer is it depends.

I don't think 5 mph will make much difference. I like to fly Vbg with a little extra for energy to control the flare and a little fast is better than a little slow. The drag curve for one is steeper on the back side.

I go right to 100 mph because it's a round number and easy to remember. I also know unless I am super light or have a big tail wind, 90 or high 80's is a min. At least that is what I think. I am not that good to fly a perfect speed and land in an emergency. I am going to be flying looking out the canopy and only glance at the airspeed. Close is good enough. All I want to do is get on the ground, under complete control, preferably at a reasonably slow speed at a place, my chosen touch down point, which I hope will not hurt too much.

[fodrv7]

All good stuff George.
Another handy thing to know, particularly with a high performance aircraft like an RV, is how much trim is required between Cruise and Best LD Speed.

If you know that 4 sec of back trim will take you from in trim at cruise speed to in trim at best LD, you can be applying that whilst you are trying to relight the fire, look for a field whilst wetting your pants.

If you don't do this you will probaably intially be throwing away altitude whilst descending at cruise speed.

Pete.

[n5lp]

Originally Posted by gmcjetpilot

...Also some recommend going to high pitch or low RPM if constant speed prop equipped. I say it can't hurt; put the throttle and prop full forward...

I'm sure you meant prop full rearward.