Stick & Rudder

Test Pilot

IN JUNE WE STARTED DISCUSSINGengine-out glide performanceby explaining the forces thatact on an airplane during aglide and illustrating howthese forces are identical tothose that affect an airplane ina steady climb (with the obvi-ous exception of the power de-livered by the engine/propeller). Weexplained the flight test techniquethat provides the data to determineyour airplane's maximum rangeglide speed and showed how this op-timum condition occurs when theplane is flown at its maximum lift-to-drag ratio (L/D).

This month we'll use data gath-ered during the descent performanceflight test in the EAA Young Eagles'RV-6A to show how to turn the flighttest data into a couple of the mostimportant airspeeds you'll ever needto know about your airplane.

We performed our test by timingdescents through a 500-foot altitudeblock from 3,750 to 3,250 feet pres-sure altitude. The mid-block pressurealtitude was 3,500 feet, which iswhere we recorded the outside airtemperature during each run. Aver-age airplane weight during the testwas 1,435 pounds, or 215 poundsbelow maximum. Our center of grav-ity was right in the middle of the al-lowable range.

Grading GlideDescent performance data reduction

ED KOLANO

During the flight I recorded theflight test data on kneeboard cardsand transcribed them to a worksheetfor the data reduction. The work-sheet has a single matrix of flighttest data that's easier to work withthan a bunch of test cards and sepa-rate worksheets. Figure 1 is the work-sheet with the raw test data (yellowcolumns) and the numbers we calcu-lated in the data reduction (bluecolumns). We'll use the 100-mphdata in the first row of our work-sheet as an example during our datareduction explanation.

Block column (3750 - 3250 =500). We entered the altitudeblock as a negative number toshow it was a descent.

Determine the average de-scent rate through the altitudeblock by dividing the altitudeblock height by the time ittook to descend through it.

Our block height is in feet, and ourelapsed time is in seconds, so we'llhave to multiply by 60 to have theanswer in feet per minute.

-500———— x 60 = -833

36

NumbersHere's the basic idea for data reduc-tion. If we know the descent rateand airspeed during the test, we candetermine the flight path angle, andthe shallower the flight path angle,the farther the airplane can glide.

Let's start with some easy math.Subtract the End Pressure Altitudefrom the Start Pressure Altitude andrecord the difference in the Altitude

Converting the average rate ofdescent to feet per minute can alsoprovide a data check. Had we notedthe descent rate on the verticalspeed indicator (VSI) during thetest, we could compare it to our cal-culated value. A significant differ-ence could help identify a waywarddata point caused by a timing erroror even a faulty VSI. Enter the calcu-lated descent rate in the Avg ROD(average rate of descent) column ofthe worksheet.

We now know how fast we camedown and how fast we flew alongthe flight path while we were timingthe descent. We'll have to take a lit-tle detour here because our averagedescent rate is a true airspeed, andour forward speed along the flight

TestOrder

12345678

ObservecAirspeed

(mph)

100801107012065130140

StartPressAlt(ffi

37503750375037503750350037503750

EndPress

Alt(ft)

32503250325032503250320032503250

AltBlock

(ft)-500-500-500-500-500-300-500-500

Mid-Block

Art(ft)

35003500350035003500335035003500

Time(sec)

3644304828282018

AV9 OATRODSU (de9C)

-833 9-682 11

-1000 11-625 11

-1071 10-643 11-1500 11-1667 11

Density TrueAlt Airspeed(ft) (mph)

36633794379438593728360937943794

106851167412769138148

FlightPath

Angle(deo.)-5.15-5.25-5.60-5.50-5.51-6.12-7.12-7.35

Remarks

Low confidenceShort run - good data

Figure 1

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Test Pilot

path is observed airspeed.To convert observed airspeed to true airspeed, first

convert it to calibrated airspeed using your airspeed cali-bration data (See "Test Pilot" January to March 2001).Then use the calibrated airspeed and the density altitudeto find the true airspeed along the flight path.

We'll use our recorded outside air temperature andpressure altitude to find the density altitude for the mid-point of the test block. The midpoint of our pressure alti-tude block is found by adding the start altitude to theend altitude and then dividing by two.

3750 + 3250———————— = 3500

2

Using the 9° C outside air temperature we recordednear the altitude block midpoint and the 3,500 feet pres-sure altitude of the midpoint, we determine the densityaltitude to be 3,663 feet. We now use this density altitudeand our calibrated airspeed for this test run to arrive at atrue airspeed along the flight path of 106 mph, which weentered in the True Airspeed worksheet column.

Now let's convert this true airspeed from miles perhour to feet per minute.

5280106 x ———— = 9328

60

If our airspeed were in knots instead of mph, wewould have used 6,076 in place of 5,280.

At this point we have both the vertical speed andflight path speed as true airspeeds in feet per minute.Figure 2 shows the trigonometric relationship betweenthese two speeds and the flight path angle. The trig sinefunction lets us use these two airspeeds to determine theflight path angle.

