Effect of simple flaps

What percentage will regular flaps inboard of ailerons reduce the landing speed?

A friend ordered an airplane expected to have a wing loading of 50oz per sq ft. Reference material from RCM indicates a relative minimum speed of 38 mph and a stall speed of 45 mph. Flaps are not considered in the reference material.

I am simply attempting to determine the flight envelope and landing speed prior initial flight-testing. How much will the landing speed be reduced with the use of flaps?

Thanks.

Bill

Ok boys have at it--
My feeling is that there is really no way of knowing without actual flight testing.

Dick is right, there's no real way of knowing (Without having a phenominal understanding of aeronautical physics)

But I can tell you this from my own experience...

I have added flaps to several planes by simply cutting about 1/3 of the inboard side of strip ailerons, and each time I have done it, they worked incredibly well, and DID NOT make any significant loss of aileron control.

A simple flap of 30.058% chord along 17.3% span with a modified clark Y airfoil and 2.4% washout at the right wing tip and 1.2% at the left tip (we think it's cool when it spins to the left...) should decrease the stall speed by 4.6% of Vne. This is only if the engine spins a three bladed wooden prop and the elevator servo has 9.8% exponential in the wrong way. Mix the flaps with full left rudder and use right aileron to keep the heading right. (just cause the spin to the left is cool, right?)

No, seriously- there's no way to give you an accurate number without more information than you have and more than the manufacturer cares to publish. Lots and lots of variables.

It was hoped that someone had put the radar gun on a heavy gal to measure flap ON vs. flap OFF minimum speed to check effectiveness.

Bill

I'm curious what this means?

Twenty percent was added to relative minimum speed to arrive at stall or estimate minimum landing speed.

Bill

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Not a radar gun... but reading speed from these videos passes with a Senior Kadet... Flaps up, fast as possible; 70 mph, Flaps up, slow as possible; 20mph, Flaps down, slow as possible; 14 mph.

That's a really cool comparison video. I'm thoroughly impressed.

what if--- instead of flaps on that Kadet -- you simply used an in flight adjustable stabilizer and kept adding AOA to it and changing power to hold altitude?
OR shove CG back to say 40% of MAC
all rather pointless except that I bet the setup which sets entire wing (or as some call it "the main(?) wing") at a higher AOA will fly slower yet.
I don't hate flaps - just musing---
My own Seniorita Kadet is soooo much fun to really slow fly - I setup the elevator for lots of throw - it has a screamin K&B Sportster 20 on it running a 12x4 prop and sounds like a muffled electric -
I can literally run alongside it in a gentle breeze.

BTW, knowing the speed still wouldn't do much good. Most of us have no idea what 14mph looks like compared to 20mph. And even if any of us did, none of us would have any experience making the model fly at either speed.

This has always been a fun idea, to know what the numbers should be, but it's never been possible to do anything with those numbers.

More to the point, if the flaps in question could do what the flaps in TallPaul's experiment did, how would anyone use that info ahead of time? Let's pretend.....

New plane is lined up on approach of very first landing attempt. Pilot (standing out on the runway next to pilot) tells copilot to drop flaps. Copilot reaches over to pilots TX, flips the switch and drops flaps. Copilot tells pilot airplane is now flying at 18mph and the planned threshold speed is 14mph. Pilot looks at airplane and decides it's actually flying at 16mph not 18mph so gives the TX throttle two clicks because he thinks a click should give about 2mph less. Plane stalls and crashes nose first into the runway. Darn. They were seeing ground speed but thinking indicated airspeed and the airplane was doing true airspeed and they hadn't paid attention to the wind which had shifted. ............ hmmmmm...........

14 mph looks kinda like 20 mph, less 6 mph.

sorta

Is that AGL or BGL?

Thats BEAGLE

Well yes I have practiced landings against the radar gun in the over 40 mph range. The mind learns to discern different speeds quickly when a measuring reference exists. Whatever the landing speed turns out to be we can practice landing at that speed with another airplane that has already had it’s share of abuse.

The more one knows about the characteristics of his airplane the more likely he is to carry it home in one piece. Most of us like to see and observe the stall speed and characteristics on the initial flights. At a wing loading of 50oz per sq ft it seems necessary to know what to expect in advance of testing and without risking the airplane.

Bill

Airplanes don't stall due to airspeed, but rather due to the angle of attack of the wing.

True, but we don't `see' that angle as easily as we `see' the airspeed of the model, and as the weight of our model is (usually) fixed, then the airspeed at which this angle is reached is also the same every time, so you can say that it stalls at an airspeed, and for our purposes it is sufficient. The plain flaps mentioned at the beginning are likely to make the model pitch up, will make the model slow down a bit (extra drag), and may reduce the stall speed `a bit'. That is assuming a moderate amount (45 deg.) of droop. More will produce more drag, but probably not much `more lift', less just less of everything up to some `best angle'. Much experimentation will be required.
Evan.

