Here's another one to cogitate. After takeoff, climbing out, at what altitude is it safe to hang a 180 and return to the field intending to land downwind? The assumption is that even with a tailwind, a safe landing can be made on the selected runway. I've practiced this at altitude and would recommend that everyone try it. I have my minimum altitude already figured out. Hint: If you unload and use about 45 degrees of bank, that is about optimum. But still, what is the minimum altitude that you would use? This is one of those bacon-savers. Bart In a message dated 8/3/2010 7:54:19 P.M. Eastern Daylight Time, [email protected] writes:
I think both Ed and Bart "nailed" the subject but I'll elaborate a bit just for fun. Bob is asking for a single "Best Glide Speed". In the absolute sense, such a single speed does not exist. For a given coupe there can be variations in indicated height AGL depending on whether one just took off with a fresh altimeter setting or if the tower took that reading 59 minutes ago and there was a frontal passage, or if the pilot set the altimeter to known ground elevation at takeoff. If one suffers power loss on departure, I greatly prefer the latter. If there is significant wind, this speed will be higher (for greater penetration and greater range) into the wind and lower downwind because even though you will be sinking faster your angle of descent from a given altitude to touchdown will be lower to such extent as your glide is extended as the wind carries your plane with it (as compared to calm conditions). As Ed pointed out, Air Speed Indicators can have significant error and a crack can develop between annuals or even in flight in one's pitot or static line so frequent GPS verification is prudent for the serious pilot. It is, therefore, worthwhile to verify and understand "what you have" [true air speed] versus what you see [indicated air speed] and be aware of wind speed and direction [water calm, wave direction and/or presence of whitecaps]. On the other hand, I think Bob's desire for a single speed to have in mind if or when the "fire goes out" is logical. Ed explains well how to derive that speed (over time). Let's assume, however, you just bought a coupe and need to fly it home. With absolutely no experience in that bird you need a number to have in mind before takeoff. Bart said "70 [mph] is close. I think." I agree with him. The Ercoupe Instruction Manual suggests approach speeds between 60 and 70 mph and describe what I interpret to be a power-off approach in a glide in a 415-C (1260 Gross Wt. and 13¼Êup elevator available). It also describes a "steepest [best] angle of climb...at an airspeed [between] 50 to 60 mph", clearly below that necessary to achieve best glide range with a coupe. The "best rate of climb...is obtained at...about 70 mph near sea level. The "Approved" Flight Manual for the 415-D (1400 Gross Wt. and 9¼Êup elevator available) suggests an approach speed of 75 mph. Best "rate of climb" was charted at 69 mph. It is logical to presume that the "best angle of climb" would still be somewhere between 50 and 60 mph. The "Approved" Flight Manual for the 415-E &G (1400 Gross Wt. and 20¼Êup (split) elevator available) suggests an approach speed of 72 mph..."a conservative glide speed higher than recommended for small fields". Best "rate of climb" was charted at 70 mph. Again, it is logical to presume that the "best angle of climb" would still be somewhere between 50 and 60 mph. Now, everyone, go out and play until you know the airspeed that yields the best glide range in your coupe (and report back!). ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ Let's take a step further. The prudent pilot in a glide is always mindful of the dangers of stalling. The Ercoupe Instruction Manual describes a 415-C in a straight glide with the control wheel full back at "about 45 mph". They use a term "minimum speed". This would seem to be used in the same sense as the term "stalling speed" is used in "Approved" Flight Manuals for later models. The 415-D has a straight glide "stalling speed" of 58 TIAS. It stalls at 66 mph at 40¼Êand 72 mph at 50¼ of bank. I like knowing I can't stall it in up to a 45¼Êbank at an Indicated Airspeed of 67mph and a True Indicated Airspeed of 70mph. The 415-E & G have a "stalling speed" of 64 mph at 40¼Êand 70 mph at 50¼ of bank and, once again, you can safely execute a 45¼Êbank at an Indicated Airspeed of 67mph and a True Indicated Airspeed of 70mph. So what? Well, I would try a 45¼Ê360¼Êturn sometime [at about 4000' AGL where there is no traffic] so I, as a pilot, gain personal experience as to what the world looks like in that configuration. Thereafter I could take comfort in knowing you can't stall a Ercoupe in a glide at 45¼Êor less angle of bank at any airspeed below 70 mph! That should take a LOT off of one's mind if a forced landing ever becomes necessary. ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ Let's further contemplate the "minimum speed" described in the Ercoupe Instruction Manual. Level flight in this configuration is accompanied by a rate of descent that should frighten those unfamiliar with it. Yet it is both possible and desirable to touch down at "minimum speed", the proverbial "full stall" landing [even in an Ercoupe!]. A pilot descending in a level power off "glide" at "minimum speed" to touchdown is a hapless victim soon to witness the main landing gear coming right up through the wings! I say this because the aircraft is "balanced in such a glide with NO "excess" energy with which to reduce the normal yet excessive rate of descent. There is one way to avoid certain disaster, and only one. The application of full power before impact, if successful, will arrest such descent almost immediately. This requires a very large withdrawal from the bag of luck every new pilot is issued ;<) The pilot should instead lower the nose no less than 200' AGL such that altitude is exchanged to increase airspeed to 60 mph or more. The "excess" forward speed (that over the "minimum speed" is then exchanged again as the yoke is moved back slowly so as to just hold the wheels off. This time "excess" airspeed (from 60 mph to the "minimum") is used to reduce the rate of descent to essentially zero by the time the control wheel is full back. Big difference in "arrivals" at the SAME INDICATED AIR SPEED! The difference is between success and failure. Regards, William R. Bayne .____|-(o)-|____. (Copyright 2010) -- On Aug 3, 2010, at 08:57, Ed Burkhead wrote: > > Bob Stearns asked: >> Is there such a thing as "Best Glide Speed" >> for the Ercoupe - 415D? I find quite a bit on >> "Glide Ratio" but nothing on speed. > > > Glide Ratio is key to define best glide speed. The speed at which you > get > the best glide ratio, power off, engine windmilling, is normally your > best > glide speed. > > The Ercoupe shares the short, thick wing type with planes like the > Tripacer > and several other planes supported by the "Short Wing Piper" club as > well as > the original Cherokee and some other planes. > > Like those other planes, the Coupe's wing is quite efficient at higher > airspeeds but at low airspeeds these planes all develop extreme "sink" > rates. (The sink rate of a Tripacer gliding at very low airspeeds > feels > like it's approaching helicopter autorotation sink! Coupes can > approach > this, too, at minimum flying speed.) > > The glide ratio curve for the Coupes (and those other planes) is very > steep > on the low speed side, has a nicely rounded, fairly wide peak around > 75-85 > mph and has a more gently sloping tail off on the high speed side. > > I've found that the Coupe's best glide speed / best glide ratio is > about 1.3 > to 1.5 times the minimum flying speed as indicated on YOUR airspeed > indicator. See the articles on airspeed indicator errors. > http://edburkhead.com/Ercoupe/airspeed_indicator_errors.htm > > This gives a decent glide ratios even with the all too common aberrant > airspeed indicators. I strongly recommend you test your ASI and get it > repaired/replaced if it's faulty. (See the testing/calibration method > at > the bottom of that article.) > > I'd recommend that you start by going up high - on your next flight, > before > your next landing - and find out the indicated airspeed on YOUR ASI at > minimum flying speed, power off, yoke all the way back. Multiply that > times > 1.4 and use that as your approach speed till you can test your ASI. > With a > good ASI, that 1.4 times minimum may give you a number as low as 70 > mph. (A > lot of Coupe owners fly approach 5 mph faster than that.) > > As I said, the top of the glide ratio curve is fairly wide. If you are > gliding toward a target at the best glide speed you determine and find > yourself coming down short of the target DO NOT EVER pull back on the > yoke - > that'll guarantee that you'll be short. > > If there's a moderate to strong headwind, you might extend the glide by > dropping the nose (big grin as this is not easy when you're gliding and > already short of your target) and pick up 5-10 mph. If you are still > coming > down short of your target you'll have to change your target. (I > remember > guidance when I was taking lessons 30+ years ago that you should only > change > your emergency landing spot once - don't dither and keep changing it.) > > Slightly increasing your glide speed will allow you to penetrate a > headwind > better while not losing much in the way of still-air-glide-ratio. > > So, the correct answer to your question is for you to go out and do > glide > ratio testing at a range of airspeeds (after calibrating your ASI). > The > results of this testing will tell you the best glide speed for your > aircraft > with your ASI. (Please report your results as we need exact, measured > data > for several aircraft to create charts that otherwise don't exist.) > > Well, that's the short answer. <grin> > > If that doesn't explain it well, or if I messed up the explanation, > please > ask further. > > Here's a bit more information: > http://edburkhead.com/Ercoupe/performance_information.htm > > Ed > > Ed Burkhead