-833SIN(y) = ———— = -0.0016

9328

Using a calculator or trig table, we find the anglewhose sine is -0.0016 is -5.15 degrees, which we enteredin the Flight Path Angle column of the worksheet.

We repeated this series of conversions and calcula-tions for every test and completed the worksheet. At thispoint it might be tempting to merely note which testedairspeed produced the shallowest flight path angle, but alittle more work might reveal some interesting and po-tentially life-saving results.

PicturesFigure 1 indicates our shallowest flight path angle occursat 100 mph, but we can't tell from the worksheetwhether 95 mph or 105 mph might be better. If we plotairspeed versus flight path angle, we can fair a curvethrough the data points to fill the gaps in our tested datapoints, and Figure 3 is the result. Notice that we plotted

134 JULY 2002

Vertical Speed-833 ft/min

Flight Path Airspeed9328 ft/min

Figure 2

observed airspeed versus flight path angle. We couldhave plotted true airspeed, but we want the curve's infor-mation useful to us in the cockpit. Incidentally, if weplotted curves for observed and true airspeed on thesame graph, they'd be identical, with the true airspeedcurve displaced slightly to the right.

Descent Performance

20Observed Airspeed (mph)

40 60 80 100 120 140 160

££ -5

E

Rgure 3

In Figure 3 you can see that we achieve our absolutebest glide performance at about 92 mph. The flight pathangle at 92 mph is just barely shallower than our work-sheet indicates for the 100 mph test run. Our curve im-plies that you could fly any observed airspeed between80 mph and 100 mph without a significant increase inflight path angle and consequent loss of glide range.

Not all airplanes have such a versatile glide curve. Theflatter the curve's peak, the less the range penalty will befor flying an off-optimum airspeed. Having this airspeedlatitude is a bonus because you can loosen your atten-tion grip on the airspeed indicator and tend to othertimely chores like checking switches, re-starting the en-gine, choosing a landing site, and making radio calls.

Loose EndsLast month we raised the issue of airplane weight affectingyour best glide speed and promised to expand on that

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Test Pilot

topic this time. Let's use the RV-6A fora little analysis. Our average weightduring our testing was 1,435 pounds.The maximum allowable weight is1,650 pounds, and the empty weightis 1,069 pounds. Now we'll use thefollowing relationship to see how toadjust our optimum glide speed fordifferent airplane weights.

V = VTSST X

Our VTEST is 92 mph, and ourWTFST is 1,235 pounds. Plugging theRV-6A's maximum weight in for W,the optimum glide observed air-speed is 99 mph. If we use the air-plane's empty weight plus a 170-pound pilot, the best glide speedbecomes 85 mph. Sounds like a bigdifference, and it could be if our de-scent performance curve wrere not asflat across the top.

If we know the descentrate and airspeed during

the test, we candetermine the flight pathangle, and the shallower

the flight path angle,the farther the airplane

can glide.For our airplane, 90 mph is a good

round number to memorize for itsmid-weight maximum range glidespeed. We'll keep in mind that weshould fly a few mph faster whenwe're heavy and a few mph slowerat a lighter weight.

Slower; Longer SteeperFigure 4 shows our flight path angleversus airspeed, with the airplane'sdescent rate versus airspeed on thesame graph. Notice that the slowerwe flew the RV-6A, the slower it de-scended. This is valuable informa-tion should you ever find yourselfneeding maximum glide endurance.

136 JULY 2002

20Observed Airspeed (mph)

40 60 80 100 120 140 160

-1 -

-2 -

-3 -

-4 -

Ia

- - -400

-- -800

-- -1200 eo

-- -1600

-2000

Figure 4

The slower you come down, thelonger you stay up. If you're fortu-nate enough to have the engine quitright above the perfect landing field,why rush the landing? Use the extratime to troubleshoot the problem,call for help, take a couple of calm-ing breaths, and review your emer-gency procedures.

Not all airplanes exhibit the RV-6A's slower-takes-longer behavior.For some planes the left side of thedescent rate curve drops, meaningthat flying slower results in a fasterrate of descent. This alone is a com-pelling reason to create this com-posite plot for your airplane, butthere's also a paramount reason fordoing it.

In Figure 4, notice what happensto the flight path angle at speeds be-low 80 mph. It increases dramati-cally. Flying this airplane in a glideat slower than 80 mph will result ina steeper descent and substantial re-duction in glide range. For example,if a pilot flew 65 mph because hemistakenly believed the minimumdescent rate means he'll get themaximum glide range, he'd actuallytravel 16 percent less than if he flewat the best speed of 90 mph. Thatcuts more than 1/3 mile off the gliderange if the engine quit at 1,000 feetAGL or 1.75 miles from 5,000 feet.