We don't see airspeed when flying RC models, we see ground speed. Watching the angle of attack is much better, especially when flying a highly loaded high performance model.

That sentence was thrown together during an attempt at humor, but has a lot of applicability in spite of it's obvious flaws.

It's an excellent idea to have as much good information as possible before acting. So let's take this idea to the next level, because the information level it's at is shortfall for a significant use.

For us to benefit from knowing the speed difference, we're going to need a couple more bits of information. One important detail will be the wind speed and direction in the approach and landing area. The difference in true airspeed of our model's two wing configurations is not very wide. So the actual windspeed and direction encountered will have significant effect and some method of measuring those wind values would be needed. The differences in airspeed that matter could be as low as one or two mph, so any windspeed at all should be known and applied.

I've found an excellent hand-held "weather station" that can readout pertinent information such as wind speed, pressure, and temperature. It also reads other information (that wouldn't be applicable to this application) but that's not applicable to this discussion. I'd ignore the effect of pressure and temperature on the performance envelope of the model however. That device would be of more value if it were held out in the approach area and it's readings relayed to the pilot to be of most value. The holder could simply shout out the readings as they changed. Some method of concisely communicating speed and direction would benefit the pilot obviously. BTW, that little device is worth it's weight in gold for learning to judge wind speeds more accurately. It's moot what value that skill actually is, however.

With the information as to wind speed and direction, the other bits of information would have lots more value.

I often do utilize the windspeed and direction during my landing approaches. I use a less exotic device than used for wind speed when judging wind direction. The feel of the wind on my ears works wonders. However, my left ear sticks out more than the right (result of a childhood injury (I got hit in the head with a rock)) so I have to adjust my observations accordingly.

With all my devices in play, I've developed such a distinctive and characteristic style of landing approach that my flying buddies have even developed a special "name" for my landings.

They have told me that I do special, "hooahh approaches". They say that they spell it: H U A

Another factor that hasn't been discussed here is the wind gradient factor. The wind are faster at higher altitude and slow as you descent. Judging airspeed isn't really practical by just looking at how fast the plane is moving is relation with the winds at ground level.

I have flown full scale for 45 years and also fly sailplanes. You have to add a percentage of indicated airspeed to your approach speed as you make turns during your approaches. With those long wings, the lower wing, during a turn may be in significant slower relative airspeed than the upper wing is. You can stall the lower wing even though your indicated airspeed is shows you are above stall speed.

While it is not as noticeable with our models (because we fly at lower altitudes) it is still a factor, especially with higher wind speeds, not to mention gusts.

I have found that judging angles of attack is fairly easy by simply looking at the wings. If you see the top of the wing during approach.... go get the shovel. If you see the bottom of the wing, then you are in a nose high attitude. By learning to look at the wing and judging the angle of attack by the width of the area you see, you can accurately judge your airspeed. Since angle of attack determines stall speed, knowing what angle your plane is at, allows you to use the elevator to control this angle allows you precise control during approach.

It works for me.

Rod

Tall Paul,

Thanks for the empirical information, which is very helpful. Based on the 6 mph delta it is guessed that the flaps were deployed to about 30 degrees. If you measure the minimum speed on other airplanes the data would be helpful.

Thanks again.

Bill

Bill, testing for minimum airspeeds is kinda like winning the contest to see who can lean out a window the furthest.
The "winner" gets a long trip with a very abrupt termination.
There's so many ways to get low speeds, practically, that testing for which works "best" would only determine what works for a specific configuration.
The actual numbers don't mean anything, we fly by interpreting what the airplane is doing.
I love to fly the Kadet in relatively strong winds.. that ground most other fliers.
The zero ground distance takeoff and hover to landing is just plain fun.

Tall Paul,

Don’t ever want anyone to break anything. The question was not asked without a purpose.

The quest is to understand in advance of testing enough about the flight characteristics to help someone get his overweight, beautiful and reasonably high dollar gal to the twentieth flight with as little damage as possible. The airplane design characteristics are frankly outside the wing loading design parameters with which I am comfortable. If it lives to flight 20 a turn on the buddy box will be required.

The initial landing attempt would likely be several passes at the radar gun probably scarring the hell out of the radar operator. Current thinking is that the minimum landing speed will be in the mid 30’s.

Bill

Bill,

Maybe it would be helpful just to see the ballpark in which your stall speed reduction might be. Flaps reduce stall speed by increasing the stall coefficient of lift (CL). The stall speed is inversely proportional to the square root of the stall CL. If you were able to double your stall CL by using flaps, your stall speed would go down by 1/1.414, or a 30% reduction. There is little chance that you will be able to double your CL with flaps, so your landing speed reduction will be less than 30%. If your flaps give you 50% higher CL, your landing speed reduction would be about 20%. If your flaps give you a 30% increase in CL, which is getting more realistic, then your landing speed would be reduced by about 12%.

Good luck,

banktoturn