The Young Eagles' RV-6A has alift-to-drag ratio (L/D) of slightly

better than 11 to 1 when flown at itsbest glide speed of 92 mph. Some pi-lots like to have this ratio in theirheads for a quick calculation be-cause it's also the airplane's glide ra-tio. This airplane can glide almost11 miles from 5,000 feet (almost amile) AGL or a little farther than 2miles (11,000 feet) for every 1,000feet AGL. That's a nice easy memoryitem. We determined the glide ratiousing our shallowest flight path an-gle and this formula:

L 1= Glide Ratio =

D tan(y)

This month we showed how totake raw data from our descent per-formance tests and churn out a sin-gle composite plot with vital engine-out performance information. Welearned the airplane's best glidespeed and how flying off-optimumspeed affects the glide range. We cal-culated the L/D ratio and saw thebenefit of flying slow for enduranceand the potential catastrophe of fly-ing too slow for range, with theYoung Eagles' RV-6A classically illus-trating the old adage, "You can'tstretch a glide."

Thanks for your comments andsuggestions. The address is Test Pi-lot, EAA Publications, P.O. Box 3086,Oshkosh, WI 54903-3086 or editorial@eaa.org with TEST PILOT as thesubject of your e-mail.

Sport Aviation 137

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Letters to the Editor

Flight Advisor VisibilityI read and like the Flight

Advisor and TechnicalCounselor column in theExperimenter. As a FlightAdvisor I've noticed that fewbuilders seem to haveknowledge about the pro-gram and what it attemptsto accomplish. Last fall Iaddressed a group of RVbuilders and was sur-prised and somewhatdismayed about how fewknew about the Flight Advisor (FA)program. Most had utilized the ser-vices of a Technical Counselor, butwhen it comes time to fly, they doso frequently on their own.

It appears to me that EAA is notgetting the word out. Could this bean oversight? I had assumed wrong-ly that anyone building a planeknew about the FA program. EAASport Aviation is the voice of EAAand has the broadest circulation yetdoes not feature FA info and activi-ty. Wondered if you had discussedthis with Editor in Chief ScottSpangler and what his spin was onit. It seems that safety should havea high priority but languishessomewhere down on the list. I'm alittle puzzled and feel that ifbuilders don't know they will notparticipate.

Jack BriggsCicero, New York

You're right that the Flight Advisorand Technical Counselor safety pro-grams do not receive the visibility inthe magazine that they deserve. We'readdressing this and hope to have asolution soon that consistently pro-vides useful information about theseprograms.—Scott

Descent Angle MeasurementJune's "Test Pilot" ("Descent

Performance Testing") suggests aflight-testing protocol to determine

the airspeed that maxi-mizes our engine-out enduranceand range. Discussion of the testdata instructs the reader to "findthe airspeed that produces the leastelapsed time" (to obtain an idle-power descent of a predeterminedchange in altitude). The shallowestdescent angle corresponds to a min-imum descent rate and therefore amaximum, not a minimum elapsedtime.

Giles HendersonCharleston, Illinois

The suggested data check containsan error. It should be "Find the air-speed that produced the longestelapsed time (this would be the slow-est descent rate airspeed among thespeeds tested). Now look at airspeedsfaster and slower than that speed.These should have shorter elapsedtimes." Sorry about the mix-up.—EdKolano

Heritage BoostIt was a surprise and an ego

booster to see a picture of me work-ing on my EAA Biplane, taken some40 years ago, in the March"Homebuilding's Heritage." Irecently completed a BushbyMustang II and now have 23.5hours on it.

Russ Norman, EAA 4338Hamilton, Ontario, Canada

Flying Is FunI thoroughly enjoy

flying, and it has beengreat fun and still is, butmay I add a personalnote, which may bepeculiar to me? After thelanding—always an expe-rience—and I'm off therunway and safelystopped on my tiedown, Ishut the engine, and as itdies, I finally totally relaxand pause for a moment.Exaltation! I did it again!And this may be my great-

est time. Am I the only one whofeels this way?

Don TaylorVia e-mail

More Altitude RecordsI have been reading for several

months about the altitude attemptby Bruce Bohannon and the ExxonFlyin' Tiger. I agree that this is agreat feat, but let's not forget thatmore than 35 years ago in the C-l.C Group 1 Class that WalterCable set and then broke his ownaltitude records.

This was in a non-modified pro-duction aircraft without any specialpreparation. The record was first setin a non-turbo Cessna 210 onJanuary 11, 1966, at 39,344 feet,and a year later broken in a CessnaT210G when he reached 42,344feet. These records are accredited byNAA and FAI.

Walter Cable was the son ofDewey Cable, founder of CableAirport (CCB), the world's largestprivately owned general aviationairport. For more information viewthe website at www.cableairport.com.

Roger Cable, EAA 652459Via e-mail

No Doubts About the DC-3In the June EAA Sport Aviation,

Lauran Paine Jr. writes in "Round

8 AUGUST 2002